CVE-2026-33006: 
Apache HTTP Server vulnerability analysis and mitigation

CVE-2026-33006 is a timing attack vulnerability in the mod_auth_digest module of Apache HTTP Server that allows a remote attacker to bypass Digest authentication. It affects Apache HTTP Server versions through 2.4.66, and was reported on 2026-03-09 by researcher Nitescu Lucian, with a fix released on 2026-05-04 in version 2.4.67. The vulnerability carries a CVSS v3.1 base score of 4.8 (Medium) (GitHub Advisory, Openwall OSS-Sec).

The root cause is an observable timing discrepancy (CWE-208) in the Digest authentication verification process within mod_auth_digest. By measuring the time differences in server responses to authentication attempts, a remote attacker can infer whether a credential guess is partially or fully correct, enabling a practical authentication bypass without valid credentials. Exploitation requires network access to a server running mod_auth_digest and the ability to send multiple crafted authentication requests to measure timing differences; no privileges or user interaction are required, though the attack complexity is rated High due to the precision needed. A public proof-of-concept Python exploit script is available on GitHub (GitHub Advisory, Openwall OSS-Sec, PoC GitHub).

Successful exploitation allows an unauthenticated remote attacker to bypass Digest authentication and gain unauthorized access to resources protected by mod_auth_digest. The primary impacts are low-level confidentiality and integrity compromise — an attacker can read and potentially modify protected content they should not have access to. Availability is not directly impacted, but unauthorized access to protected resources could facilitate further lateral movement or data exfiltration depending on what is protected (GitHub Advisory, Openwall OSS-Sec).

A public proof-of-concept exploit (ctt_apache_digest_timing.py) is available on GitHub, implementing a full timing-based authentication bypass workflow against Apache 2.4.66 and earlier with mod_auth_digest enabled (PoC GitHub). As of the time of reporting, there is no evidence of active in-the-wild exploitation, and no threat actor attribution has been made. The EPSS score is approximately 0.117–0.246%, indicating a relatively low near-term exploitation probability. The vulnerability is not listed in the CISA Known Exploited Vulnerabilities (KEV) catalog. Detection plugins are available in Nessus (IDs: 312236, 312249, 312666, 313221, 313207, 316988) and Qualys (IDs: 6032801, 288795) (GitHub Advisory).

1. Reconnaissance: Identify Apache HTTP Server instances (version ≤ 2.4.66) with mod_auth_digest enabled using tools like Shodan, Censys, or Nmap with HTTP banner grabbing. Look for WWW-Authenticate: Digest headers in server responses.
2. Baseline timing measurement: Send a series of authentication requests with known-invalid credentials to the target endpoint and record server response times to establish a baseline timing profile.
3. Credential enumeration via timing: Using the PoC script (python3 ctt_apache_digest_timing.py <target_ip> [options]), systematically vary credential components (e.g., username, password characters) and measure response time differences. Statistically significant timing variations reveal which guesses are closer to the correct value.
4. Authentication bypass: Leverage the recovered credential information to construct a valid or near-valid Digest authentication response, bypassing the authentication gate and gaining access to protected resources (PoC GitHub, Openwall OSS-Sec).

• Network: High volume of repeated HTTP requests to Digest-authenticated endpoints from a single source IP, especially with varying Authorization: Digest header values; unusual patterns of authentication failures followed by a successful authentication from the same IP.
• Logs: Apache access logs showing many 401 responses to the same resource from one IP in rapid succession, followed by a 200 response; requests with subtly varying response fields in the Authorization: Digest header.
• Process/Behavior: Unexpected access to resources protected by mod_auth_digest from IPs or user agents not previously seen in access logs; access at unusual hours inconsistent with normal usage patterns.

The primary remediation is to upgrade Apache HTTP Server to version 2.4.67 or later, which contains the fix for this vulnerability (commit r1933356) (Openwall OSS-Sec, Apache Vulnerabilities). If immediate patching is not feasible, consider disabling mod_auth_digest and replacing it with mod_auth_basic over HTTPS (TLS), which is the recommended modern alternative and not susceptible to this timing attack. Additionally, network-level controls such as IP allowlisting or rate limiting on authenticated endpoints can reduce exposure while a patch is applied. Distribution-specific patches are available for Debian, Ubuntu, SUSE, openSUSE, Slackware, Mageia, Amazon Linux 2/2023, and Fedora (GitHub Advisory).

SecurityWeek covered the vulnerability as part of a broader report on critical and high-severity flaws patched in Apache HTTP Server and MINA (SecurityWeek). Heise reported on the Apache HTTP Server release noting highly critical flaws allowing malicious code injection alongside this moderate issue (Heise). The Belgian Centre for Cybersecurity (CCB) issued a warning about multiple Apache HTTP Server vulnerabilities including this one. Community discussion on oss-security and infosec.exchange noted the moderate severity and the availability of a patch, with no significant alarm given the High attack complexity rating.