CVE-2024-38474 in HTTP Server
Summary
by MITRE • 07/01/2024
Substitution encoding issue in mod_rewrite in Apache HTTP Server 2.4.59 and earlier allows attacker to execute scripts in directories permitted by the configuration but not directly reachable by any URL or source disclosure of scripts meant to only to be executed as CGI.
Users are recommended to upgrade to version 2.4.60, which fixes this issue.
Some RewriteRules that capture and substitute unsafely will now fail unless rewrite flag "UnsafeAllow3F" is specified.
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Analysis
by VulDB Data Team • 01/04/2026
The vulnerability described in CVE-2024-38474 represents a critical substitution encoding issue within the mod_rewrite module of the Apache HTTP Server version 2.4.59 and earlier. This flaw manifests as a security bypass that allows attackers to execute scripts in directories that are properly configured for access but not directly reachable through standard URL paths. The vulnerability specifically targets the handling of rewrite rules that perform unsafe substitutions, creating a pathway for unauthorized script execution that could potentially lead to full system compromise. The issue is particularly concerning because it affects directories that are intended to be protected from direct web access but may still be accessible through carefully crafted rewrite operations.
The technical implementation of this vulnerability stems from improper handling of substitution encoding within mod_rewrite's URL rewriting functionality. When rewrite rules contain unsafe capture and substitution patterns, the module fails to properly sanitize or validate the substituted content, allowing malicious input to be interpreted as executable code. This flaw operates at the intersection of web server configuration management and input validation, where the rewrite engine's substitution mechanism does not adequately protect against potentially harmful characters or sequences that could be interpreted as executable instructions. The vulnerability is categorized under CWE-155 as "Improper Neutralization of Special Elements in Output Used by a Downstream Component" and aligns with ATT&CK technique T1059.007 for "Command and Scripting Interpreter: JavaScript" and T1566.002 for "Phishing: Spearphishing Attachment", as it enables attackers to execute malicious code through web server configuration manipulation.
The operational impact of this vulnerability extends beyond simple script execution, as it can potentially allow attackers to access files and execute commands that should remain protected within the server's directory structure. Systems running affected Apache versions may experience unauthorized access to sensitive data, potential privilege escalation, and complete system compromise if proper mitigation is not implemented. The vulnerability specifically affects environments where mod_rewrite is actively used to manage URL routing and where scripts are configured to execute as CGI but should not be directly accessible through web requests. Organizations may face significant security implications as attackers can exploit this weakness to bypass traditional access controls and gain unauthorized execution capabilities within their web server environments.
Organizations should immediately implement the recommended mitigation by upgrading to Apache HTTP Server version 2.4.60, which includes the necessary fixes to address the substitution encoding issue. The upgrade process should be prioritized across all affected systems and thoroughly tested to ensure compatibility with existing configurations. For environments where immediate upgrade is not feasible, administrators should review all existing rewrite rules to identify and disable any patterns that may be susceptible to the vulnerability. The introduction of the "UnsafeAllow3F" rewrite flag provides a temporary workaround for specific scenarios, though this should be used cautiously and only when absolutely necessary. Additionally, organizations should implement comprehensive monitoring of rewrite rule usage and conduct security audits to identify any potentially vulnerable configurations that may not have been immediately apparent. The vulnerability demonstrates the critical importance of proper input validation and the need for continuous security assessment of web server components that handle user-supplied data through configuration parameters.