CVE-2011-2766 in FCGI
Summary
by MITRE
The FCGI (aka Fast CGI) module 0.70 through 0.73 for Perl, as used by CGI::Fast, uses environment variable values from one request during processing of a later request, which allows remote attackers to bypass authentication via crafted HTTP headers.
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Analysis
by VulDB Data Team • 01/12/2025
The vulnerability identified as CVE-2011-2766 represents a critical security flaw in the FastCGI module version 0.70 through 0.73 for Perl, specifically affecting applications utilizing the CGI::Fast module. This issue stems from improper handling of environment variables during request processing, creating a cross-request contamination scenario that fundamentally undermines the security model of web applications. The flaw exists at the intersection of CGI processing and FastCGI protocol implementation, where the module fails to properly isolate environment variables between consecutive requests, allowing data from one request to persist and influence subsequent processing.
The technical root cause of this vulnerability lies in the improper management of environment variable scope within the FastCGI processing loop. When multiple requests are handled sequentially by the same FastCGI process, the module fails to clear or properly reset environment variables between requests. This creates a condition where values from one HTTP request can leak into the processing context of a subsequent request, enabling attackers to manipulate authentication mechanisms by crafting specific HTTP headers that persist across request boundaries. The vulnerability operates at the application layer and can be classified under CWE-200, which addresses improper handling of environment variables, and more specifically under CWE-284, addressing improper access control mechanisms.
The operational impact of this vulnerability is severe and directly enables authentication bypass attacks. An attacker can craft malicious HTTP headers that, when processed by the vulnerable FastCGI module, carry environment variable values from one request to another. This persistence allows unauthorized access to protected resources, potentially enabling full system compromise or data exfiltration. The vulnerability is particularly dangerous in environments where authentication tokens or session identifiers are stored in environment variables, as these can be manipulated to gain elevated privileges. The attack vector requires only remote access to the web application and knowledge of the specific HTTP header manipulation techniques needed to exploit the environment variable leakage.
Mitigation strategies for CVE-2011-2766 focus on immediate version updates and architectural modifications. The primary recommendation is to upgrade to FastCGI module version 0.74 or later, where the environment variable handling has been corrected to properly isolate request contexts. Organizations should also implement proper input validation and sanitization for HTTP headers, particularly those that might influence environment variable values. Network-level protections such as web application firewalls can help detect and block suspicious header patterns, though these are not complete solutions. The vulnerability demonstrates the importance of proper request isolation in multi-request processing environments and aligns with ATT&CK technique T1078.004, which covers valid accounts and credential manipulation through environment variable exploitation. Additionally, implementing proper process isolation and ensuring that environment variables are explicitly cleared between requests provides defense in depth against similar vulnerabilities.