CVE-2025-55753

Executive Summary

CVE-2025-55753 is a denial-of-service (DoS) vulnerability in the Apache HTTP Server's ACME (Automatic Certificate Management Environment) certificate renewal module. The flaw is an integer overflow in the backoff timer calculation when ACME certificate renewal repeatedly fails. After approximately 30 days of continuous failures (in default configurations), the backoff timer wraps to zero, causing the server to attempt certificate renewal in an infinite loop without any delay. This results in excessive CPU consumption and potential service disruption, effectively creating a self-inflicted DoS condition.

The vulnerability affects Apache HTTP Server versions from 2.4.30 up to, but not including, 2.4.66. The fix is included in version 2.4.66. No public proof-of-concept (PoC) or exploit code is currently available. The EPSS score is 0.0%, indicating a very low probability of exploitation in the wild at this time.

CVSS Breakdown & Threat Level

CVSS Score: 0.0 (NVD has not yet assigned a score; the following is an analyst-derived assessment)

Derived CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

Breakdown:

• Attack Vector (AV): Network – The vulnerability is triggered by the server's own ACME renewal process, which communicates over the network with a CA.
• Attack Complexity (AC): Low – No special conditions are required; the failure of ACME renewal is the only prerequisite.
• Privileges Required (PR): None – An attacker does not need any authentication; they only need to cause ACME renewal to fail (e.g., by blocking network access to the CA).
• User Interaction (UI): None – No user action is required.
• Scope (S): Unchanged – The vulnerability does not affect resources beyond the vulnerable component.
• Confidentiality (C): None – No data is exposed.
• Integrity (I): None – No data is modified.
• Availability (A): Low – The vulnerability can cause high CPU usage and potential service degradation, but not a complete crash.

Threat Level: Medium (due to the potential for resource exhaustion and service disruption, despite low exploitability).

Deep Dive & Mechanics

The vulnerability resides in the ACME client module (mod_md) of Apache HTTP Server. When the server attempts to renew a certificate via ACME, it uses an exponential backoff algorithm to space out retry attempts after failures. The backoff timer is stored as an integer.

Root Cause: An integer overflow occurs when the backoff timer value exceeds the maximum value that can be stored in the integer variable. After a large number of consecutive failures (approximately 30 days in default configurations), the timer wraps around to zero.

Trigger Condition: The vulnerability is triggered when ACME certificate renewal fails repeatedly. This can happen due to:

• Network connectivity issues preventing the server from reaching the Certificate Authority (CA).
• The CA being temporarily unavailable.
• An attacker deliberately blocking or interfering with the ACME renewal process (e.g., by performing a DoS on the CA or manipulating DNS).

Impact: Once the backoff timer becomes zero, the server attempts to renew the certificate in a tight loop without any delay. This results in:

• High CPU usage: The server continuously processes renewal requests.
• Resource exhaustion: Other server processes may be starved of CPU time.
• Service disruption: The server may become unresponsive to legitimate HTTP requests.

Affected Component: mod_md (the ACME module) in Apache HTTP Server.

Vulnerable vs. Patched Code

Vulnerable Code (Apache HTTP Server < 2.4.66): The vulnerable code is in the backoff timer calculation. The exact location is in the mod_md module, likely in a function that calculates the delay before the next retry. The integer overflow occurs when the delay value is incremented without proper bounds checking.

// Vulnerable pseudocode
int backoff = 1;
while (renewal_fails) {
    sleep(backoff);
    backoff = backoff * 2; // No overflow check
    if (backoff < 0) { // Integer overflow wraps to negative or zero
        backoff = 0; // This may not be present or may be incorrect
    }
}

Patched Code (Apache HTTP Server 2.4.66): The fix introduces a cap on the backoff timer to prevent it from overflowing. The maximum delay is limited to a safe value (e.g., 86400 seconds, or 24 hours).

// Patched pseudocode
int backoff = 1;
int max_backoff = 86400; // 24 hours
while (renewal_fails) {
    sleep(backoff);
    backoff = backoff * 2;
    if (backoff > max_backoff) {
        backoff = max_backoff; // Cap the backoff
    }
}

Key Difference: The patched code ensures that the backoff timer never exceeds a predefined maximum, preventing the integer overflow and the subsequent infinite loop.

Exploitation & Lab Setup

Red Team (Exploitation)

Prerequisites:

• An Apache HTTP Server running a vulnerable version (2.4.30 to 2.4.64) with mod_md enabled and configured for ACME certificate renewal.
• The ability to cause ACME renewal to fail repeatedly (e.g., by blocking outbound connections to the CA or by manipulating the server's network configuration).

Exploitation Steps:

1. Identify Target: Scan for Apache servers with mod_md enabled. Use Shodan dorks like "Apache/2.4.30" or "Server: Apache/2.4.30".
2. Trigger ACME Failure: Block the server's outbound traffic to the ACME CA (e.g., using a firewall rule or ARP spoofing). Alternatively, if you have network access, you can perform a DoS on the CA itself.
3. Wait for Backoff Overflow: The server will retry ACME renewal with increasing delays. After approximately 30 days (in default configurations), the backoff timer will overflow to zero.
4. Observe DoS: The server will enter an infinite loop of renewal attempts, consuming 100% CPU on one core. The server may become unresponsive to legitimate HTTP requests.

Note: No public exploit code is available. This is a theoretical exploitation scenario.

Blue Team (Defense & Detection)

Detection:

• Monitor CPU Usage: Sudden, sustained high CPU usage on the Apache process (especially on a single core) could indicate this vulnerability being triggered.
• Monitor ACME Logs: Look for repeated ACME renewal failure messages in the Apache error log. A high frequency of these messages (e.g., multiple per second) after a period of failures is a strong indicator.
• Network Monitoring: Detect outbound connections to ACME CAs that are being blocked or failing.

Mitigation:

• Patch Immediately: Upgrade to Apache HTTP Server 2.4.66 or later.
• Network Segmentation: Restrict outbound traffic from the Apache server to only necessary CAs.
• Rate Limiting: Implement rate limiting on ACME renewal attempts at the network level.
• Monitoring: Set up alerts for high CPU usage on Apache processes and for a high rate of ACME renewal failures.

Lab Setup:

1. Install Vulnerable Apache: Use a Docker container with Apache 2.4.30.
2. Enable mod_md: Configure ACME certificate renewal.
3. Block CA Access: Use iptables to block outbound traffic to the ACME CA.
4. Monitor Logs: Watch the Apache error log for renewal failures.
5. Simulate Attack: Wait for the backoff timer to overflow (may take days in real time; for testing, you can modify the source code to reduce the backoff multiplier).

Indicators of Compromise (IOCs)

• Log Entries:
  • Repeated ACME renewal failure messages in /var/log/apache2/error.log or similar.
  • Messages indicating "ACME renewal failed" or "certificate renewal error" occurring at a high frequency (multiple per second).
• Network Traffic:
  • Outbound connections to ACME CAs (e.g., Let's Encrypt) that are failing or being blocked.
  • High volume of outbound traffic to the same CA IP address.
• System Metrics:
  • High CPU usage (near 100%) on the Apache HTTP Server process.
  • Increased memory usage due to the infinite loop.
• File System:
  • Temporary files related to ACME renewal that are not being cleaned up.

Detection Engineering

YARA Rule

rule CVE_2025_55753_Apache_ACME_Backoff_Overflow {
    meta:
        description = "Detects Apache HTTP Server versions vulnerable to CVE-2025-55753"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-55753"
        severity = "medium"
    strings:
        $version_pattern = /Apache\/2\.4\.(3[0-9]|4[0-9]|5[0-9]|6[0-4])/
        $mod_md = "mod_md.c"
    condition:
        $version_pattern and $mod_md
}

Sigma Rule

title: CVE-2025-55753 - Apache HTTP Server ACME Backoff Overflow DoS
id: 8a4b3c2d-1e5f-4a7b-9c8d-0e1f2a3b4c5d
status: experimental
description: Detects repeated ACME renewal failures that may indicate exploitation of CVE-2025-55753
author: Valters IT
date: 2025-04-10
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2025-55753
logsource:
    product: apache
    service: httpd
detection:
    selection:
        - EventID: 30200  # Example: Apache error log event ID
        - Message: "*ACME renewal failed*"
    timeframe: 5m
    condition: selection | count() > 100
falsepositives:
    - Legitimate network issues causing temporary ACME failures
level: medium

Suricata Rule

alert tcp $HOME_NET any -> $EXTERNAL_NET 80 (msg:"CVE-2025-55753 - Apache ACME Renewal Attempt"; flow:to_server; content:"POST"; http_method; content:"/acme"; http_uri; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Upgrade Apache HTTP Server: Update to version 2.4.66 or later. This is the only complete fix.
2. Apply Workaround: If upgrading is not immediately possible, disable the ACME module (mod_md) if it is not required. Alternatively, configure a firewall to rate-limit outbound connections to ACME CAs.
3. Monitor: Implement monitoring for high CPU usage and repeated ACME renewal failures.

Long-Term Mitigations:

• Patch Management: Establish a process for timely application of security patches.
• Network Segmentation: Isolate the Apache server from untrusted networks.
• Defense in Depth: Use Web Application Firewalls (WAF) and intrusion detection systems (IDS) to detect and block exploitation attempts.

Valters IT Signature

This report was generated by Valters IT Cyber Threat Intelligence. For further analysis or remediation support, contact your Valters IT representative.