CVE-2026-72701: Observable Timing Discrepancy in getgrav grav

The vulnerability exists in Grav CMS's core function Grav\Common\Utils::verifyNonce(), which validates CSRF nonces using PHP's === operator for string comparison. Since === short-circuits on the first differing byte, the comparison time varies depending on how many leading bytes match, leaking timing information (CWE-208). Grav CMS uses hash_equals() for similar secret comparisons elsewhere, but this function still uses ===. The nonce is an md5 hash derived from a secret installation-level key and other parameters. Timing experiments confirm that the string comparison is not constant time, producing measurable timing differences at the nonce length of 32 hex characters. The affected versions are all versions before 2.0.16.

An attacker able to measure response times precisely could use the timing discrepancy to incrementally guess valid CSRF nonces by observing how many leading bytes are correct. This could facilitate bypassing CSRF protections in Grav CMS. However, exploitation requires precise timing measurement and is not confirmed in the wild. The vulnerability is rated medium severity with a CVSS 4.0 score of 6.3.

A fix is available in Grav CMS version 2.0.16 and later, which presumably replaces the vulnerable === comparison with a constant-time comparison function like hash_equals(). Users should upgrade to version 2.0.16 or later to remediate this issue. No other mitigation is explicitly stated or required.