CVE-2026-33006 Overview
CVE-2026-33006 is a timing attack vulnerability in mod_auth_digest within Apache HTTP Server 2.4.66. The flaw allows a remote attacker to bypass Digest authentication by measuring response time differences during credential validation. The Apache Software Foundation has released version 2.4.67 to address the issue.
The vulnerability is tracked under CWE-208: Observable Timing Discrepancy. Successful exploitation requires high attack complexity but no privileges or user interaction. Attackers can leverage the side channel to compromise confidentiality and integrity of resources protected by Digest authentication.
Critical Impact
Remote attackers can bypass Digest authentication on Apache HTTP Server 2.4.66 by exploiting timing variations in mod_auth_digest, gaining unauthorized access to protected resources.
Affected Products
- Apache HTTP Server 2.4.66 with mod_auth_digest enabled
- Deployments relying on Digest authentication for access control
- Web infrastructure built on Apache 2.4.x prior to 2.4.67
Discovery Timeline
- 2026-05-04 - CVE-2026-33006 published to the National Vulnerability Database
- 2026-05-04 - Last updated in NVD database
Technical Details for CVE-2026-33006
Vulnerability Analysis
The vulnerability resides in the credential comparison logic of mod_auth_digest, the Apache module that implements HTTP Digest authentication as defined in RFC 7616. Digest authentication validates client-supplied response hashes against server-computed values. When the comparison routine returns early upon detecting the first mismatched byte, the execution time correlates with how many leading bytes match.
A remote attacker can issue large numbers of authentication requests and statistically analyze response times. By iteratively refining guesses based on observed timing differences, the attacker can reconstruct a valid Digest response without knowing the underlying credentials. The attack is network-reachable and requires no prior authentication.
The vulnerability impacts confidentiality and integrity but does not directly affect availability. High attack complexity reflects the statistical nature of timing analysis and sensitivity to network jitter. EPSS data lists the probability of observed exploitation at 0.117%.
Root Cause
The root cause is the use of a non-constant-time comparison function during Digest credential validation. Standard byte-by-byte comparison routines such as memcmp short-circuit when bytes differ, leaking partial-match information through measurable execution time. Constant-time comparison is required for any cryptographic equality check.
Attack Vector
Exploitation occurs over the network against any HTTP endpoint protected by mod_auth_digest. The attacker sends crafted Digest authentication headers, measures server response time, and refines guesses across iterations. No code execution or credential prerequisites are needed. Refer to the Apache HTTP Server Vulnerabilities advisory for vendor technical details.
// No verified proof-of-concept code is publicly available.
// See the Apache advisory and OSS-Security post for technical context.
Detection Methods for CVE-2026-33006
Indicators of Compromise
- High volumes of HTTP 401 responses originating from a small number of source IPs against Digest-protected endpoints
- Repeated authentication requests with systematically varying response values in the Authorization: Digest header
- Successful authentication events from clients that previously generated large numbers of failed Digest attempts
Detection Strategies
- Inspect Apache access_log and error_log for sustained authentication failure patterns followed by a successful 200 response from the same client
- Deploy web application firewall rules that rate-limit Digest authentication attempts per source address
- Correlate timing-based anomalies in authentication latency metrics with request volume from individual clients
Monitoring Recommendations
- Track the apache:http_server component version inventory and flag instances running 2.4.66
- Alert on traffic patterns showing thousands of Digest authentication attempts within short time windows
- Forward Apache logs to a centralized SIEM and apply detections for credential-bruteforce patterns against mod_auth_digest realms
How to Mitigate CVE-2026-33006
Immediate Actions Required
- Upgrade Apache HTTP Server to version 2.4.67 or later as the primary remediation
- Inventory all hosts running Apache 2.4.66 and prioritize internet-facing systems for patching
- Audit configurations for AuthType Digest directives to identify exposed authentication realms
Patch Information
The Apache Software Foundation fixed the timing attack in Apache HTTP Server 2.4.67. The patched build replaces the vulnerable comparison in mod_auth_digest with a constant-time equivalent. Patch availability and changelog details are published in the Apache HTTP Server 2.4 Vulnerabilities page and discussed in the OpenWall OSS-Security announcement.
Workarounds
- Replace Digest authentication with stronger mechanisms such as mutual TLS or token-based authentication where feasible
- Place Digest-protected endpoints behind a reverse proxy or WAF that enforces strict rate limiting on authentication attempts
- Restrict access to Digest-protected resources by source IP using Require ip directives until patching completes
# Verify installed Apache version and upgrade
httpd -v
# Debian/Ubuntu
sudo apt update && sudo apt install --only-upgrade apache2
# RHEL/CentOS
sudo dnf update httpd
# Confirm upgraded version
httpd -v
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
