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

CVE-2024-38475 is an improper escaping of output vulnerability in Apache HTTP Server's mod_rewrite module. It lets an attacker craft requests that map URLs to filesystem locations the server is allowed to serve but that were never meant to be reachable that way. The result can be source code disclosure or code execution. Because Apache HTTP Server is widely used to host web applications and static content, unpatched instances can expose sensitive files or allow further compromise of the host.

Defenders should treat this as a high-priority configuration and patching issue. Confirm exact impact, affected releases, and fixes against the official Apache HTTP Server advisory; do not rely solely on secondary summaries.

How it works

The weakness is classified as CWE-116 (Improper Encoding or Escaping of Output). In mod_rewrite, output that should be safely escaped is not handled correctly. An attacker can therefore supply specially formed input that causes the rewrite engine to resolve a URL to an unintended but still permitted filesystem path.

Once that mapping succeeds, the server may serve the contents of that path (source disclosure) or, depending on what is present and how the server is configured, allow execution of code that was never intended to be reachable via any public URL. No further exploit mechanics are detailed in the available summary; treat any public proof-of-concept claims as unverified until validated against the vendor advisory and your own test environment.

Am I affected? How to find it in your systems

Apache HTTP Server is commonly found on Linux and Unix web tiers, reverse-proxy layers, container images, and cloud virtual machines that serve HTTP/HTTPS traffic. Inventory every host, container, and load-balancer backend that runs httpd or the apache2 package.

• Enumerate installed Apache HTTP Server packages and binaries; record the exact version string and whether mod_rewrite is loaded (check LoadModule directives and the output of apachectl -M or httpd -M).
• Review all RewriteRule, RewriteMap, and related configuration that touch user-controlled path or query data; pay special attention to rules that map into filesystem locations outside the normal document root or that use flags allowing further processing.
• Confirm against the vendor advisory which releases are vulnerable; do not assume a version is safe merely because it is recent.
• Look for anomalous access-log entries that request unusual paths, encoded sequences, or resources that should not be publicly reachable. Correlate with error-log messages related to rewrite processing or unexpected file opens. Network telemetry showing repeated probes against rewrite-heavy endpoints may also indicate reconnaissance.

How to remediate

Apply the vendor-supplied update for Apache HTTP Server as soon as it is available and tested in your environment. Follow the exact upgrade path and configuration notes published by the Apache HTTP Server project; the CISA-required action is to apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.

• After patching, restart the service cleanly and verify that mod_rewrite still behaves as expected for legitimate traffic.
• Re-audit rewrite rules for unnecessary complexity or overly broad filesystem mappings; restrict rewrite targets to the minimum set of directories required by the application.
• Ensure that directories containing source code, configuration files, or executable scripts are not reachable via any rewrite path and that filesystem permissions prevent the web-server user from reading or executing unintended content.
• Enable and retain access and error logging at a level sufficient for later forensic review.

If you can't patch immediately

Until the vendor update can be deployed, reduce exposure with compensating controls.

• Segment web servers so that only necessary clients can reach them; place them behind a reverse proxy or WAF that can inspect and block suspicious rewrite-related request patterns.
• If a virtual-patching or WAF rule set covering this class of improper-escaping / path-mapping issues is available, enable it after testing for false positives.
• Temporarily disable mod_rewrite if the application can function without it, or disable the specific RewriteRule sets that map into sensitive filesystem areas.
• Increase monitoring of access logs, process execution, and file-access events on the web-server host; alert on any unexpected file reads outside the document root or on the appearance of new processes spawned by the Apache worker.
• If the service is not business-critical, consider taking it offline until a patch can be applied.

If your data may have been exposed

Vulnerabilities that permit source-code disclosure or code execution can lead to credential theft, further lateral movement, or data exfiltration. Known ransomware use of this CVE is not documented, but any successful exploitation should still be treated as a potential breach. Review logs for evidence of successful mapping to sensitive paths, rotate any secrets that may have been present in disclosed files, and perform a full host and application compromise assessment. You can also run a free exposure scan of your email addresses against known breach data sets to determine whether related credentials have already appeared in public dumps.