Overview
A critical security vulnerability, identified as CVE-2025-55753, has been discovered in the Apache HTTP Server. This integer overflow issue affects versions 2.4.30 through 2.4.65. The vulnerability occurs during ACME certificate renewal, potentially leading to denial-of-service-like behavior due to excessive renewal attempts. It is highly recommended that all users upgrade to version 2.4.66 as soon as possible.
Technical Details
The vulnerability stems from an integer overflow in the backoff timer mechanism used during ACME certificate renewal. When an ACME certificate renewal fails, Apache HTTP Server employs a backoff timer to prevent repeated attempts from overwhelming the ACME server. However, after approximately 30 days of continuous failed renewal attempts (using default configurations), the integer overflow occurs, causing the backoff timer to reset to 0. This effectively eliminates the delay between renewal attempts.
Consequently, the server will relentlessly attempt to renew the certificate without any delay, potentially impacting server performance and potentially leading to rate limiting or other issues with the ACME provider.
CVSS Analysis
Currently, the CVSS score for CVE-2025-55753 is not yet available (N/A). However, due to the potential for denial-of-service and impact on server resources, it is anticipated that the score will be rated as medium to high severity once assigned.
Possible Impact
- Denial of Service (DoS): Continuous and rapid certificate renewal attempts can consume significant server resources, potentially impacting the availability of the web server for legitimate users.
- ACME Rate Limiting: Repeated failed attempts may trigger rate limiting mechanisms implemented by ACME certificate authorities, preventing the server from obtaining valid certificates in the future.
- Performance Degradation: The excessive load from repeated renewal attempts can lead to overall performance degradation of the Apache HTTP Server.
Mitigation and Patch Steps
The primary mitigation is to upgrade your Apache HTTP Server to version 2.4.66 or later. This version contains the fix for the integer overflow vulnerability.
- Backup your configuration: Before upgrading, create a backup of your Apache HTTP Server configuration files.
- Upgrade Apache HTTP Server: Use your distribution’s package manager or follow the official Apache HTTP Server upgrade instructions to upgrade to version 2.4.66.
- Verify the upgrade: After the upgrade, verify that the server is running version 2.4.66 by using the
httpd -vcommand. - Monitor your server: After the upgrade, monitor your server for any unexpected behavior.
