CVE-2026-56449: Apache HTTP Server Buffer Overflow Flaw

CVE-2026-56449 Overview

CVE-2026-56449 is an out-of-bounds write vulnerability [CWE-787] in the mod_proxy_html module of Apache HTTP Server. The flaw affects Apache HTTP Server versions 2.4.0 through 2.4.68. An attacker can trigger the condition by causing the proxied origin to return a crafted HTTP response body that mod_proxy_html parses and rewrites.

The vulnerability is network-exploitable without authentication or user interaction. According to the CVSS vector, the primary impact is to confidentiality, enabling exposure of process memory contents through the HTML rewriting pipeline.

Critical Impact
Remote, unauthenticated attackers can trigger an out-of-bounds write in mod_proxy_html through crafted upstream HTTP response bodies, risking memory disclosure in deployments that reverse-proxy and rewrite HTML content.

Affected Products

Apache HTTP Server 2.4.0 through 2.4.68
Deployments using the mod_proxy_html module for HTML link rewriting
Reverse proxy configurations proxying untrusted or attacker-influenced upstreams

Discovery Timeline

2026-10-01 - CVE-2026-56449 published to NVD
2026-10-01 - Coordinated disclosure posted to the OpenWall oss-security mailing list
2026-10-05 - Last updated in NVD database

Technical Details for CVE-2026-56449

Vulnerability Analysis

The mod_proxy_html module rewrites HTML links in responses passing through Apache acting as a reverse proxy. It parses upstream HTML bodies, matches URL attributes, and emits a modified response to the client. The vulnerability occurs when the parser writes past the bounds of an internal buffer while processing specific crafted input.

Because the module operates on response bodies returned by upstream servers, exploitation requires an attacker to control or influence content served by a proxied origin. The impact is limited to confidentiality under the published CVSS vector, which is consistent with disclosure of adjacent heap or stack memory into the rewritten HTML stream delivered to the client.

The flaw is reachable without authentication to the Apache front-end, as any client request that triggers a proxied fetch of malicious HTML can exercise the vulnerable code path. See the Apache HTTP Server Security Vulnerabilities advisory for authoritative detail.

Root Cause

The root cause is an out-of-bounds write [CWE-787] in the HTML parsing and rewriting logic of mod_proxy_html. Insufficient boundary enforcement allows crafted response content to drive the parser into writing beyond an allocated buffer during URL substitution.

Attack Vector

Exploitation requires a reverse-proxy deployment that loads mod_proxy_html and rewrites responses from an upstream the attacker can influence. The attacker causes a client-visible request to reach an origin that returns a crafted HTML body. When Apache rewrites the response, the vulnerable code path is executed. No credentials or user interaction with the Apache server are required. Refer to the OpenWall OSS Security advisory for additional technical context.

Detection Methods for CVE-2026-56449

Indicators of Compromise

Apache error_log entries showing segmentation faults or child process crashes correlated with requests routed through mod_proxy_html
Anomalous response bodies emitted to clients containing non-HTML byte sequences or truncated markup from proxied endpoints
Access log entries pointing to proxied URIs returning unusually large or malformed HTML payloads

Detection Strategies

Inventory Apache HTTP Server instances and identify any loading mod_proxy_html via httpd -M or configuration review of LoadModule proxy_html_module
Compare installed Apache versions against the vulnerable range 2.4.0 through 2.4.68 using package manager queries or httpd -v
Enable core dump collection on Apache workers and alert on repeated crashes tied to the mod_proxy_html module

Monitoring Recommendations

Monitor proxied upstream destinations for integrity and apply allowlisting where feasible to constrain attacker-controlled HTML content
Alert on sudden increases in Apache worker restarts, SIGSEGV events, or response size anomalies through reverse-proxy paths
Correlate web application firewall logs with Apache error logs to identify request patterns that precede parser failures

How to Mitigate CVE-2026-56449

Immediate Actions Required

Upgrade Apache HTTP Server to the fixed version listed in the Apache HTTP Server Security Vulnerabilities advisory
If immediate patching is not possible, disable mod_proxy_html in affected deployments until the upgrade is applied
Audit reverse-proxy configurations to identify upstreams that are attacker-influenced or serve untrusted HTML

Patch Information

The Apache Software Foundation addresses the issue in the mod_proxy_html module. Administrators should consult the Apache HTTP Server 2.4 Security Vulnerabilities page for the exact fixed version and apply the vendor-supplied update through the operating system package manager or source build.

Workarounds

Unload mod_proxy_html by commenting out the LoadModule proxy_html_module directive and restarting Apache
Restrict reverse-proxy targets to trusted internal origins that do not return attacker-controlled HTML
Deploy a web application firewall rule to block or sanitize responses from untrusted upstreams before they reach mod_proxy_html

bash

# Configuration example: disable mod_proxy_html until patched
# In httpd.conf or the relevant conf.modules.d file, comment out:
# LoadModule proxy_html_module modules/mod_proxy_html.so

# Validate configuration and restart Apache
httpd -t
systemctl restart httpd

# Verify the module is no longer loaded
httpd -M | grep -i proxy_html