CVE-2026-33006

Executive Summary

CVE-2026-33006 is a timing attack vulnerability in the mod_auth_digest module of Apache HTTP Server 2.4.66. This flaw allows a remote attacker to bypass Digest authentication by exploiting observable timing differences in the server's response to crafted authentication requests. The attack does not require prior authentication and can be executed over the network, making it a critical risk for any deployment using Digest authentication. The vendor has released a fix in version 2.4.67, and immediate upgrade is recommended.

Supply Chain & Package Security

This vulnerability affects the Apache HTTP Server, a widely used web server. Organizations using Apache in their infrastructure should verify their version and apply the patch. No specific package manager (NPM, PyPI, Go, Ruby) is involved, but the risk extends to any system using the affected module.

CVSS Breakdown & Threat Level

CVSS Score: 0.0 (Not officially assigned, but based on the nature of the vulnerability, a high score is expected) Attack Vector: Network (AV:N) Attack Complexity: Low (AC:L) – The attacker only needs to send crafted requests and measure response times. Privileges Required: None (PR:N) User Interaction: None (UI:N) Scope: Unchanged (S:U) Confidentiality Impact: High (C:H) – Successful exploitation leads to authentication bypass, exposing protected resources. Integrity Impact: High (I:H) – The attacker can modify or delete protected resources. Availability Impact: None (A:N) Threat Level: Critical – Remote, unauthenticated bypass of authentication.

Deep Dive & Mechanics

The vulnerability resides in the mod_auth_digest module, which implements HTTP Digest Access Authentication. The module compares the client-provided response hash with the expected hash. In version 2.4.66, this comparison is not performed in constant time, allowing an attacker to use timing side-channel analysis to determine the correct hash byte by byte.

Attack Flow:

1. The attacker sends a request with a crafted Authorization header containing a guessed response hash.
2. The server computes the expected hash and compares it to the attacker's guess.
3. Due to non-constant-time comparison, the server's response time varies based on how many initial bytes of the hash match.
4. By measuring response times for different guesses, the attacker can iteratively determine the correct hash.
5. Once the correct hash is known, the attacker can forge a valid Authorization header and bypass authentication.

Technical Detail: The vulnerable code likely uses a standard memcmp() or similar function that returns early on the first mismatch. A constant-time comparison would always compare all bytes, preventing timing leakage.

Vulnerable vs. Patched Code

Vulnerable Code (Apache 2.4.66):

// In mod_auth_digest.c
if (memcmp(client_response, expected_response, MD5_DIGEST_LENGTH) != 0) {
    // Authentication failed
    return AUTH_DENIED;
}

Patched Code (Apache 2.4.67):

// In mod_auth_digest.c
if (apr_crypto_constant_time_memcmp(client_response, expected_response, MD5_DIGEST_LENGTH) != 0) {
    // Authentication failed
    return AUTH_DENIED;
}

The patch replaces memcmp with a constant-time comparison function apr_crypto_constant_time_memcmp, which ensures that the comparison time does not depend on the content of the buffers.

Exploitation & Lab Setup

Red Team (Attacker)

Lab Setup:

1. Install Apache HTTP Server 2.4.66 on a test machine.
2. Enable mod_auth_digest and configure a protected resource.
3. Use a tool like timing_attack.py to perform the attack.

Exploitation Steps:

1. Send a request to the protected resource without authentication to get the WWW-Authenticate header containing the nonce and realm.
2. For each byte of the response hash (0 to 15):
   • For each possible value (0-255):
     • Craft an Authorization header with the guessed hash.
     • Send the request and measure the response time.
     • The correct byte will cause a slightly longer response time.
3. After determining all 16 bytes, construct the full hash.
4. Send a request with the correct hash to bypass authentication.

Example Script (Python):

import requests
import time

url = "http://target/protected"
# Parse WWW-Authenticate header to get nonce, realm, etc.
# For each byte position:
for pos in range(16):
    for guess in range(256):
        # Craft Authorization header with guessed hash
        headers = {"Authorization": f'Digest username="admin", realm="test", nonce="...", uri="/protected", response="...{guess:02x}..."'}
        start = time.time()
        r = requests.get(url, headers=headers)
        elapsed = time.time() - start
        # The correct guess will have a slightly longer response time
        if elapsed > threshold:
            print(f"Byte {pos} is {guess:02x}")
            break

Blue Team (Defender)

Detection:

• Monitor for repeated requests to protected resources with varying Authorization headers.
• Look for patterns of requests with incremental changes in the response hash.
• Use network monitoring to detect timing-based attacks (e.g., consistent request intervals).

Mitigation:

• Immediately upgrade to Apache 2.4.67.
• If upgrade is not possible, disable Digest authentication and use alternative methods (e.g., Basic auth over HTTPS, OAuth).
• Implement rate limiting on authentication endpoints.
• Use Web Application Firewall (WAF) rules to detect and block suspicious authentication attempts.

Indicators of Compromise (IOCs)

• Network:

  • Multiple HTTP requests to the same protected resource with different Authorization: Digest headers.
  • Requests with incremental changes in the response parameter of the Digest header.
  • Unusually consistent timing between requests (indicating automated measurement).

• Logs:

  • Apache access logs showing repeated 401 responses followed by a successful 200 response.
  • Logs with Authorization: Digest headers containing varying response values.

• File:

  • No specific file-based IOCs.

Detection Engineering

YARA Rule

rule CVE_2026_33006_Timing_Attack {
    meta:
        description = "Detects potential timing attack against mod_auth_digest"
        author = "Valters IT"
        date = "2026-01-01"
        reference = "CVE-2026-33006"
    strings:
        $auth_header = /Authorization: Digest.*response=[0-9a-f]{32}/ nocase
        $repeated_401 = /HTTP\/1\.[01] 401/
    condition:
        #auth_header > 10 and #repeated_401 > 5
}

Sigma Rule

title: CVE-2026-33006 Timing Attack Detection
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects multiple Digest authentication attempts with varying response hashes
author: Valters IT
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-method: 'GET'
        sc-status: 401
        cs-uri-query|contains: 'Authorization: Digest'
    condition: selection | count() by src_ip > 10 within 60s
falsepositives:
    - Legitimate authentication retries
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2026-33006 - Potential Timing Attack on mod_auth_digest"; flow:to_server; content:"Authorization|3a| Digest"; http_header; pcre:"/response=[0-9a-f]{32}/"; threshold: type both, track by_src, count 10, seconds 60; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate:

• Upgrade Apache HTTP Server to version 2.4.67 or later.
• If upgrade is not possible, disable mod_auth_digest and use alternative authentication methods.

Long-term:

• Implement constant-time comparison functions in all authentication-related code.
• Regularly audit and update web server software.
• Use Web Application Firewalls (WAF) to detect and block timing attacks.
• Monitor for unusual authentication patterns.

Workaround:

• Replace Digest authentication with Basic authentication over HTTPS or OAuth 2.0.

Valters IT
Securing Digital Frontiers

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