What is the Security Tango?The Security Tango is my name for the dance you have to do every time you want to assure yourself that your computer is free of viruses, spyware, keystroke loggers, backdoors, trojans, and other forms of malware (click the Definitions button in the menu to see what all those things mean). It's something you need to do regularly and often - daily is not too often! The simple act of getting on the Internet and downloading email or going to a Web page can expose your computer to malicious crackers who would love to take over your machine for their own use.
To dance the Security Tango, click the Let's Dance link up above.
Two left feet? Don't worry - it's not as hard as you might think!
Which Operating System Do You Use?
Originally, the Security Tango was mostly for Windows-based computers. I'm sure that those of you running Linux or a Macintosh used to laugh yourselves sick at all the machinations that your Windows-using friends had to go through to keep themselves safe. But don't get too complacent - your time is here! As Linux and the Mac have become more popular, we've see more viruses for them. Yes, there are verified malware programs out there for both the Macintosh and for Linux. You need to protect yourself. Equally importantly, if you don't at least run an antivirus program, you run the risk of passing a virus on to your Windows friends (assuming any of them actually talk to you). And that's just not being a good net citizen!
So I've split the Tango into parts - Windows, Linux, the Macintosh, etc. I'll add more as changes in technology warrant. But you get to all of them by that same "Let's Dance!" button in the menu!
Latest Virus Alerts
All Symantec and Norton branded antivirus products
Symantec and Norton branded antivirus products contain multiple vulnerabilities. Some of these products are in widespread use throughout government and industry. Exploitation of these vulnerabilities could allow a remote attacker to take control of an affected system.
The vulnerabilities are listed below:
Symantec Antivirus multiple remote memory corruption unpacking RAR 
- Symantec antivirus products use common unpackers to extract malware binaries when scanning a system. A heap overflow vulnerability in the ASPack unpacker could allow an unauthenticated remote attacker to gain root privileges on Linux or OSX platforms. The vulnerability can be triggered remotely using a malicious file (via email or link) with no user interaction. 
- Symantec: PowerPoint misaligned stream-cache remote stack buffer overflow 
- Symantec: Remote Stack Buffer Overflow in dec2lha library 
- Symantec: Symantec Antivirus multiple remote memory corruption unpacking MSPACK Archives 
- Symantec: Heap overflow modifying MIME messages 
- Symantec: Integer Overflow in TNEF decoder 
- Symantec: missing bounds checks in dec2zip ALPkOldFormatDecompressor::UnShrink 
The large number of products affected (24 products), across multiple platforms (OSX, Windows, and Linux), and the severity of these vulnerabilities (remote code execution at root or SYSTEM privilege) make this a very serious event. A remote, unauthenticated attacker may be able to run arbitrary code at root or SYSTEM privileges by taking advantage of these vulnerabilities. Some of the vulnerabilities require no user interaction and are network-aware, which could result in a wormable-event.
US-CERT encourages users and network administrators to patch Symantec or Norton antivirus products immediately. While there has been no evidence of exploitation, the ease of attack, widespread nature of the products, and severity of the exploit may make this vulnerability a popular target.
-  Symantec Antivirus multiple remote memory corruption unpacking RAR
-  How to Compromise the Enterprise Endpoint
-  Symantec: PowerPoint misaligned stream-cache remote stack buffer overflow
-  Symantec: Remote Stack Buffer Overflow in dec2lha library
-  Symantec: Symantec Antivirus multiple remote memory corruption unpacking MSPACK Archives
-  Symantec: Heap overflow modifying MIME messages
-  Symantec: Integer Overflow in TNEF decoder
-  Symantec: missing bounds checks in dec2zip ALPkOldFormatDecompressor::UnShrink
-  Symantec SYM16-008 security advisory
-  Symantec SYM16-010 security advisory
- July 5, 2016: Initial Release
- Windows, OS X, Linux systems, and web browsers with WPAD enabled
- Networks using unregistered or unreserved TLDs
Web Proxy Auto-Discovery (WPAD) Domain Name System (DNS) queries that are intended for resolution on private or enterprise DNS servers have been observed reaching public DNS servers . In combination with the new generic top level domain (gTLD) program’s incorporation of previously undelegated gTLDs for public registration, leaked WPAD queries could result in domain name collisions with internal network naming schemes  . Opportunistic domain registrants could abuse these collisions by configuring external proxies for network traffic and enabling man-in-the-middle (MitM) attacks across the Internet.
WPAD is a protocol used to ensure all systems in an organization use the same web proxy configuration. Instead of individually modifying configurations on each device connected to a network, WPAD locates a proxy configuration file and applies the configuration automatically.
The use of WPAD is enabled by default on all Microsoft Windows operating systems and Internet Explorer browsers. WPAD is supported but not enabled by default on Mac OS X and Linux-based operating systems, as well as Safari, Chrome, and Firefox browsers.
With the New gTLD program, previously undelegated gTLD strings are now being delegated for public domain name registration . These strings may be used by private or enterprise networks, and in certain circumstances, such as when a work computer is connected from a home or external network, WPAD DNS queries may be made in error to public DNS servers. Attackers may exploit such leaked WPAD queries by registering the leaked domain and setting up MitM proxy configuration files on the Internet.
Other services (e.g., mail and internal web sites) may also perform DNS queries and attempt to automatically connect to supposedly internal DNS names .
Leaked WPAD queries could result in domain name collisions with internal network naming schemes. If an attacker registers a domain to answer leaked WPAD queries and configures a valid proxy, there is potential to conduct man-in-the-middle (MitM) attacks across the Internet.
The WPAD vulnerability is significant to corporate assets such as laptops. In some cases, these assets are vulnerable even while at work, but observations indicate that most assets become vulnerable when used outside an internal network (e.g., home networks, public Wi-Fi networks).
The impact of other types of leaked DNS queries and connection attempts varies depending on the type of service and its configuration.
US-CERT encourages users and network administrators to implement the following recommendations to provide a more secure and efficient network infrastructure:
- Consider disabling automatic proxy discovery/configuration in browsers and operating systems unless those systems will only be used on internal networks.
- Consider using a registered and fully qualified domain name (FQDN) from global DNS as the root for enterprise and other internal namespace.
- Consider using an internal TLD that is under your control and restricted from registration with the new gTLD program. Note that there is no assurance that the current list of “Reserved Names” from the new gTLD Applicant Guidebook (AGB) will remain reserved with subsequent rounds of new gTLDs .
- Configure internal DNS servers to respond authoritatively to internal TLD queries.
- Configure firewalls and proxies to log and block outbound requests for wpad.dat files.
- Identify expected WPAD network traffic and monitor the public namespace or consider registering domains defensively to avoid future name collisions.
- File a report with ICANN if your system is suffering demonstrable severe harm due to name collision by visiting https://forms.icann.org/en/help/name-collision/report-problems.
-  Verisign – MitM Attack by Name Collision: Cause Analysis and Vulnerability Assessment in the New gTLD Era
-  ICANN – Name Collision Resources & Information
-  ICANN – New gTLDs
-  US-CERT – Controlling Outbound DNS Access
-  ICANN – gTLD Applicant Guidebook
- May 23, 2016: Initial Release
- June 1, 2016: Added information on using TLDs restricted from registration with the gTLD program
Outdated or misconfigured SAP systems
At least 36 organizations worldwide are affected by an SAP vulnerability . Security researchers from Onapsis discovered indicators of exploitation against these organizations’ SAP business applications.
The observed indicators relate to the abuse of the Invoker Servlet, a built-in functionality in SAP NetWeaver Application Server Java systems (SAP Java platforms). The Invoker Servlet contains a vulnerability that was patched by SAP in 2010. However, the vulnerability continues to affect outdated and misconfigured SAP systems.
SAP systems running outdated or misconfigured software are exposed to increased risks of malicious attacks.
The Invoker Servlet vulnerability affects business applications running on SAP Java platforms.
SAP Java platforms are the base technology stack for many SAP business applications and technical components, including:
- SAP Enterprise Resource Planning (ERP),
- SAP Product Lifecycle Management (PLM),
- SAP Customer Relationship Management (CRM),
- SAP Supply Chain Management (SCM),
- SAP Supplier Relationship Management (SRM),
- SAP NetWeaver Business Warehouse (BW),
- SAP Business Intelligence (BI),
- SAP NetWeaver Mobile Infrastructure (MI),
- SAP Enterprise Portal (EP),
- SAP Process Integration (PI),
- SAP Exchange Infrastructure (XI),
- SAP Solution Manager (SolMan),
- SAP NetWeaver Development Infrastructure (NWDI),
- SAP Central Process Scheduling (CPS),
- SAP NetWeaver Composition Environment (CE),
- SAP NetWeaver Enterprise Search,
- SAP NetWeaver Identity Management (IdM), and
- SAP Governance, Risk & Control 5.x (GRC).
The vulnerability resides on the SAP application layer, so it is independent of the operating system and database application that support the SAP system.
Exploitation of the Invoker Servlet vulnerability gives unauthenticated remote attackers full access to affected SAP platforms, providing complete control of the business information and processes on these systems, as well as potential access to other systems.
In order to mitigate this vulnerability, US-CERT recommends users and administrators implement SAP Security Note 1445998 and disable the Invoker Servlet. For more mitigation details, please review the Onapsis threat report .
In addition, US-CERT encourages that users and administrators:
- Scan systems for all known vulnerabilities, such as missing security patches and dangerous system configurations.
- Identify and analyze the security settings of SAP interfaces between systems and applications to understand risks posed by these trust relationships.
- Analyze systems for malicious or excessive user authorizations.
- Monitor systems for indicators of compromise resulting from the exploitation of vulnerabilities.
- Monitor systems for suspicious user behavior, including both privileged and non-privileged users.
- Apply threat intelligence on new vulnerabilities to improve the security posture against advanced targeted attacks.
- Define comprehensive security baselines for systems and continuously monitor for compliance violations and remediate detected deviations.
These recommendations apply to SAP systems in public, private, and hybrid cloud environments.
Note: The U.S. Government does not endorse or support any particular product or vendor.
-  Onapsis Threat Report: Wild Exploitation & Cyber-Attacks on SAP Business Applications
-  SAP: Invoker Servlet
- May 11, 2016: Initial Release
Microsoft Windows with Apple QuickTime installed
According to Trend Micro, Apple will no longer be providing security updates for QuickTime for Windows, leaving this software vulnerable to exploitation. 
All software products have a lifecycle. Apple will no longer be providing security updates for QuickTime for Windows. 
Computer systems running unsupported software are exposed to elevated cybersecurity dangers, such as increased risks of malicious attacks or electronic data loss. Exploitation of QuickTime for Windows vulnerabilities could allow remote attackers to take control of affected systems.
Computers running QuickTime for Windows will continue to work after support ends. However, using unsupported software may increase the risks from viruses and other security threats. Potential negative consequences include loss of confidentiality, integrity, or availability of data, as well as damage to system resources or business assets. The only mitigation available is to uninstall QuickTime for Windows. Users can find instructions for uninstalling QuickTime for Windows on the Apple Uninstall QuickTime page. 
-  Trend Micro - Urgent Call to Action: Uninstall QuickTime for Windows Today
-  Zero Day Initiative Advisory ZDI 16-241: (0Day) Apple QuickTime moov Atom Heap Corruption Remote Code Execution Vulnerabilit
-  Zero Day Initiative Advisory ZDI 16-242: (0Day) Apple QuickTime Atom Processing Heap Corruption Remote Code Execution Vulner
-  Apple - Uninstall QuickTime 7 for Windows
- April 14, 2016: Initial Release
In early 2016, destructive ransomware variants such as Locky and Samas were observed infecting computers belonging to individuals and businesses, which included healthcare facilities and hospitals worldwide. Ransomware is a type of malicious software that infects a computer and restricts users’ access to it until a ransom is paid to unlock it.
The United States Department of Homeland Security (DHS), in collaboration with Canadian Cyber Incident Response Centre (CCIRC), is releasing this Alert to provide further information on ransomware, specifically its main characteristics, its prevalence, variants that may be proliferating, and how users can prevent and mitigate against ransomware.
WHAT IS RANSOMWARE?
Ransomware is a type of malware that infects computer systems, restricting users’ access to the infected systems. Ransomware variants have been observed for several years and often attempt to extort money from victims by displaying an on-screen alert. Typically, these alerts state that the user’s systems have been locked or that the user’s files have been encrypted. Users are told that unless a ransom is paid, access will not be restored. The ransom demanded from individuals varies greatly but is frequently $200–$400 dollars and must be paid in virtual currency, such as Bitcoin.
Ransomware is often spread through phishing emails that contain malicious attachments or through drive-by downloading. Drive-by downloading occurs when a user unknowingly visits an infected website and then malware is downloaded and installed without the user’s knowledge.
Crypto ransomware, a malware variant that encrypts files, is spread through similar methods and has also been spread through social media, such as Web-based instant messaging applications. Additionally, newer methods of ransomware infection have been observed. For example, vulnerable Web servers have been exploited as an entry point to gain access into an organization’s network.
WHY IS IT SO EFFECTIVE?
The authors of ransomware instill fear and panic into their victims, causing them to click on a link or pay a ransom, and users systems can become infected with additional malware. Ransomware displays intimidating messages similar to those below:
- “Your computer has been infected with a virus. Click here to resolve the issue.”
- “Your computer was used to visit websites with illegal content. To unlock your computer, you must pay a $100 fine.”
- “All files on your computer have been encrypted. You must pay this ransom within 72 hours to regain access to your data.”
PROLIFERATION OF VARIANTS
In 2012, Symantec, using data from a command and control (C2) server of 5,700 computers compromised in one day, estimated that approximately 2.9 percent of those compromised users paid the ransom. With an average ransom of $200, this meant malicious actors profited $33,600 per day, or $394,400 per month, from a single C2 server. These rough estimates demonstrate how profitable ransomware can be for malicious actors.
This financial success has likely led to a proliferation of ransomware variants. In 2013, more destructive and lucrative ransomware variants were introduced, including Xorist, CryptorBit, and CryptoLocker. Some variants encrypt not just the files on the infected device, but also the contents of shared or networked drives. These variants are considered destructive because they encrypt users’ and organizations’ files, and render them useless until criminals receive a ransom.
Samas, another variant of destructive ransomware, was used to compromise the networks of healthcare facilities in 2016. Unlike Locky, Samas propagates through vulnerable Web servers. After the Web server was compromised, uploaded Ransomware-Samas files were used to infect the organization’s networks.
LINKS TO OTHER TYPES OF MALWARE
Systems infected with ransomware are also often infected with other malware. In the case of CryptoLocker, a user typically becomes infected by opening a malicious attachment from an email. This malicious attachment contains Upatre, a downloader, which infects the user with GameOver Zeus. GameOver Zeus is a variant of the Zeus Trojan that steals banking information and is also used to steal other types of data. Once a system is infected with GameOver Zeus, Upatre will also download CryptoLocker. Finally, CryptoLocker encrypts files on the infected system, and requests that a ransom be paid.
The close ties between ransomware and other types of malware were demonstrated through the recent botnet disruption operation against GameOver Zeus, which also proved effective against CryptoLocker. In June 2014, an international law enforcement operation successfully weakened the infrastructure of both GameOver Zeus and CryptoLocker.
Ransomware not only targets home users; businesses can also become infected with ransomware, leading to negative consequences, including
- temporary or permanent loss of sensitive or proprietary information,
- disruption to regular operations,
- financial losses incurred to restore systems and files, and
- potential harm to an organization’s reputation.
Paying the ransom does not guarantee the encrypted files will be released; it only guarantees that the malicious actors receive the victim’s money, and in some cases, their banking information. In addition, decrypting files does not mean the malware infection itself has been removed.
Infections can be devastating to an individual or organization, and recovery can be a difficult process that may require the services of a reputable data recovery specialist.
US-CERT recommends that users and administrators take the following preventive measures to protect their computer networks from ransomware infection:
- Employ a data backup and recovery plan for all critical information. Perform and test regular backups to limit the impact of data or system loss and to expedite the recovery process. Note that network-connected backups can also be affected by ransomware; critical backups should be isolated from the network for optimum protection.
- Use application whitelisting to help prevent malicious software and unapproved programs from running. Application whitelisting is one of the best security strategies as it allows only specified programs to run, while blocking all others, including malicious software.
- Keep your operating system and software up-to-date with the latest patches. Vulnerable applications and operating systems are the target of most attacks. Ensuring these are patched with the latest updates greatly reduces the number of exploitable entry points available to an attacker.
- Maintain up-to-date anti-virus software, and scan all software downloaded from the internet prior to executing.
- Restrict users’ ability (permissions) to install and run unwanted software applications, and apply the principle of “Least Privilege” to all systems and services. Restricting these privileges may prevent malware from running or limit its capability to spread through the network.
- Avoid enabling macros from email attachments. If a user opens the attachment and enables macros, embedded code will execute the malware on the machine. For enterprises or organizations, it may be best to block email messages with attachments from suspicious sources. For information on safely handling email attachments, see Recognizing and Avoiding Email Scams. Follow safe practices when browsing the Web. See Good Security Habits and Safeguarding Your Data for additional details.
- Do not follow unsolicited Web links in emails. Refer to the US-CERT Security Tip on Avoiding Social Engineering and Phishing Attacks or the Security Publication on Ransomware for more information.
Individuals or organizations are discouraged from paying the ransom, as this does not guarantee files will be released. Report instances of fraud to the FBI at the Internet Crime Complaint Center.
- Kaspersky Lab, Kaspersky Lab detects mobile Trojan Svpeng: Financial malware with ransomware capabilities now targeting U.S.
- Sophos / Naked Security, What’s next for ransomware? CryptoWall picks up where CryptoLocker left off
- Symantec, CryptoDefence, the CryptoLocker Imitator, Makes Over $34,000 in One Month
- Symantec, Cryptolocker: A Thriving Menace
- Symantec, Cryptolocker Q&A: Menace of the Year
- Symantec, International Takedown Wounds Gameover Zeus Cybercrime Network
- Sophos / Naked Security, “Locky” ransomware – what you need to know
- McAfee Labs Threat Advisory: Ransomware-Locky. March 9, 2016
- SamSam: The Doctor Will See You, After He Pays The Ransom
- March 31, 2016: Initial publication
- May 6, 2016: Clarified guidance on offline backups
- July 11, 2016: Added link to governmental interagency guidance on ransomware
Dorkbot is a botnet used to steal online payment, participate in distributed denial-of-service (DDoS) attacks, and deliver other types of malware to victims’ computers. According to Microsoft, the family of malware used in this botnet “has infected more than one million personal computers in over 190 countries over the course of the past year.” The United States Department of Homeland Security (DHS), in collaboration with the Federal Bureau of Investigation (FBI) and Microsoft, is releasing this Technical Alert to provide further information about Dorkbot.
Dorkbot-infected systems are used by cyber criminals to steal sensitive information (such as user account credentials), launch denial-of-service (DoS) attacks, disable security protection, and distribute several malware variants to victims’ computers. Dorkbot is commonly spread via malicious links sent through social networks instant message programs or through infected USB devices.
In addition, Dorkbot’s backdoor functionality allows a remote attacker to exploit infected system. According to Microsoft’s analysis, a remote attacker may be able to:
- Download and run a file from a specified URL;
- Collect logon information and passwords through form grabbing, FTP, POP3, or Internet Explorer and Firefox cached login details; or
- Block or redirect certain domains and websites (e.g., security sites).
A system infected with Dorkbot may be used to send spam, participate in DDoS attacks, or harvest users' credentials for online services, including banking services.
Users are advised to take the following actions to remediate Dorkbot infections:
- Use and maintain anti-virus software – Anti-virus software recognizes and protects your computer against most known viruses. Even though Dorkbot is designed to evade detection, security companies are continuously updating their software to counter these advanced threats. Therefore, it is important to keep your anti-virus software up-to-date. If you suspect you may be a victim of Dorkbot, update your anti-virus software definitions and run a full-system scan. (See Understanding Anti-Virus Software for more information.)
- Change your passwords – Your original passwords may have been compromised during the infection, so you should change them. (See Choosing and Protecting Passwords for more information.)
- Keep your operating system and application software up-to-date – Install software patches so that attackers cannot take advantage of known problems or vulnerabilities. You should enable automatic updates of the operating system if this option is available. (See Understanding Patches for more information.)
- Use anti-malware tools – Using a legitimate program that identifies and removes malware can help eliminate an infection. Users can consider employing a remediation tool (see example below) to help remove Dorkbot from their systems.
- Disable Autorun – Dorkbot tries to use the Windows Autorun function to propagate via removable drives (e.g., USB flash drive). You can disable Autorun to stop the threat from spreading.
The above example does not constitute an exhaustive list. The U.S. Government does not endorse or support any particular product or vendor.
- Microsoft Malware Protection Center – Worm: Win32/Dorkbot
- Microsoft Malware Protection Center – Microsoft assists law enforcement to help disrupt Dorkbot botnets
- December 3, 2015: Initial Publication
Compromised web servers with malicious web shells installed
This alert describes the frequent use of web shells as an exploitation vector. Web shells can be used to obtain unauthorized access and can lead to wider network compromise. This alert outlines the threat and provides prevention, detection, and mitigation strategies.
Consistent use of web shells by Advanced Persistent Threat (APT) and criminal groups has led to significant cyber incidents.
This product was developed in collaboration with US-CERT partners in the United Kingdom, Australia, Canada, and New Zealand based on activity seen targeting organizations across these countries. The detection and mitigation measures outlined in this document represent the shared judgement of all participating agencies.
Web Shell Description
A web shell is a script that can be uploaded to a web server to enable remote administration of the machine. Infected web servers can be either Internet-facing or internal to the network, where the web shell is used to pivot further to internal hosts.
A web shell can be written in any language that the target web server supports. The most commonly observed web shells are written in languages that are widely supported, such as PHP and ASP. Perl, Ruby, Python, and Unix shell scripts are also used.
Using network reconnaissance tools, an adversary can identify vulnerabilities that can be exploited and result in the installation of a web shell. For example, these vulnerabilities can exist in content management systems (CMS) or web server software.
Once successfully uploaded, an adversary can use the web shell to leverage other exploitation techniques to escalate privileges and to issue commands remotely. These commands are directly linked to the privilege and functionality available to the web server and may include the ability to add, delete, and execute files as well as the ability to run shell commands, further executables, or scripts.
How and why are they used by malicious adversaries?
Web shells are frequently used in compromises due to the combination of remote access and functionality. Even simple web shells can have a considerable impact and often maintain minimal presence.
Web shells are utilized for the following purposes:
- To harvest and exfiltrate sensitive data and credentials;
- To upload additional malware for the potential of creating, for example, a watering hole for infection and scanning of further victims;
- To use as a relay point to issue commands to hosts inside the network without direct Internet access;
- To use as command-and-control infrastructure, potentially in the form of a bot in a botnet or in support of compromises to additional external networks. This could occur if the adversary intends to maintain long-term persistence.
While a web shell itself would not normally be used for denial of service (DoS) attacks, it can act as a platform for uploading further tools, including DoS capability.
Web shells such as China Chopper, WSO, C99 and B374K are frequently chosen by adversaries; however these are just a small number of known used web shells. (Further information linking to IOCs and SNORT rules can be found in the Additional Resources section).
- China Chopper – A small web shell packed with features. Has several command and control features including a password brute force capability.
- WSO – Stands for “web shell by orb” and has the ability to masquerade as an error page containing a hidden login form.
- C99 – A version of the WSO shell with additional functionality. Can display the server’s security measures and contains a self-delete function.
- B374K – PHP based web shell with common functionality such as viewing processes and executing commands.
Web shells can be delivered through a number of web application exploits or configuration weaknesses including:
- Cross-Site Scripting;
- SQL Injection;
- Vulnerabilities in applications/services (e.g., WordPress or other CMS applications);
- File processing vulnerabilities (e.g., upload filtering or assigned permissions);
- Remote File Include (RFI) and Local File Include (LFI) vulnerabilities;
- Exposed Admin Interfaces (possible areas to find vulnerabilities mentioned above).
The above tactics can be and are combined regularly. For example, an exposed admin interface also requires a file upload option, or another exploit method mentioned above, to deliver successfully.
A successfully uploaded shell script may allow a remote attacker to bypass security restrictions and gain unauthorized system access.
Prevention and Mitigation
Installation of a web shell is commonly accomplished through web application vulnerabilities or configuration weaknesses. Therefore, identification and closure of these vulnerabilities is crucial to avoiding potential compromise. The following suggestions specify good security and web shell specific practices:
- Employ regular updates to applications and the host operating system to ensure protection against known vulnerabilities.
- Implement a least-privileges policy on the web server to:
- Reduce adversaries’ ability to escalate privileges or pivot laterally to other hosts.
- Control creation and execution of files in particular directories.
- If not already present, consider deploying a demilitarized zone (DMZ) between your webfacing systems and the corporate network. Limiting the interaction and logging traffic between the two provides a method to identify possible malicious activity.
- Ensure a secure configuration of web servers. All unnecessary services and ports should be disabled or blocked. All necessary services and ports should be restricted where feasible. This can include whitelisting or blocking external access to administration panels and not using default login credentials.
- Utilize a reverse proxy or alternative service, such as mod_security, to restrict accessible URL paths to known legitimate ones.
- Establish, and backup offline, a “known good” version of the relevant server and a regular change-management policy to enable monitoring for changes to servable content with a file integrity system.
- Employ user input validation to restrict local and remote file inclusion vulnerabilities.
- Conduct regular system and application vulnerability scans to establish areas of risk. While this method does not protect against zero day attacks it will highlight possible areas of concern.
- Deploy a web application firewall and conduct regular virus signature checks, application fuzzing, code reviews and server network analysis.
Due to the potential simplicity and ease of modification of web shells, they can be difficult to detect. For example, anti-virus products sometimes produce poor results in detecting web shells.
The following may be indicators that your system has been infected by a web shell. Note a number of these indicators are common to legitimate files. Any suspected malicious files should be considered in the context of other indicators and triaged to determine whether further inspection or validation is required.
- Abnormal periods of high site usage (due to potential uploading and downloading activity);
- Files with an unusual timestamp (e.g., more recent than the last update of the web applications installed);
- Suspicious files in Internet-accessible locations (web root);
- Files containing references to suspicious keywords such as cmd.exe or eval;
- Unexpected connections in logs. For example:
- A file type generating unexpected or anomalous network traffic (e.g., a JPG file making requests with POST parameters);
- Suspicious logins originating from internal subnets to DMZ servers and vice versa.
- Any evidence of suspicious shell commands, such as directory traversal, by the web server process.
For investigating many types of shells, a search engine can be very helpful. Often, web shells will be used to spread malware onto a server and the search engines are able to see it. But many web shells check the User-Agent and will display differently for a search engine spider (a program that crawls through links on the Internet, grabbing content from sites and adding it to search engine indexes) than for a regular user. To find a shell, you may need to change your User-Agent to one of the search engine bots. Some browsers have plugins that allow you to easily switch a User-Agent. Once the shell is detected, simply delete the file from the server.
Client characteristics can also indicate possible web shell activity. For example, the malicious actor will often visit only the URI where the web shell script was created, but a standard user usually loads the webpage from a linked page/referrer or loads additional content/resources. Thus, performing frequency analysis on the web access logs could indicate the location of a web shell. Most legitimate URI visits will contain varying user-agents, whereas a web shell is generally only visited by the creator, resulting in limited user-agent variants.
- Australian Cyber Security Centre – Securing Content Management Systems (CMS)
- FireEye China Chopper – The Little Malware That Could. Detecting and Defeating the China Chopper Web Shell
- MANDIANT – Old Web Shells New Tricks
- FireEye – Breaking Down the China Chopper Web Shell Part I
- FireEye – Breaking Down the China Chopper Web Shell Part II
- WSO Information
- Exploit-db – China Chopper
- INFOSEC Institute – Web Shell Detection
- November 10, 2015: Initial Release
- November 13, 2015: Changes to Title and Systems Affected sections
Dridex, a peer-to-peer (P2P) bank credential-stealing malware, uses a decentralized network infrastructure of compromised personal computers and web servers to execute command-and-control (C2). The United States Department of Homeland Security (DHS), in collaboration with the Federal Bureau of Investigation (FBI) and the Department of Justice (DOJ), is releasing this Technical Alert to provide further information about the Dridex botnet.
Dridex is a multifunctional malware package that leverages obfuscated macros in Microsoft Office and extensible markup language (XML) files to infect systems. The primary goal of Dridex is to infect computers, steal credentials, and obtain money from victims’ bank accounts. Operating primarily as a banking Trojan, Dridex is generally distributed through phishing email messages. The emails appear legitimate and are carefully crafted to entice the victim to click on a hyperlink or to open a malicious attached file. Once a computer has been infected, Dridex is capable of stealing user credentials through the use of surreptitious keystroke logging and web injects.
A system infected with Dridex may be employed to send spam, participate in distributed denial-of-service (DDoS) attacks, and harvest users' credentials for online services, including banking services.
Users are recommended to take the following actions to remediate Dridex infections:
- Use and maintain anti-virus software - Anti-virus software recognizes and protects your computer against most known viruses. Even though Dridex is designed to evade detection, security companies are continuously updating their software to counter these advanced threats. Therefore, it is important to keep your anti-virus software up-to-date (see Understanding Anti-Virus Software for more information).
- Change your passwords - Your original passwords may have been compromised during the infection, so you should change them (see Choosing and Protecting Passwords for more information).
- Keep your operating system and application software up-to-date - Install software patches so that attackers can't take advantage of known problems or vulnerabilities. Many operating systems offer automatic updates. You should enable automatic updates if this option is available (see Understanding Patches for more information).
- Use anti-malware tools - Using a legitimate program that identifies and removes malware can help eliminate an infection. Users can consider employing a remediation tool (examples below) to help remove Dridex from your system.
The above are examples only and do not constitute an exhaustive list. The U.S. Government does not endorse or support any particular product or vendor.
- Initial Publication - October 13, 2015
US-CERT has observed an increase in Domain Name System (DNS) traffic from client systems within internal networks to publically hosted DNS servers. Direct client access to Internet DNS servers, rather than controlled access through enterprise DNS servers, can expose an organization to unnecessary security risks and system inefficiencies. This Alert provides recommendations for improving security related to outbound DNS queries and responses.
Client systems and applications may be configured to send DNS requests to servers other than authorized enterprise DNS caching name servers (also called resolving, forwarding or recursive name servers). This type of configuration poses a security risk and may introduce inefficiencies to an organization.
Unless managed by perimeter technical solutions, client systems and applications may connect to systems outside the enterprise’s administrative control for DNS resolution. Internal enterprise systems should only be permitted to initiate requests to and receive responses from approved enterprise DNS caching name servers. Permitting client systems and applications to connect directly to Internet DNS infrastructure introduces risks and inefficiencies to the organization, which include:
- Bypassed enterprise monitoring and logging of DNS traffic; this type of monitoring is an important tool for detecting potential malicious network activity.
- Bypassed enterprise DNS security filtering (sinkhole/redirect or blackhole/block) capabilities; this may allow clients to access malicious domains that would otherwise be blocked.
- Client interaction with compromised or malicious DNS servers; this may cause inaccurate DNS responses for the domain requested (e.g., the client is sent to a phishing site or served malicious code).
- Lost protections against DNS cache poisoning and denial-of-service attacks. The mitigating effects of a tiered or hierarchical (e.g., separate internal and external DNS servers, split DNS, etc.) DNS architecture used to prevent such attacks are lost.
- Reduced Internet browsing speed since enterprise DNS caching would not be utilized.
Implement the recommendations below to provide a more secure and efficient DNS infrastructure. Please note that these recommendations focus on improving the security of outbound DNS query or responses and do not encompass all DNS security best practices.
- Configure operating systems and applications (including lower-tier DNS servers intended to forward queries to controlled enterprise DNS servers) to use only authorized DNS servers within the enterprise for outbound DNS resolution.
- Configure enterprise perimeter network devices to block all outbound User Datagram Protocol (UDP) and Transmission Control Protocol (TCP) traffic to destination port 53, except from specific, authorized DNS servers (including both authoritative and caching/forwarding name servers).
- Additionally, filtering inbound destination port 53 TCP and UDP traffic to only allow connections to authorized DNS servers (including both authoritative and caching/forwarding name servers) will provide additional protections.
- Refer to Section 12 of the NIST Special Publication 800-81-2 for guidance when configuring enterprise recursive DNS resolvers. 
- August 28, 2015: Initial Release
Microsoft Windows Systems, Adobe Flash Player, and Linux
Between June and July 2015, the United States Computer Emergency Readiness Team (US-CERT) received reports of multiple, ongoing and likely evolving, email-based phishing campaigns targeting U.S. Government agencies and private sector organizations. This alert provides general and phishing-specific mitigation strategies and countermeasures.
US-CERT is aware of three phishing campaigns targeting U.S. Government agencies and private organizations across multiple sectors. All three campaigns leveraged website links contained in emails; two sites exploited a recent Adobe Flash vulnerability (CVE-2015-5119) while the third involved the download of a compressed (i.e., ZIP) file containing a malicious executable file. Most of the websites involved are legitimate corporate or organizational sites that were compromised and are hosting malicious content.
Systems infected through targeted phishing campaigns act as an entry point for attackers to spread throughout an organization’s entire enterprise, steal sensitive business or personal information, or disrupt business operations.
Phishing Mitigation and Response Recommendations
- Implement perimeter blocks for known threat indicators:
- Email server or email security gateway filters for email indicators
- Web proxy and firewall filters for websites or Internet Protocol (IP) addresses linked in the emails or used by related malware
- DNS server blocks (blackhole) or redirects (sinkhole) for known related domains and hostnames
- Remove malicious emails from targeted user mailboxes based on email indicators (e.g., using Microsoft ExMerge).
- Identify recipients and possible infected systems:
- Search email server logs for applicable sender, subject, attachments, etc. (to identify users that may have deleted the email and were not identified in purge of mailboxes)
- Search applicable web proxy, DNS, firewall or IDS logs for activity the malicious link clicked.
- Search applicable web proxy, DNS, firewall or IDS logs for activity to any associated command and control (C2) domains or IP addresses associated with the malware.
- Review anti-virus (AV) logs for alerts associated with the malware. AV products should be configured to be in quarantine mode. It is important to note that the absence of AV alerts or a clean AV scan should not be taken as conclusive evidence a system is not infected.
- Scan systems for host-level indicators of the related malware (e.g., YARA signatures)
- For systems that may be infected:
- Capture live memory of potentially infected systems for analysis
- Take forensic images of potentially infected systems for analysis
- Isolate systems to a virtual local area network (VLAN) segmented form the production agency network (e.g., an Internet-only segment)
- Report incidents, with as much detail as possible, to the NCCIC.
Educate Your Users
Organizations should remind users that they play a critical role in protecting their organizations form cyber threats. Users should:
- Exercise caution when opening email attachments, even if the attachment is expected and the sender appears to be known. Be particularly wary of compressed or ZIP file attachments.
- Avoid clicking directly on website links in emails; attempts to verify web addresses independently (e.g., contact your organization’s helpdesk or search the Internet for the main website of the organization or topic mentioned in the email).
- Report any suspicious emails to the information technology (IT) helpdesk or security office immediately.
Basic Cyber Hygiene
Practicing basic cyber hygiene would address or mitigate the vast majority of security breaches handled by today’s security practitioners:
- Privilege control (i.e., minimize administrative or superuser privileges)
- Application whitelisting / software execution control (by file or location)
- System application patching (e.g., operating system vulnerabilities, third-party vendor applications)
- Security software updating (e.g., AV definitions, IDS/IPS signatures and filters)
- Network segmentation (e.g., separate administrative networks from business-critical networks with physical controls and virtual local area networks)
- Multi-factor authentication (e.g., one-time password tokens, personal identity verification (PIV cards)
For more information on cybersecurity best practices, users and administrators are encouraged to review US-CERT Security Tip: Handling Destructive Malware to evaluate their capabilities encompassing planning, preparation, detection, and response. Another resource is ICS-CERT Recommended Practice: Improving Industrial Control Systems Cybersecurity with Defense-In-Depth Strategies.
- Executive Order 13636: Cybersecurity Framework
- US-CERT Security Tip: Handling Destructive Malware
- ICS-CERT Recommended Practice: Improving Industrial Control Systems Cybersecurity with Defense-In-Depth Strategies
- August 1, 2015: Initial Release
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