{ "id": "02df3795-8821-4762-8992-d4e51a3de90e", "created_at": "2026-04-06T00:06:38.084003Z", "updated_at": "2026-04-10T13:12:49.894807Z", "deleted_at": null, "sha1_hash": "80ef7d551c46edf1143c1375be3b94454237a8db", "title": "Compromised Web Servers and Web Shells - Threat Awareness and Guidance | CISA", "llm_title": "", "authors": "", "file_creation_date": "0001-01-01T00:00:00Z", "file_modification_date": "0001-01-01T00:00:00Z", "file_size": 78155, "plain_text": "Compromised Web Servers and Web Shells - Threat Awareness\r\nand Guidance | CISA\r\nPublished: 2017-08-09 · Archived: 2026-04-05 16:17:39 UTC\r\nSystems Affected\r\nCompromised web servers with malicious web shells installed\r\nOverview\r\nThis alert describes the frequent use of web shells as an exploitation vector. Web shells can be used to obtain\r\nunauthorized access and can lead to wider network compromise. This alert outlines the threat and provides\r\nprevention, detection, and mitigation strategies.\r\nConsistent use of web shells by Advanced Persistent Threat (APT) and criminal groups has led to significant cyber\r\nincidents.\r\nThis product was developed in collaboration with US-CERT partners in the United Kingdom, Australia, Canada,\r\nand New Zealand based on activity seen targeting organizations across these countries. The detection and\r\nmitigation measures outlined in this document represent the shared judgement of all participating agencies.\r\nWeb Shell Description\r\nA web shell is a script that can be uploaded to a web server to enable remote administration of the machine.\r\nInfected web servers can be either Internet-facing or internal to the network, where the web shell is used to pivot\r\nfurther to internal hosts.\r\nA web shell can be written in any language that the target web server supports. The most commonly observed web\r\nshells are written in languages that are widely supported, such as PHP and ASP. Perl, Ruby, Python, and Unix\r\nshell scripts are also used.\r\nUsing network reconnaissance tools, an adversary can identify vulnerabilities that can be exploited and result in\r\nthe installation of a web shell. For example, these vulnerabilities can exist in content management systems (CMS)\r\nor web server software.\r\nOnce successfully uploaded, an adversary can use the web shell to leverage other exploitation techniques to\r\nescalate privileges and to issue commands remotely. These commands are directly linked to the privilege and\r\nfunctionality available to the web server and may include the ability to add, delete, and execute files as well as the\r\nability to run shell commands, further executables, or scripts.\r\nHow and why are they used by malicious adversaries?\r\nhttps://www.us-cert.gov/ncas/alerts/TA15-314A\r\nPage 1 of 5\n\nWeb shells are frequently used in compromises due to the combination of remote access and functionality. Even\r\nsimple web shells can have a considerable impact and often maintain minimal presence.\r\nWeb shells are utilized for the following purposes:\r\n1. To harvest and exfiltrate sensitive data and credentials;\r\n2. To upload additional malware for the potential of creating, for example, a watering hole for infection and\r\nscanning of further victims;\r\n3. To use as a relay point to issue commands to hosts inside the network without direct Internet access;\r\n4. To use as command-and-control infrastructure, potentially in the form of a bot in a botnet or in support of\r\ncompromises to additional external networks. This could occur if the adversary intends to maintain long-term persistence.\r\nWhile a web shell itself would not normally be used for denial of service (DoS) attacks, it can act as a platform for\r\nuploading further tools, including DoS capability.\r\nExamples\r\nWeb shells such as China Chopper, WSO, C99 and B374K are frequently chosen by adversaries; however these\r\nare just a small number of known used web shells. (Further information linking to IOCs and SNORT rules can be\r\nfound in the Additional Resources section).\r\nChina Chopper – A small web shell packed with features. Has several command and control features\r\nincluding a password brute force capability.\r\nWSO – Stands for “web shell by orb” and has the ability to masquerade as an error page containing a\r\nhidden login form.\r\nC99 – A version of the WSO shell with additional functionality. Can display the server’s security measures\r\nand contains a self-delete function.\r\nB374K – PHP based web shell with common functionality such as viewing processes and executing\r\ncommands.\r\nDelivery Tactics\r\nWeb shells can be delivered through a number of web application exploits or configuration weaknesses including:\r\nCross-Site Scripting;\r\nSQL Injection;\r\nVulnerabilities in applications/services  (e.g., WordPress or other CMS applications);\r\nFile processing vulnerabilities (e.g., upload filtering or assigned permissions);\r\nRemote File Include (RFI) and Local File Include (LFI) vulnerabilities;\r\nExposed Admin Interfaces (possible areas to find vulnerabilities mentioned above).\r\nThe above tactics can be and are combined regularly. For example, an exposed admin interface also requires a file\r\nupload option, or another exploit method mentioned above, to deliver successfully.\r\nhttps://www.us-cert.gov/ncas/alerts/TA15-314A\r\nPage 2 of 5\n\nImpact\r\nA successfully uploaded shell script may allow a remote attacker to bypass security restrictions and gain\r\nunauthorized system access.\r\nSolution\r\nPrevention and Mitigation\r\nInstallation of a web shell is commonly accomplished through web application vulnerabilities or configuration\r\nweaknesses. Therefore, identification and closure of these vulnerabilities is crucial to avoiding potential\r\ncompromise. The following suggestions specify good security and web shell specific practices:\r\nEmploy regular updates to applications and the host operating system to ensure protection against known\r\nvulnerabilities.\r\nImplement a least-privileges policy on the web server to:\r\nReduce adversaries’ ability to escalate privileges or pivot laterally to other hosts.\r\nControl creation and execution of files in particular directories.\r\nIf not already present, consider deploying a demilitarized zone (DMZ) between your webfacing systems\r\nand the corporate network. Limiting the interaction and logging traffic between the two provides a method\r\nto identify possible malicious activity.\r\nEnsure a secure configuration of web servers. All unnecessary services and ports should be disabled or\r\nblocked. All necessary services and ports should be restricted where feasible. This can include whitelisting\r\nor blocking external access to administration panels and not using default login credentials.\r\nUtilize a reverse proxy or alternative service, such as mod_security, to restrict accessible URL paths to\r\nknown legitimate ones.\r\nEstablish, 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.\r\nEmploy user input validation to restrict local and remote file inclusion vulnerabilities.\r\nConduct regular system and application vulnerability scans to establish areas of risk. While this method\r\ndoes not protect against zero day attacks it will highlight possible areas of concern.\r\nDeploy a web application firewall and conduct regular virus signature checks, application fuzzing, code\r\nreviews and server network analysis.\r\nDetection\r\nDue to the potential simplicity and ease of modification of web shells, they can be difficult to detect. For example,\r\nanti-virus products sometimes produce poor results in detecting web shells.\r\nThe following may be indicators that your system has been infected by a web shell. Note a number of these\r\nindicators are common to legitimate files. Any suspected malicious files should be considered in the context of\r\nother indicators and triaged to determine whether further inspection or validation is required.\r\nAbnormal periods of high site usage (due to potential uploading and downloading activity);\r\nhttps://www.us-cert.gov/ncas/alerts/TA15-314A\r\nPage 3 of 5\n\nFiles with an unusual timestamp (e.g., more recent than the last update of the web applications installed);\r\nSuspicious files in Internet-accessible locations (web root);\r\nFiles containing references to suspicious keywords such as cmd.exe or eval;\r\nUnexpected connections in logs. For example:\r\nA file type generating unexpected or anomalous network traffic (e.g., a JPG file making requests\r\nwith POST parameters);\r\nSuspicious logins originating from internal subnets to DMZ servers and vice versa.\r\nAny evidence of suspicious shell commands, such as directory traversal, by the web server process. \r\nFor investigating many types of shells, a search engine can be very helpful. Often, web shells will be used to\r\nspread malware onto a server and the search engines are able to see it. But many web shells check the User-Agent\r\nand will display differently for a search engine spider (a program that crawls through links on the Internet,\r\ngrabbing content from sites and adding it to search engine indexes) than for a regular user. To find a shell, you\r\nmay need to change your User-Agent to one of the search engine bots. Some browsers have plugins that allow you\r\nto easily switch a User-Agent. Once the shell is detected, simply delete the file from the server.\r\nClient characteristics can also indicate possible web shell activity. For example, the malicious actor will often visit\r\nonly the URI where the web shell script was created, but a standard user usually loads the webpage from a linked\r\npage/referrer or loads additional content/resources. Thus, performing frequency analysis on the web access logs\r\ncould indicate the location of a web shell. Most legitimate URI visits will contain varying user-agents, whereas a\r\nweb shell is generally only visited by the creator, resulting in limited user-agent variants.\r\nReferences\r\nAustralian Cyber Security Centre – Securing Content Management Systems (CMS)\r\nFireEye China Chopper – The Little Malware That Could. Detecting and Defeating the China Chopper Web Shell\r\nMANDIANT – Old Web Shells New Tricks\r\nFireEye – Breaking Down the China Chopper Web Shell Part I\r\nFireEye – Breaking Down the China Chopper Web Shell Part II\r\nWSO Information\r\nExploit-db – China Chopper\r\nC99\r\nINFOSEC Institute – Web Shell Detection\r\nRevisions\r\nNovember 10, 2015: Initial Release|November 13, 2015: Changes to Title and Systems Affected sections|August\r\n9, 2017: Updated c99 link\r\nhttps://www.us-cert.gov/ncas/alerts/TA15-314A\r\nPage 4 of 5\n\nSource: https://www.us-cert.gov/ncas/alerts/TA15-314A\r\nhttps://www.us-cert.gov/ncas/alerts/TA15-314A\r\nPage 5 of 5", "extraction_quality": 1, "language": "EN", "sources": [ "MITRE" ], "origins": [ "web" ], "references": [ "https://www.us-cert.gov/ncas/alerts/TA15-314A" ], "report_names": [ "TA15-314A" ], "threat_actors": [ { "id": "d90307b6-14a9-4d0b-9156-89e453d6eb13", "created_at": "2022-10-25T16:07:23.773944Z", "updated_at": "2026-04-10T02:00:04.746188Z", "deleted_at": null, "main_name": "Lead", "aliases": [ "Casper", "TG-3279" ], "source_name": "ETDA:Lead", "tools": [ "Agentemis", "BleDoor", "Cobalt Strike", "CobaltStrike", "RbDoor", "RibDoor", "Winnti", "cobeacon" ], "source_id": "ETDA", "reports": null } ], "ts_created_at": 1775433998, "ts_updated_at": 1775826769, "ts_creation_date": 0, "ts_modification_date": 0, "files": { "pdf": "https://archive.orkl.eu/80ef7d551c46edf1143c1375be3b94454237a8db.pdf", "text": "https://archive.orkl.eu/80ef7d551c46edf1143c1375be3b94454237a8db.txt", "img": "https://archive.orkl.eu/80ef7d551c46edf1143c1375be3b94454237a8db.jpg" } }