CVE-2025-55753: Apache HTTP Server DoS Vulnerability

CVE-2025-55753 Overview

CVE-2025-55753 is an Integer Overflow vulnerability affecting Apache HTTP Server's ACME certificate renewal mechanism. When certificate renewal fails repeatedly over an extended period (approximately 30 days in default configurations), an integer overflow occurs in the backoff timer calculation, causing the timer value to become zero. This results in the server making continuous, unthrottled certificate renewal attempts until successful, potentially causing resource exhaustion and enabling information disclosure through excessive network traffic patterns.

Critical Impact

This vulnerability can lead to significant resource consumption and potential information disclosure through uncontrolled network requests when ACME certificate renewals fail, affecting server availability and potentially exposing certificate management infrastructure details.

Affected Products

• Apache HTTP Server versions 2.4.30 through 2.4.65
• Systems using ACME protocol for automated certificate management
• Deployments with Let's Encrypt or other ACME-based certificate authorities

Discovery Timeline

• 2025-12-05 - CVE-2025-55753 published to NVD
• 2025-12-10 - Last updated in NVD database

Technical Details for CVE-2025-55753

Vulnerability Analysis

The vulnerability resides in Apache HTTP Server's implementation of ACME (Automatic Certificate Management Environment) protocol handling, specifically within the certificate renewal retry logic. The ACME module includes a backoff mechanism designed to prevent overwhelming certificate authorities when renewal requests fail. However, this backoff calculation is susceptible to an integer overflow condition.

When certificate renewal fails, the server implements an exponential backoff strategy to space out retry attempts. After approximately 30 days of continuous failures in default configurations, the accumulated backoff value exceeds the maximum integer boundary, causing it to wrap around to zero. With a zero-value backoff timer, the server enters a tight loop, continuously attempting certificate renewals without any delay between attempts.

This behavior can lead to information disclosure as the high volume of network traffic may reveal details about the certificate infrastructure to network observers. Additionally, the constant renewal attempts consume server resources and generate excessive load on ACME certificate authority endpoints.

Root Cause

The root cause is classified as CWE-190: Integer Overflow or Wraparound. The backoff timer variable lacks proper boundary checking to prevent overflow conditions. When the exponentially increasing backoff value exceeds the maximum representable integer value, it wraps to zero rather than being clamped to a maximum safe value. This oversight in integer arithmetic handling allows the retry mechanism to bypass its intended throttling behavior.

Attack Vector

The attack vector is network-based and does not require authentication or user interaction. An attacker could potentially trigger this condition by:

1. Interfering with legitimate ACME certificate renewal processes through network-level attacks
2. Causing persistent DNS resolution failures to the ACME endpoint
3. Manipulating firewall rules or network configurations to block ACME traffic
4. Exploiting the condition on already-affected servers to amplify denial of service impacts

Once the integer overflow occurs, the server will generate a high volume of outbound HTTPS requests to the configured ACME certificate authority, potentially revealing infrastructure details through traffic analysis and causing resource exhaustion on both the web server and the certificate authority.

Detection Methods for CVE-2025-55753

Indicators of Compromise

• Unusually high volume of outbound HTTPS connections to ACME certificate authority endpoints (e.g., acme-v02.api.letsencrypt.org)
• Rapid succession of TLS certificate requests visible in network traffic logs
• Apache error logs showing continuous ACME renewal failures followed by immediate retry attempts without delays
• CPU and network utilization spikes correlated with certificate renewal processes

Detection Strategies

• Monitor Apache error logs for patterns indicating ACME certificate renewal failures exceeding typical retry intervals
• Implement network traffic analysis to detect abnormal volumes of requests to ACME endpoints
• Configure alerting for certificate-related processes consuming excessive system resources
• Review ACME account rate limiting notifications from certificate authorities indicating potential overflow conditions

Monitoring Recommendations

• Enable verbose logging for mod_md (ACME module) to capture certificate renewal timing information
• Deploy network intrusion detection rules to identify rapid ACME request patterns
• Implement threshold-based alerts for certificate renewal retry frequencies
• Monitor for rate limit errors returned from ACME certificate authorities

How to Mitigate CVE-2025-55753

Immediate Actions Required

• Upgrade Apache HTTP Server to version 2.4.66 or later immediately
• Review current certificate renewal status and check for signs of failed renewals exceeding 30 days
• Monitor server logs for evidence of integer overflow conditions in ACME handling
• Consider temporarily switching to manual certificate management if upgrades cannot be performed immediately

Patch Information

Apache has released version 2.4.66 which addresses this integer overflow vulnerability in the ACME certificate renewal backoff mechanism. The fix implements proper boundary checking to prevent the backoff timer from overflowing to zero. Users should upgrade to this version or later as soon as possible.

For detailed information about the fix and other security updates, refer to the Apache HTTP Server Security Vulnerabilities page. Additional discussion of this vulnerability is available on the OpenWall OSS Security Mailing List.

Workarounds

• Implement external monitoring to detect certificate renewal failures and trigger manual intervention before the 30-day overflow threshold
• Configure network-level rate limiting for outbound connections to ACME endpoints to mitigate impact if overflow occurs
• Deploy a reverse proxy or firewall rule to throttle certificate renewal requests if patching is not immediately feasible
• Consider temporarily disabling automatic ACME certificate renewal and using manual certificate deployment until the patch is applied

# Verify Apache HTTP Server version
httpd -v

# Check for mod_md (ACME module) status
httpd -M | grep md

# Monitor ACME renewal logs for anomalies
tail -f /var/log/httpd/error_log | grep -i "md\|acme\|certificate"

# Restart Apache after upgrading to 2.4.66
systemctl restart httpd