In early 2025, a new vulnerability was disclosed in the Apache HTTP Server, one of the world’s most popular web servers. This vulnerability, tracked as CVE-2025-55753, affects the Automatic Certificate Management Environment (ACME) certificate renewal feature. If exploited, it could cause your server to hammer certificate authorities with rapid-fire renewal requests, possibly getting you rate-limited, banned, or even exposing your system to denial of service.

Let’s break down what happened, how this bug works, and most importantly, how you can protect your systems.

Main issue: Integer overflow when tracking failed certificate renewal attempts

- Impact: After a certain number of failures (about a month’s worth by default), the system’s retry delay becomes —causing the server to immediately and repeatedly try certificate renewal, flooding the certificate authority

How Does the Bug Happen?

ACME modules are designed to periodically renew TLS certificates. If a renewal fails, the server backs off, adding extra delay before each next attempt—a common approach called *exponential backoff*. Unfortunately, in Apache HTTP Server up to 2.4.65, the integer used to track the backoff timer could overflow given enough failures.

When this happens, the delay value “rolls over” to . That means—no waiting. The server starts hammering renewal attempts as fast as it can, instead of waiting minutes or hours between tries.

Here’s a simplified code sketch (not the real Apache source!)

int backoff = 60; // start with 1-minute delay

while (!certificate_renewed) {
    int rc = try_renew();
    if (rc != ) {
        backoff = backoff * 2; // exponential backoff
        if (backoff > MAX_BACKOFF) backoff = MAX_BACKOFF;
    } else {
        backoff = 60; // reset on success
    }
    sleep(backoff);
}

Now imagine that, internally, backoff is a signed 32-bit integer and never checked for overflow. After enough failed attempts (for instance, 1-minute -> 2, 4, 8, … up to a month or so), backoff overflows to or a negative number. The next sleep(backoff) is either immediately executed or skipped, so renewal attempts loop instantly.

Exploit Scenario: How Would an Attacker Abuse This?

The vulnerability mostly causes *self-inflicted* denial of service, but it could be manipulated by malicious actors:

1. Force Renewal Failures: If an attacker can block outbound connections to the ACME provider (like Let’s Encrypt), the server piles up renewal failures.

Wait for Overflow: After about a month, the backoff timer goes negative (overflow).

3. Flood on Unblocking: Once the block is lifted, the server instantly fires repeated renewal requests until the certificate is issued or the authority rate-limits the server.
4. Collateral Damage: CA authorities may temporarily ban your domain or IP, preventing further renewals for days or weeks.

Detecting The Issue

Are you still on Apache HTTP Server pre-2.4.66 and using ACME renewal? You should check your error logs for repeated ACME renewal failures or an abnormal spike in connection attempts to your CA’s endpoint.

Look for log entries like

[error] [acme] Certificate renewal failed, retrying in  seconds...

Fix and Mitigation

The Apache Software Foundation released 2.4.66, which corrects this overflow bug by capping and properly checking the backoff timer.

Upgrade now

- Apache HTTP Server downloads
- Release Notes for 2.4.66

References

- Apache HTTP Server Announcement for CVE-2025-55753
- CERT/CC Vulnerability Note VU#XXXXX
- Let’s Encrypt Rate Limits

Conclusion

CVE-2025-55753 is a classic example of why robust integer handling matters, especially with timers and backoff algorithms. While it’s not a remote code execution bug, it can result in denial of service and operational headaches. And as always, patch early, patch often.

Upgrade to Apache HTTP Server 2.4.66 as soon as possible.

If you want to dive deeper, read the official advisory and consider subscribing to the Apache announce list to stay ahead of future issues.

Staff and sysadmins: Stay safe, and keep your certs happy!

Timeline

Published on: 12/05/2025 10:12:22 UTC
Last modified on: 12/10/2025 16:39:14 UTC