CVE-2025-55753 is a vulnerability classified under CWE-190 (Integer Overflow or Wraparound) affecting the Apache HTTP Server, specifically versions from 2.4.30 up to but not including 2.4.66. The flaw arises in the ACME certificate renewal mechanism, which uses a backoff timer to space out retry attempts after failures. Under normal operation, if certificate renewal fails, the server waits progressively longer before retrying, preventing excessive load. However, due to an integer overflow triggered after approximately 30 days of continuous failures (default configuration), the backoff timer value wraps around to zero. This causes the server to attempt certificate renewal repeatedly without any delay until it succeeds. Such behavior can lead to excessive CPU and network resource consumption, potentially resulting in denial of service conditions on the affected server. The vulnerability does not require authentication or user interaction to be exploited, making it easier for attackers or misconfigurations to trigger. While no public exploits are known at this time, the issue affects a core component of many web infrastructures worldwide. The recommended remediation is to upgrade to Apache HTTP Server version 2.4.66, where the issue has been fixed. This vulnerability highlights the risks of integer overflows in retry logic, especially in critical security functions like certificate management.

For European organizations, the impact of CVE-2025-55753 primarily concerns availability and operational stability of web services relying on Apache HTTP Server. Organizations using affected versions risk service disruptions due to resource exhaustion caused by rapid, repeated certificate renewal attempts. This can lead to denial of service, affecting customer-facing websites, internal portals, and APIs. The vulnerability could also increase operational costs due to higher CPU and network usage. Additionally, if attackers deliberately induce certificate renewal failures, they could exploit this flaw to degrade service availability. Given Apache HTTP Server's widespread adoption in Europe across public sector, financial institutions, and enterprises, the potential impact is significant. The integrity and confidentiality of data are less directly affected, but service unavailability can indirectly impact business continuity and trust. Organizations with automated certificate management relying on ACME protocols are especially vulnerable. The absence of known exploits reduces immediate risk but does not eliminate the threat, particularly as attackers may develop exploits once the vulnerability is publicly known.

1. Upgrade all Apache HTTP Server instances to version 2.4.66 or later immediately to apply the official fix for this vulnerability. 2. Monitor ACME certificate renewal logs for unusual patterns, such as repeated rapid retries, which may indicate exploitation or misconfiguration. 3. Implement resource usage monitoring and alerting on web servers to detect abnormal CPU or network spikes related to certificate renewal processes. 4. Consider implementing external rate limiting or circuit breakers on renewal attempts if possible, to prevent rapid retry loops. 5. Review and test backup and failover mechanisms to ensure service continuity in case of denial of service conditions. 6. Educate system administrators about the risks of integer overflow in retry logic and the importance of timely patching. 7. For organizations using containerized or cloud deployments, ensure base images and orchestration templates are updated to include the patched Apache version. 8. Engage with certificate authority providers to understand and possibly adjust renewal failure handling to reduce prolonged failure states.