CVE-2025-55753 is an integer overflow vulnerability classified under CWE-190 affecting the Apache HTTP Server versions from 2.4.30 to before 2.4.66. The vulnerability arises in the ACME certificate renewal process, which is used to automate SSL/TLS certificate management. When the renewal process fails repeatedly over a default period of about 30 days, an integer overflow causes the backoff timer—intended to delay subsequent retry attempts—to wrap around to zero. Consequently, the server attempts to renew the certificate continuously without any delay. This behavior can lead to excessive CPU and network resource consumption, potentially degrading server performance or causing denial of service conditions. The flaw can be triggered remotely without authentication or user interaction, increasing its risk profile. Although no direct compromise of confidentiality or integrity is reported, the availability of the affected server may be impacted due to resource exhaustion. The Apache Software Foundation has addressed this issue in version 2.4.66, recommending all users upgrade to this or later versions to prevent exploitation.

For European organizations, the primary impact of this vulnerability is the risk of denial of service due to resource exhaustion on servers running vulnerable Apache HTTP Server versions. Organizations relying on automated ACME certificate renewals, especially those with default configurations, may experience continuous renewal attempts that degrade server performance or cause outages. This can disrupt web services, impacting business operations, customer access, and internal communications. Critical infrastructure providers, government agencies, and large enterprises using Apache HTTP Server for public-facing or internal services are particularly at risk. The lack of authentication or user interaction required for exploitation increases the likelihood of automated attacks. While no known exploits are currently active, the vulnerability's nature and ease of triggering make it a significant operational risk. The impact is more pronounced in environments with strict uptime requirements or limited capacity to handle resource spikes.

The most effective mitigation is to upgrade Apache HTTP Server to version 2.4.66 or later, where the integer overflow issue has been fixed. Organizations should audit their server fleet to identify all instances running affected versions and prioritize patching those exposed to the internet or critical internal networks. Additionally, administrators can temporarily adjust ACME renewal configurations to reduce retry frequency or implement external rate limiting on renewal requests to prevent resource exhaustion. Monitoring server resource usage and renewal logs can help detect abnormal retry patterns early. Employing web application firewalls (WAFs) or network-level protections to limit excessive renewal traffic may also reduce risk. Finally, organizations should ensure robust incident response plans are in place to quickly address any service disruptions caused by this vulnerability.