CVE-2025-55753: 
Apache HTTP Server vulnerability analysis and mitigation

CVE-2025-55753 is an integer overflow vulnerability in the mod_md module of Apache HTTP Server that causes unintended ACME certificate renewal retry behavior. When ACME certificate renewal fails repeatedly (approximately 30 days in default configurations), an integer overflow causes the backoff timer to wrap around to 0, after which renewal attempts are made continuously without any delay. The vulnerability affects Apache HTTP Server versions 2.4.30 through 2.4.65 (before 2.4.66) and was reported on 2025-08-15, fixed on 2025-11-20, and publicly disclosed on 2025-12-04/05. It carries a CVSS v3.1 base score of 7.5 (High) (Feedly, oss-security).

The root cause is an integer overflow (CWE-190) in the ACME backoff timer logic within Apache's mod_md module. Under normal operation, mod_md implements exponential backoff when ACME certificate renewal fails; however, after approximately 30 days of accumulated failures in default configurations, the internal timer counter overflows and wraps to zero. Once the timer reaches zero, the module enters an unthrottled retry loop, issuing certificate renewal requests to the ACME provider without any delay between attempts. The vulnerability requires no authentication or user interaction and is exploitable over the network by an attacker capable of persistently blocking ACME challenge completion (e.g., disrupting network connectivity or DNS/HTTP challenge responses) to trigger and sustain the overflow condition (oss-security, Feedly).

The primary impact is a confidentiality concern rated High by NVD (CVSS C:H), though the practical exploitation scenario centers on resource exhaustion and potential service disruption. Once the backoff timer overflows to zero, the affected Apache server floods the ACME provider (e.g., Let's Encrypt) with continuous renewal requests, potentially exhausting rate limits and causing certificate renewal failures that could lead to TLS certificate expiry and service unavailability. Additionally, the unthrottled requests consume local system resources. Oracle's assessment of the vulnerability in Solaris context rated it with High confidentiality, High integrity, and Low availability impact, suggesting broader potential consequences depending on deployment context (Oracle, Feedly).

There is no public proof-of-concept exploit code and no evidence of in-the-wild exploitation as of the time of disclosure. The EPSS score is very low at approximately 0.011%, reflecting limited near-term exploitation likelihood. The vulnerability is not listed in the CISA Known Exploited Vulnerabilities (KEV) catalog. Exploitation requires the ability to persistently block ACME certificate renewals for approximately 30 days, making opportunistic exploitation unlikely; however, targeted attacks against specific servers could be feasible. No threat actor attribution has been reported (Feedly, oss-security).

1. Reconnaissance: Identify Apache HTTP Server instances (versions 2.4.30–2.4.65) using mod_md for automated ACME certificate management via tools like Shodan, Censys, or banner grabbing.
2. Block ACME challenges: Persistently disrupt the target server's ability to complete ACME HTTP-01 or DNS-01 challenges — for example, by blocking outbound connections to the ACME provider, poisoning DNS responses for the challenge domain, or interfering with the .well-known/acme-challenge/ endpoint.
3. Wait for overflow: Maintain the disruption for approximately 30 days (the default configuration threshold) until the mod_md backoff timer integer overflows and resets to zero.
4. Observe unthrottled retries: Once the timer reaches zero, the server begins issuing continuous, undelayed ACME renewal requests. This exhausts ACME provider rate limits (e.g., Let's Encrypt's 5 failed validations per hour per domain) and consumes server resources.
5. Achieve objective: The resulting rate-limit exhaustion prevents legitimate certificate renewal, eventually causing TLS certificate expiry and service disruption for the target domain (oss-security, Feedly).

• Logs: Apache error logs (error_log) showing repeated mod_md ACME renewal attempt messages without backoff delays; entries such as md[<domain>]: renewal attempt appearing at high frequency without exponential spacing.
• Network: Unusually high volume of outbound HTTPS connections from the Apache server to ACME provider endpoints (e.g., acme-v02.api.letsencrypt.org) in rapid succession without delay intervals.
• Application Behavior: ACME rate-limit error responses (HTTP 429) from the ACME provider logged in Apache error logs; TLS certificate approaching or past expiry despite mod_md being enabled.
• System Resources: Elevated CPU or network I/O on the Apache server correlating with continuous ACME renewal attempts, observable via netstat, ss, or system monitoring tools.

The primary remediation is to upgrade Apache HTTP Server to version 2.4.66 or later, which contains the fix (committed as r1929884 in the 2.4.x branch on 2025-11-20). Downstream distributions including Red Hat (RHSA-2025:23732, RHSA-2025:23739, RHSA-2026:0009, RHSA-2026:0012, RHSA-2026:0029, RHSA-2026:0092–0094), Oracle Linux, AlmaLinux, Rocky Linux, Debian, Ubuntu (USN-7968-1, USN-7968-2, USN-8338-1, USN-8338-2), SUSE, Slackware, Mageia, Amazon Linux 2, and Apple macOS (Sequoia 15.7.5, Sonoma 14.8.5, Tahoe 26.4) have all released updated packages. As a workaround prior to patching, administrators can disable mod_md if ACME-based certificate management is not required, or implement external monitoring to detect and alert on ACME renewal failures before the 30-day overflow threshold is reached (oss-security, Apple Sequoia, Oracle).

The Apache Software Foundation disclosed the vulnerability with a severity rating of "low" via the oss-security mailing list, crediting "Aisle Research" as the finder. Coverage was picked up by security news outlets including Heise (German-language), SecurityOnline.info, and Red Hot Cyber, which covered it as part of the broader Apache HTTP Server 2.4.66 release fixing multiple vulnerabilities. The vulnerability received routine attention from Linux distribution security teams given the wide deployment of Apache HTTP Server, with rapid patch releases across major distributions. No significant controversy or notable researcher commentary beyond standard disclosure was observed (oss-security).