CVE-2026-63292: Apache HTTP Server Buffer Overflow Flaw

CVE-2026-63292 Overview

CVE-2026-63292 is a stack-based buffer overflow [CWE-121] in the mod_vhost_alias module of Apache HTTP Server through version 2.4.68. A remote client can trigger the overflow by sending an HTTP request with a Host header exceeding 8192 bytes when VirtualDocumentRoot uses a hostname format specifier and LimitRequestFieldSize is raised above the default. Successful exploitation causes denial of service and may allow arbitrary code execution. The Apache Software Foundation released version 2.4.69 to address the issue.

Critical Impact
Remote, unauthenticated attackers can crash the Apache HTTP Server worker process and potentially execute arbitrary code on hosts running vulnerable configurations with elevated LimitRequestFieldSize values.

Affected Products

Apache HTTP Server versions through 2.4.68
Deployments using mod_vhost_alias with hostname-based VirtualDocumentRoot specifiers
Servers configured with LimitRequestFieldSize raised above the default of 8192 bytes

Discovery Timeline

2026-10-01 - CVE-2026-63292 published to the National Vulnerability Database
2026-10-05 - Last updated in NVD database

Technical Details for CVE-2026-63292

Vulnerability Analysis

The flaw resides in Apache's mod_vhost_alias module, which maps virtual hosts to filesystem paths based on components of the request's Host header. When VirtualDocumentRoot is configured with a hostname format specifier such as %0 or %1, the module expands the hostname into a stack-allocated buffer during request processing. The expansion routine assumes the hostname length is bounded by the default LimitRequestFieldSize of 8192 bytes. Administrators who raise LimitRequestFieldSize to accommodate larger headers unknowingly remove that boundary, allowing attacker-controlled data to overflow the fixed-size stack buffer. The overflow can corrupt saved return addresses and adjacent stack frames.

Root Cause

The root cause is missing bounds enforcement in the hostname substitution logic of mod_vhost_alias. The module copies hostname bytes from the request into a stack buffer sized against the default header limit rather than against the configured limit. This classic stack-based buffer overflow [CWE-121] occurs because the code trusts an implicit size assumption that configuration directives can invalidate.

Attack Vector

An unauthenticated remote attacker sends a single HTTP request containing a Host header longer than 8192 bytes to a vulnerable server. The request must reach a virtual host whose VirtualDocumentRoot directive includes a hostname format specifier. Processing the oversized header overflows the stack buffer inside mod_vhost_alias, producing a worker process crash or, depending on platform mitigations, control-flow hijack leading to arbitrary code execution in the context of the Apache worker.

No verified public proof-of-concept code is available at this time. See the Apache HTTP Server Vulnerabilities advisory and the OpenWall OSS-Security Discussion for technical details.

Detection Methods for CVE-2026-63292

Indicators of Compromise

HTTP requests containing Host headers larger than 8192 bytes recorded in access or error logs.
Apache worker process crashes, segmentation faults, or child process restarts coinciding with inbound requests.
Unexpected core dumps generated by httpd processes on servers using mod_vhost_alias.

Detection Strategies

Audit active Apache configurations for VirtualDocumentRoot directives that use hostname format specifiers combined with a non-default LimitRequestFieldSize.
Deploy web application firewall or reverse proxy rules that reject requests with Host header values exceeding a safe maximum length.
Monitor error_log for AH00052 segfault entries and correlate with upstream request metadata.

Monitoring Recommendations

Forward Apache access and error logs into a centralized analytics platform and alert on repeated oversized Host headers from the same source.
Track process stability metrics for httpd workers and alert on abnormal restart rates.
Capture packet or proxy telemetry to retain full request headers for forensic review after suspected exploitation.

How to Mitigate CVE-2026-63292

Immediate Actions Required

Upgrade Apache HTTP Server to version 2.4.69 or later on all affected hosts.
Inventory configurations for mod_vhost_alias usage and record every LimitRequestFieldSize override.
Place a reverse proxy or WAF in front of exposed Apache instances to drop Host headers larger than 8192 bytes until patching completes.

Patch Information

The Apache Software Foundation fixed this vulnerability in Apache HTTP Server 2.4.69. Administrators should install the update from their operating system vendor or build from the official release. Refer to the Apache HTTP Server Vulnerabilities advisory for release details and backport status.

Workarounds

Restore LimitRequestFieldSize to the default value of 8192 bytes where feasible.
Replace hostname format specifiers in VirtualDocumentRoot with static paths or non-hostname specifiers until the patch is applied.
Disable mod_vhost_alias entirely on servers that do not require dynamic virtual host mapping.

bash

# Configuration example: enforce the default header size limit
# in httpd.conf or a conf.d fragment, then reload Apache
LimitRequestFieldSize 8192

# Verify the running version after upgrade
httpd -v
# Expected: Server version: Apache/2.4.69 (or later)

# Reload configuration after changes
apachectl configtest && apachectl graceful