“Avalanche” refers to a large global network hosting infrastructure used by cyber criminals to conduct phishing and malware distribution campaigns and money mule schemes. The United States Department of Homeland Security (DHS), in collaboration with the Federal Bureau of Investigation (FBI), is releasing this Technical Alert to provide further information about Avalanche.
Cyber criminals utilized Avalanche botnet infrastructure to host and distribute a variety of malware variants to victims, including the targeting of over 40 major financial institutions. Victims may have had their sensitive personal information stolen (e.g., user account credentials). Victims’ compromised systems may also have been used to conduct other malicious activity, such as launching denial-of-service (DoS) attacks or distributing malware variants to other victims’ computers.
In addition, Avalanche infrastructure was used to run money mule schemes where criminals recruited people to commit fraud involving transporting and laundering stolen money or merchandise.
Avalanche used fast-flux DNS, a technique to hide the criminal servers, behind a constantly changing network of compromised systems acting as proxies.
The following malware families were hosted on the infrastructure:
Avalanche was also used as a fast flux botnet which provides communication infrastructure for other botnets, including the following:
A system infected with Avalanche-associated malware may be subject to malicious activity including the theft of user credentials and other sensitive data, such as banking and credit card information. Some of the malware had the capability to encrypt user files and demand a ransom be paid by the victim to regain access to those files. In addition, the malware may have allowed criminals unauthorized remote access to the infected computer. Infected systems could have been used to conduct distributed denial-of-service (DDoS) attacks.
Users are advised to take the following actions to remediate malware infections associated with Avalanche:
ESET Online Scanner
Microsoft Safety Scanner
Norton Power Eraser
Trend Micro HouseCall
Internet of Things (IoT)—an emerging network of devices (e.g., printers, routers, video cameras, smart TVs) that connect to one another via the Internet, often automatically sending and receiving data
Recently, IoT devices have been used to create large-scale botnets—networks of devices infected with self-propagating malware—that can execute crippling distributed denial-of-service (DDoS) attacks. IoT devices are particularly susceptible to malware, so protecting these devices and connected hardware is critical to protect systems and networks.
On September 20, 2016, Brian Krebs’ security blog (krebsonsecurity.com) was targeted by a massive DDoS attack, one of the largest on record, exceeding 620 gigabits per second (Gbps).[1 (link is external)] An IoT botnet powered by Mirai malware created the DDoS attack. The Mirai malware continuously scans the Internet for vulnerable IoT devices, which are then infected and used in botnet attacks. The Mirai bot uses a short list of 62 common default usernames and passwords to scan for vulnerable devices. Because many IoT devices are unsecured or weakly secured, this short dictionary allows the bot to access hundreds of thousands of devices.[2 (link is external)] The purported Mirai author claimed that over 380,000 IoT devices were enslaved by the Mirai malware in the attack on Krebs’ website.[3 (link is external)]
In late September, a separate Mirai attack on French webhost OVH broke the record for largest recorded DDoS attack. That DDoS was at least 1.1 terabits per second (Tbps), and may have been as large as 1.5 Tbps.[4 (link is external)]
The IoT devices affected in the latest Mirai incidents were primarily home routers, network-enabled cameras, and digital video recorders.[5 (link is external)] Mirai malware source code was published online at the end of September, opening the door to more widespread use of the code to create other DDoS attacks.
In early October, Krebs on Security reported on a separate malware family responsible for other IoT botnet attacks.[6 (link is external)] This other malware, whose source code is not yet public, is named Bashlite. This malware also infects systems through default usernames and passwords. Level 3 Communications, a security firm, indicated that the Bashlite botnet may have about one million enslaved IoT devices.[7 (link is external)]
With the release of the Mirai source code on the Internet, there are increased risks of more botnets being generated. Both Mirai and Bashlite can exploit the numerous IoT devices that still use default passwords and are easily compromised. Such botnet attacks could severely disrupt an organization’s communications or cause significant financial harm.
Software that is not designed to be secure contains vulnerabilities that can be exploited. Software-connected devices collect data and credentials that could then be sent to an adversary’s collection point in a back-end application.
In late November 2016, a new Mirai-derived malware attack actively scanned TCP port 7547 on broadband routers susceptible to a Simple Object Access Protocol (SOAP) vulnerability. [8 (link is external)] Affected routers use protocols that leave port 7547 open, which allows for exploitation of the router. These devices can then be remotely used in DDoS attacks. [9, 10 (links are external)]
Cybersecurity professionals should harden networks against the possibility of a DDoS attack. For more information on DDoS attacks, please refer to US-CERT Security Publication DDoS Quick Guide and the US-CERT Alert on UDP-Based Amplification Attacks.
In order to remove the Mirai malware from an infected IoT device, users and administrators should take the following actions:
In order to prevent a malware infection on an IoT device, users and administrators should take following precautions:
Network Infrastructure Devices
The advancing capabilities of organized hacker groups and cyber adversaries create an increasing global threat to information systems. The rising threat levels place more demands on security personnel and network administrators to protect information systems. Protecting the network infrastructure is critical to preserve the confidentiality, integrity, and availability of communication and services across an enterprise.
To address threats to network infrastructure devices, this Alert provides information on recent vectors of attack that advanced persistent threat (APT) actors are targeting, along with prevention and mitigation recommendations.
Network infrastructure consists of interconnected devices designed to transport communications needed for data, applications, services, and multi-media. Routers and firewalls are the focus of this alert; however, many other devices exist in the network, such as switches, load-balancers, intrusion detection systems, etc. Perimeter devices, such as firewalls and intrusion detection systems, have been the traditional technologies used to secure the network, but as threats change, so must security strategies. Organizations can no longer rely on perimeter devices to protect the network from cyber intrusions; organizations must also be able to contain the impact/losses within the internal network and infrastructure.
For several years now, vulnerable network devices have been the attack-vector of choice and one of the most effective techniques for sophisticated hackers and advanced threat actors. In this environment, there has never been a greater need to improve network infrastructure security. Unlike hosts that receive significant administrative security attention and for which security tools such as anti-malware exist, network devices are often working in the background with little oversight—until network connectivity is broken or diminished. Malicious cyber actors take advantage of this fact and often target network devices. Once on the device, they can remain there undetected for long periods. After an incident, where administrators and security professionals perform forensic analysis and recover control, a malicious cyber actor with persistent access on network devices can reattack the recently cleaned hosts. For this reason, administrators need to ensure proper configuration and control of network devices.
In September 2015, an attack known as SYNful Knock was disclosed. SYNful Knock silently changes a router’s operating system image, thus allowing attackers to gain a foothold on a victim’s network. The malware can be customized and updated once embedded. When the modified malicious image is uploaded, it provides a backdoor into the victim’s network. Using a crafted TCP SYN packet, a communication channel is established between the compromised device and the malicious command and control (C2) server. The impact of this infection to a network or device is severe and most likely indicates that there may be additional backdoors or compromised devices on the network. This foothold gives an attacker the ability to maneuver and infect other hosts and access sensitive data.
The initial infection vector does not leverage a zero-day vulnerability. Attackers either use the default credentials to log into the device or obtain weak credentials from other insecure devices or communications. The implant resides within a modified IOS image and, when loaded, maintains its persistence in the environment, even after a system reboot. Any further modules loaded by the attacker will only exist in the router’s volatile memory and will not be available for use after the device reboots. However, these devices are rarely or never rebooted.
To prevent the size of the image from changing, the malware overwrites several legitimate IOS functions with its own executable code. The attacker examines the functionality of the router and determines functions that can be overwritten without causing issues on the router. Thus, the overwritten functions will vary upon deployment.
The attacker can utilize the secret backdoor password in three different authentication scenarios. In these scenarios the implant first checks to see if the user input is the backdoor password. If so, access is granted. Otherwise, the implanted code will forward the credentials for normal verification of potentially valid credentials. This generally raises the least amount of suspicion. Cisco has provided an alert on this attack vector. For more information, see the Cisco SYNful Knock Security Advisory.
Other attacks against network infrastructure devices have also been reported, including more complicated persistent malware that silently changes the firmware on the device that is used to load the operating system so that the malware can inject code into the running operating system. For more information, please see Cisco's description of the evolution of attacks on Cisco IOS devices.
A Cisco ASA device is a network device that provides firewall and Virtual Private Network (VPN) functionality. These devices are often deployed at the edge of a network to protect a site’s network infrastructure, and to give remote users access to protected local resources.
In June 2016, NCCIC received several reports of compromised Cisco ASA devices that were modified in an unauthorized way. The ASA devices directed users to a location where malicious actors tried to socially engineer the users into divulging their credentials.
It is suspected that malicious actors leveraged CVE-2014-3393 to inject malicious code into the affected devices. The malicious actor would then be able to modify the contents of the Random Access Memory Filing System (RAMFS) cache file system and inject the malicious code into the appliance’s configuration. Refer to the Cisco Security Advisory Multiple Vulnerabilities in Cisco ASA Software for more information and for remediation details.
In August 2016, a group known as “Shadow Brokers” publicly released a large number of files, including exploitation tools for both old and newly exposed vulnerabilities. Cisco ASA devices were found to be vulnerable to the released exploit code. In response, Cisco released an update to address a newly disclosed Cisco ASA Simple Network Management Protocol (SNMP) remote code execution vulnerability (CVE-2016-6366). In addition, one exploit tool targeted a previously patched Cisco vulnerability (CVE-2016-6367). Although Cisco provided patches to fix this Cisco ASA command-line interface (CLI) remote code execution vulnerability in 2011, devices that remain unpatched are still vulnerable to the described attack. Attackers may target vulnerabilities for months or even years after patches become available.
If the network infrastructure is compromised, malicious hackers or adversaries can gain full control of the network infrastructure enabling further compromise of other types of devices and data and allowing traffic to be redirected, changed, or denied. Possibilities of manipulation include denial-of-service, data theft, or unauthorized changes to the data.
Intruders with infrastructure privilege and access can impede productivity and severely hinder re-establishing network connectivity. Even if other compromised devices are detected, tracking back to a compromised infrastructure device is often difficult.
Malicious actors with persistent access to network devices can reattack and move laterally after they have been ejected from previously exploited hosts.
Proper network segmentation is a very effective security mechanism to prevent an intruder from propagating exploits or laterally moving around an internal network. On a poorly segmented network, intruders are able to extend their impact to control critical devices or gain access to sensitive data and intellectual property. Security architects must consider the overall infrastructure layout, segmentation, and segregation. Segregation separates network segments based on role and functionality. A securely segregated network can contain malicious occurrences, reducing the impact from intruders, in the event that they have gained a foothold somewhere inside the network.
Local Area Network (LAN) segments are separated by traditional network devices such as routers. Routers are placed between networks to create boundaries, increase the number of broadcast domains, and effectively filter users’ broadcast traffic. These boundaries can be used to contain security breaches by restricting traffic to separate segments and can even shut down segments of the network during an intrusion, restricting adversary access.
As technologies change, new strategies are developed to improve IT efficiencies and network security controls. Virtual separation is the logical isolation of networks on the same physical network. The same physical segmentation design principles apply to virtual segmentation but no additional hardware is required. Existing technologies can be used to prevent an intruder from breaching other internal network segments.
Allowing unfiltered workstation-to-workstation communications (as well as other peer-to-peer communications) creates serious vulnerabilities, and can allow a network intruder to easily spread to multiple systems. An intruder can establish an effective “beach head” within the network, and then spread to create backdoors into the network to maintain persistence and make it difficult for defenders to contain and eradicate.
A fundamental way to enhance network infrastructure security is to safeguard networking devices with secure configurations. Government agencies, organizations, and vendors supply a wide range of resources to administrators on how to harden network devices. These resources include benchmarks and best practices. These recommendations should be implemented in conjunction with laws, regulations, site security policies, standards, and industry best practices. These guides provide a baseline security configuration for the enterprise that protects the integrity of network infrastructure devices. This guidance supplements the network security best practices supplied by vendors.
Administrative privileges on infrastructure devices allow access to resources that are normally unavailable to most users and permit the execution of actions that would otherwise be restricted. When administrator privileges are improperly authorized, granted widely, and/or not closely audited, intruders can exploit them. These compromised privileges can enable adversaries to traverse a network, expanding access and potentially allowing full control of the infrastructure backbone. Unauthorized infrastructure access can be mitigated by properly implementing secure access policies and procedures.
Out-of-Band (OoB) management uses alternate communication paths to remotely manage network infrastructure devices. These dedicated paths can vary in configuration to include anything from virtual tunneling to physical separation. Using OoB access to manage the network infrastructure will strengthen security by limiting access and separating user traffic from network management traffic. OoB management provides security monitoring and can implement corrective actions without allowing the adversary who may have already compromised a portion of the network to observe these changes.
OoB management can be implemented physically or virtually, or through a hybrid of the two. Building additional physical network infrastructure is the most secure option for the network managers, although it can be very expensive to implement and maintain. Virtual implementation is less costly, but still requires significant configuration changes and administration. In some situations, such as access to remote locations, virtual encrypted tunnels may be the only viable option.
Products purchased through unauthorized channels are often known as “counterfeit,” “secondary,” or “grey market” devices. There have been numerous reports in the press regarding grey market hardware and software being introduced into the marketplace. Grey market products have not been thoroughly tested to meet quality standards and can introduce risks to the network. Lack of awareness or validation of the legitimacy of hardware and software presents a serious risk to users’ information and the overall integrity of the network environment. Products purchased from the secondary market run the risk of having the supply chain breached, which can result in the introduction of counterfeit, stolen, or second-hand devices. This could affect network performance and compromise the confidentiality, integrity, or availability of network assets. Furthermore, breaches in the supply chain provide an opportunity for malicious software or hardware to be installed on the equipment. In addition, unauthorized or malicious software can be loaded onto a device after it is in operational use, so integrity checking of software should be done on a regular basis.
|Fortinet||CVE-2016-6909||EGREGIOUSBLUNDER||Authentication cookie overflow|
|WatchGuard||CVE-2016-7089||ESCALATEPLOWMAN||Command line injection via ipconfig|
|Cisco||CVE-2016-6366||EXTRABACON||SNMP remote code execution|
|Cisco||CVE-2016-6367||EPICBANANA||Command line injection remote code execution|
|TOPSEC||N/A||ELIGIBLEBACHELOR||Attack vector unknown, but has an XML-like payload|
|TOPSEC||N/A||ELIGIBLEBOMBSHELL||HTTP cookie command injection|
|TOPSEC||N/A||ELIGIBLECANDIDATE||HTTP cookie command injection|
|TOPSEC||N/A||ELIGIBLECONTESTANT||HTTP POST parameter injection|
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 
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.
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:
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:
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:
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.
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. 
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.
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.
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:
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.
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
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:
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.
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:
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:
The above example does not constitute an exhaustive list. The U.S. Government does not endorse or support any particular product or vendor.
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.
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.
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:
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).
Web shells can be delivered through a number of web application exploits or configuration weaknesses including:
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.
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:
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.
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.