CVE-2026-59685: Apache HTTP Server Buffer Overflow Vulnerability

CVE-2026-59685 Overview

CVE-2026-59685 is an out-of-bounds write vulnerability [CWE-787] in Apache HTTP Server running on Windows. The flaw occurs when the server processes paths containing 8.3 short filenames that expand to longer strings. Expansion past the allocated buffer boundary triggers memory corruption that an unauthenticated remote attacker can reach over the network.

The vulnerability affects Apache HTTP Server versions 2.4.0 through 2.4.68 on Windows platforms. The CVSS vector indicates confidentiality impact without required privileges or user interaction, consistent with information exposure through server-side memory corruption.

Critical Impact
Remote unauthenticated attackers can trigger an out-of-bounds write condition on Windows deployments of Apache HTTP Server by submitting crafted path requests, potentially exposing sensitive process memory.

Affected Products

Apache HTTP Server 2.4.0 through 2.4.68 (Windows builds)
Downstream distributions bundling vulnerable httpd versions on Windows
Web applications and reverse proxies fronted by vulnerable Apache installations

Discovery Timeline

2026-10-01 - CVE-2026-59685 published to the National Vulnerability Database
2026-10-05 - Last updated in NVD database

Technical Details for CVE-2026-59685

Vulnerability Analysis

The defect lives in Apache HTTP Server's Windows-specific path handling. When Windows resolves a request path that includes an 8.3 short filename, the operating system expands the alias to the full long filename. The expanded form can be substantially longer than the original request string. Apache's path handling code on Windows allocates buffers sized against the inbound request rather than the post-expansion length, so the expanded path overflows the allocation and writes beyond the bounds of the buffer.

Because the write crosses an allocation boundary, adjacent heap or stack metadata can be modified. The CVSS vector reports a confidentiality impact only, which aligns with scenarios where the corrupted memory causes subsequent reads to leak server state back to the attacker. The flaw requires no authentication and no user interaction, and the attack surface is any URL path reachable over HTTP.

Root Cause

The root cause is a mismatch between buffer sizing and the post-expansion length of 8.3 names on Windows. The pre-expansion length drives the allocation, while the write destination receives the longer canonical path produced by Windows filename resolution. This is a classic out-of-bounds write pattern tracked under CWE-787.

Attack Vector

An attacker reaches the vulnerability by sending an HTTP request whose URI references an 8.3 short filename that resolves to a longer path on the target file system. No credentials, headers, or prior session state are required. The attack targets Windows builds specifically because 8.3 name resolution is a Windows file system behavior. Internet-facing Apache HTTP Server instances on Windows are directly exposed. For technical specifics, see the Apache HTTP Server security advisory and the Openwall OSS-Security announcement.

Detection Methods for CVE-2026-59685

Indicators of Compromise

HTTP requests containing tilde-delimited 8.3 short filename patterns such as PROGRA~1 or DOCUME~1 in the URI path
Repeated httpd.exe process crashes or Windows Error Reporting events on Apache worker processes
Unusual 500-series responses correlating with crafted path requests from a single source

Detection Strategies

Inspect access.log and error.log for request paths containing short filename aliases followed by server faults
Deploy web application firewall rules that flag or block URIs matching 8.3 naming conventions on Windows-hosted Apache endpoints
Correlate Apache process termination events with inbound HTTP request patterns to identify exploitation attempts

Monitoring Recommendations

Enable detailed request logging including full URI and client source for all Windows Apache HTTP Server instances
Monitor Windows Application event logs for httpd.exe faults with exception codes indicating heap or stack corruption
Track EPSS and vendor advisory updates for CVE-2026-59685; the current EPSS probability is approximately 0.498%

How to Mitigate CVE-2026-59685

Immediate Actions Required

Inventory all Windows hosts running Apache HTTP Server versions 2.4.0 through 2.4.68 and prioritize internet-facing systems
Upgrade affected httpd installations to the fixed release identified on the Apache HTTP Server vulnerabilities page
Place a WAF or reverse proxy in front of unpatched instances to filter requests containing 8.3 filename patterns

Patch Information

The Apache Software Foundation addresses CVE-2026-59685 in a release later than 2.4.68. Refer to the official Apache HTTP Server 2.4 security advisory for the exact fixed version and binary download links for Windows. Rebuild or redeploy any custom distributions that embed httpd to incorporate the patched source.

Workarounds

Disable 8.3 short filename generation on the Windows volume hosting Apache content using fsutil 8dot3name set 1, then regenerate paths without short names
Restrict Apache to serve only directories with disabled 8.3 name creation and verify with dir /x
Add request filtering at the perimeter to reject URIs matching the ~[0-9] short filename pattern until the patch is applied

bash

# Configuration example
# Disable 8.3 short filename creation on the Apache content volume (run as Administrator)
fsutil 8dot3name set C: 1

# Verify current setting
fsutil 8dot3name query C:

# Example mod_rewrite rule to block 8.3-style path requests
# Add to httpd.conf inside the relevant <VirtualHost> block
RewriteEngine On
RewriteCond %{REQUEST_URI} ~[0-9] [NC]
RewriteRule .* - [F,L]