CVE-2021-41773: Apache HTTP Server Path Traversal Vulnerability

Vulnerability summary

CVE-2021-41773 is a critical (CVSS 9.8) Path Traversal vulnerability in Apache HTTP Server. It is known to be exploited: CISA lists it, first on 03-11-2021. It has been used in ransomware campaigns. EPSS estimates a 99.99% chance of exploitation in the next 30 days, higher than 99.9% of scored vulnerabilities. 10 public exploits are recorded.

Timeline

1. 05-10-2021
   PublishedVulnerability first disclosed.
2. 03-11-2021
   Added to CISA KEVApache HTTP Server Path Traversal Vulnerability
3. 17-11-2021
   CISA remediation dueApply updates per vendor instructions.
4. 21-10-2025
   Last modifiedVulnerability information updated.

Description

A flaw was found in a change made to path normalization in Apache HTTP Server 2.4.49. An attacker could use a path traversal attack to map URLs to files outside the directories configured by Alias-like directives. If files outside of these directories are not protected by the usual default configuration "require all denied", these requests can succeed. If CGI scripts are also enabled for these aliased pathes, this could allow for remote code execution. This issue is known to be exploited in the wild. This issue only affects Apache 2.4.49 and not earlier versions. The fix in Apache HTTP Server 2.4.50 was found to be incomplete, see CVE-2021-42013.

• CWE-22Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')The product uses external input to construct a pathname that is intended to identify a file or directory that is located underneath a restricted parent directory, but the product does not properly neutralize special elements within the pathname that can cause the pathname to resolve to a location that is outside of the restricted directory.