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An integer overflow vulnerability exists in the ACME certificate renewal backoff mechanism of Apache HTTP Server. When automatic certificate renewal via the ACME protocol fails, the server implements a timer to delay subsequent retry attempts. This backoff timer is designed to increase incrementally after each failure to prevent overwhelming the certificate authority. However, a flaw in this timer’s calculation causes an integer overflow after a sustained period of renewal failures. In default configurations, this occurs after approximately 30 consecutive days of failed renewal attempts. When the overflow happens, the backoff timer value wraps around and becomes zero. Once the timer is zero, the system ceases to impose any delay between renewal retries. The server then enters a state where it attempts to renew the certificate in rapid, immediate succession. This persists indefinitely until a renewal attempt finally succeeds. The issue is triggered specifically within the mod_md module responsible for managing certificates. The overflow occurs in the code handling the exponential backoff delay, likely due to multiplication without proper bounds checking. This condition does not require an attacker to have direct access to the server but relies on renewal failures. While not directly allowing code execution, it can lead to a denial-of-service condition. The resource exhaustion stems from continuous, high-frequency network requests and internal processing. It may also cause the server to be rate-limited or blocked by the ACME certificate authority. Affected versions include Apache HTTP Server from 2.4.30 up to, but not including, version 2.4.66. The fix in version 2.4.66 addresses the integer overflow by implementing proper checks on the backoff timer value.
Platform: Apache HTTP Server
Version: Before 2.4.66
Vulnerability: Integer overflow timer
Severity: Medium
date: 12/05/2025
Prediction: Fixed in 2.4.66
What Undercode Say:
Analytics:
bash commands:
httpd -v
apachectl -v
systemctl status apache2
grep -r “mod_md” /etc/apache2/
tail -f /var/log/apache2/error.log
codes:
// Example vulnerable timer logic
unsigned int backoff = initial_delay;
while (failures–) { backoff = 2; }
if (backoff == 0) { retry_immediately(); }
how Exploit:
Induce ACME renewal failures.
Spoof or block ACME server responses.
Wait for timer overflow (~30 days).
Trigger resource exhaustion loop.
Protection from this CVE:
Upgrade to 2.4.66.
Monitor renewal logs.
Ensure network stability.
Impact:
Resource exhaustion denial-of-service.
Increased server load.
Potential service disruption.
ACME server rate-limiting.
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Sources:
Reported By: nvd.nist.gov
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