CVE-2011-2767 in mod_perl
Summary
by MITRE
mod_perl 2.0 through 2.0.10 allows attackers to execute arbitrary Perl code by placing it in a user-owned .htaccess file, because (contrary to the documentation) there is no configuration option that permits Perl code for the administrator's control of HTTP request processing without also permitting unprivileged users to run Perl code in the context of the user account that runs Apache HTTP Server processes.
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Analysis
by VulDB Data Team • 05/04/2023
The vulnerability identified as CVE-2011-2767 represents a critical privilege escalation flaw within the mod_perl module version 2.0 through 2.0.10. This issue stems from a fundamental misalignment between the module's documented security model and its actual implementation. The mod_perl module serves as a powerful interface between the Apache HTTP Server and the Perl programming language, enabling developers to write server-side applications in Perl. When properly configured, mod_perl allows administrators to embed Perl code directly within Apache configuration files, providing extensive control over HTTP request processing. However, the vulnerability arises from the module's failure to properly enforce security boundaries between privileged administrative code execution and user-level code execution.
The technical flaw manifests in the module's handling of .htaccess files, which are configuration files that can be placed in any directory and are processed by Apache for that directory and its subdirectories. Under normal circumstances, .htaccess files should only be processed with the privileges of the user account running the Apache server, but mod_perl's implementation allows user-controlled Perl code to execute with the same privileges as the Apache process itself. This creates a dangerous scenario where any user with write access to a directory containing an .htaccess file can inject malicious Perl code that will execute with the elevated privileges of the Apache server process. The vulnerability exists because mod_perl lacks a proper configuration option that would allow administrators to control HTTP request processing through Perl code while simultaneously preventing unprivileged users from executing Perl code in the context of the Apache user account.
The operational impact of this vulnerability is severe and far-reaching, as it essentially provides attackers with a path to arbitrary code execution on the web server. An attacker who gains write access to a directory that can be served by Apache can simply create a malicious .htaccess file containing Perl code that executes with the privileges of the Apache process. This could result in complete server compromise, data exfiltration, privilege escalation to other system users, or even the ability to use the compromised server as a pivot point for attacking other systems within the network. The vulnerability is particularly dangerous because it can be exploited through various attack vectors, including web application vulnerabilities that allow file upload capabilities, misconfigured directory permissions, or even through legitimate administrative tasks that might grant users write access to web directories. The attack surface is broad and the potential for exploitation is high, as many web applications and server configurations inadvertently provide users with the ability to write files to directories served by Apache.
From a cybersecurity perspective, this vulnerability aligns with multiple CWE (Common Weakness Enumeration) categories including CWE-255 Credentials Management, CWE-94 Code Injection, and CWE-78 Improper Neutralization of Special Elements used in a Command. The flaw also maps to several ATT&CK (Attack Tree for Threats and Countermeasures) techniques such as T1059 Command and Scripting Interpreter and T1078 Valid Accounts, as it leverages legitimate server functionality to execute malicious code. The vulnerability demonstrates a classic case of inadequate privilege separation, where the security model fails to properly distinguish between administrative and user-level code execution contexts. Organizations should immediately apply patches or updates to mod_perl versions 2.0.11 and later, which address this specific issue by implementing proper configuration options for controlling Perl code execution. Alternative mitigations include restricting write access to directories that are served by Apache, disabling mod_perl if not strictly required, and implementing proper file permissions and access controls to prevent unauthorized users from creating or modifying .htaccess files. Additionally, security monitoring should be enhanced to detect suspicious .htaccess file modifications and unusual Perl code execution patterns, as these could indicate exploitation attempts.