CVE-2024-38475: Apache HTTP Server Improper Escaping of Output Vulnerability

Vulnerability summary

CVE-2024-38475 is a critical (CVSS 9.1) Improper Encoding or Escaping of Output vulnerability in Apache HTTP Server. It is known to be exploited: CISA lists it, first on 01-05-2025. EPSS estimates a 99.96% chance of exploitation in the next 30 days, higher than 99.9% of scored vulnerabilities.

Timeline

1. 01-07-2024
   PublishedVulnerability first disclosed.
2. 01-05-2025
   Added to CISA KEVApache HTTP Server Improper Escaping of Output Vulnerability
3. 22-05-2025
   CISA remediation dueApply mitigations per vendor instructions.
4. 03-11-2025
   Last modifiedVulnerability information updated.

Description

Improper escaping of output in mod_rewrite in Apache HTTP Server 2.4.59 and earlier allows an attacker to map URLs to filesystem locations that are permitted to be served by the server but are not intentionally/directly reachable by any URL, resulting in code execution or source code disclosure. Substitutions in server context that use a backreferences or variables as the first segment of the substitution are affected.  Some unsafe RewiteRules will be broken by this change and the rewrite flag "UnsafePrefixStat" can be used to opt back in once ensuring the substitution is appropriately constrained.

• CWE-116Improper Encoding or Escaping of OutputThe product prepares a structured message for communication with another component, but encoding or escaping of the data is either missing or done incorrectly. As a result, the intended structure of the message is not preserved.