CVE-2021-42013: Apache HTTP Server Path Traversal Vulnerability

CVE-2021-42013 is a path traversal vulnerability in Apache HTTP Server. It can let an attacker reach files outside the directories intended by Alias-like configuration and, under certain conditions, achieve remote code execution. This matters because Apache HTTP Server is widely deployed as a front-end web server; successful abuse can lead to full compromise of the host and has been associated with ransomware activity. The issue also addresses an incomplete fix for the earlier CVE-2021-41773, so systems that were only partially updated may still be exposed. Confirm exact affected releases and fixed packages against the vendor advisory.

How it works

The weakness is CWE-22 (path traversal). In Apache HTTP Server, certain request handling around Alias-like directives failed to fully normalize or restrict path components. An attacker who can send crafted requests may traverse outside the directories the administrator intended to expose. According to the CISA summary, remote code execution becomes possible when files outside those directories are not protected by a default “require all denied” posture, or when CGI scripts are enabled. In practical terms, the attacker maps a URL path that escapes the intended document root or alias mapping, reads or executes content that should have been inaccessible, and—if CGI or equivalent execution is available—runs code in the context of the web server process. Exact request patterns and bypass techniques must be taken from the vendor advisory rather than assumed.

Am I affected? How to find it in your systems

Apache HTTP Server commonly runs on Linux and Unix-like systems as the system httpd/apache2 package, in containers, on reverse-proxy or load-balancer tiers, and on appliances that embed it. Inventory every host and image that listens on HTTP/HTTPS and identify the binary and package version (for example via package manager queries, httpd -v/apache2 -v, or configuration management databases). Compare those versions with the ranges listed in the official Apache advisory for CVE-2021-42013; do not rely on partial upgrades that only addressed CVE-2021-41773.

Review configuration for Alias, ScriptAlias, and similar directives, and verify that directories outside the intended aliases are covered by restrictive authorization (such as require all denied) and that CGI or other execution handlers are disabled unless required. On the detection side, examine access and error logs for unusual path sequences, requests that attempt to escape document roots, or unexpected hits on CGI endpoints. Network and WAF telemetry showing anomalous URL encoding or traversal-style paths aimed at Apache listeners should be treated as suspicious and correlated with host logs. Confirm any version- or config-specific indicators against the vendor advisory.

How to remediate

Patch first. Apply the updates published by the Apache HTTP Server project (or your OS vendor’s backported packages) exactly as directed in the advisory. CISA’s required action is to apply updates per vendor instructions. After upgrading, restart the service cleanly, verify the running binary version, and re-check that Alias-like mappings and authorization directives still match your intended policy.

Then harden for this class of flaw: ensure directories not explicitly aliased are denied by default; disable CGI and other server-side execution modules if they are not required; run the worker processes with least privilege; and keep configuration under change control so that future Alias or ScriptAlias additions cannot reintroduce open traversal paths. Re-scan or re-inventory after the change window to confirm no unpatched instances remain.

If you can't patch immediately

Reduce exposure until the vendor update can be installed. Segment Apache hosts so they are not directly reachable from untrusted networks; place a reverse proxy or WAF in front and enable rules that block path-traversal patterns and anomalous encoding (treat this as virtual patching, not a permanent fix). Disable CGI and any execution handlers that the CISA summary links to code execution. Tighten filesystem permissions and authorization so that content outside intended aliases cannot be read or executed even if a request reaches it. Increase monitoring on access logs, process creation from the httpd user, and outbound connections from web tiers, and alert on anomalies. These steps lower risk but do not replace the official patch.

If your data may have been exposed

Actively exploited vulnerabilities, including those with known ransomware use, frequently lead to broader compromise and data theft. If you have evidence of exploitation or cannot rule it out, follow your incident-response process: isolate affected hosts, preserve logs, rotate credentials that may have been accessible to the web server, and assess what data the process could reach. As an additional check, you can run a free exposure scan of your email addresses against known breach data to see whether associated accounts appear in prior incidents.