Sitecore CMS versions 7.0 through 8.2 contain a critical vulnerability in the anti-CSRF (cross-site request forgery) protection module. An attacker can forge the __CSRFTOKEN value in an HTTP POST request and include a malicious payload. Because the server trusts this token and processes it insecurely, the attacker can execute arbitrary code on the Sitecore server without any authentication. In other words, by manipulating an authentication token, an unauthenticated attacker can gain full control of a vulnerable Sitecore instance.

Technical Summary

• Vulnerable Module: The Sitecore.Security.AntiCSRF library in Sitecore is designed to deserialize a CSRF token in order to verify requests. However, it deserializes the content of the token without validating its origin or security. This means that attacker-controlled data flows directly into the deserializer.

• Exploit Vector: The exploit uses the HTTP POST parameter __CSRFTOKEN. An attacker can create a maliciously serialized .NET object and insert it as the value of __CSRFTOKEN. When Sitecore’s Anti-CSRF code deserializes this object, any command or script embedded within it is executed on the server.

• No Validation ⇒ RCE: Because there is no appropriate validation or whitelist on the deserialized content, any payload will be accepted. In practice, this allows for Remote Code Execution (RCE) on the server. The attacker’s code runs with the privileges of the Sitecore process, potentially leading to full server compromise.

• Unauthenticated Attack: Significantly, CVE-2019-9874 does not require any login. All Sitecore CMS instances from version 7.0–7.2 and XP 7.5–8.2 (including 6.6.3 according to some advisories) are vulnerable. An attacker can send the payload over the Internet to any publicly exposed Sitecore site running these versions and trigger the vulnerability.

• Proof-of-Concept: Researchers have demonstrated the exploit using tools such as ysoserial.net. For example, a malicious payload is generated (e.g., using the TypeConfuseDelegate gadget to execute a PowerShell command) and base64 encoded. By sending this value in __CSRFTOKEN (e.g., via a POST request to the Sitecore admin page at /sitecore/shell/Applications/Security/CreateNewUser.aspx), the server deserializes and executes the payload. During testing, this allowed for the opening of a shell or the execution of arbitrary commands on the target system, confirming the RCE.

Recommendations

• Apply Official Patches: Immediately install Sitecore security updates or hotfixes. (For older versions, Sitecore has released a hotfix for the Anti-CSRF module.)

• Update Sitecore: Upgrade all instances to a patched version. Sitecore XP 9.1.1 Update-1 or higher includes the fix for this issue. Keeping Sitecore on the latest supported version ensures resolution of this and other vulnerabilities.

• Restrict Administrative Endpoints: In the meantime, block or filter access to Sitecore administrative paths (e.g., the /sitecore/shell directory and related applications) from untrusted networks via a firewall. For example, deny external requests to /sitecore/shell so that only internal or authenticated access is permitted.

• Validate Inputs and Monitor: Apply strict validation on all inputs and tokens in custom code. Even after applying patches, monitor Sitecore logs and web traffic for unusual __CSRFTOKEN values or repeated POST requests to Sitecore admin pages, as these are possible indicators of an exploit. Promptly investigate any deserialization errors or traces of command execution.