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Response snapshot Oct 27, 2026, 02:47 UTC

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VulnerabilityCVE-2026-72701

Grav: Non constant time nonce comparison in Utils::verifyNonce() used for CSRF protection

`Grav\Common\Utils::verifyNonce()`, the core function Grav and its plugins use to validate CSRF nonces, compares the submitted nonce to the expected value with PHP's `===` operator instead of `hash_equals()`.

Published Updated

At a glance

Exploited
Not known
Severity
3.7 Low (CVSS 3.1)
Chance of exploitation
0.2% in the next 30 days
Sources
1 source

What to do

GitHub lists first patched versions: getgrav/grav: 2.0.16. Confirm the affected range and upgrade guidance in the exact advisory.

Read GitHub-reviewed advisory GHSA-38p6-h87p-r4cg (opens a new tab)

What happened

## Summary

`Grav\Common\Utils::verifyNonce()`, the core function Grav and its plugins use to validate CSRF nonces, compares the submitted nonce to the expected value with PHP's `===` operator instead of `hash_equals()`. `===` on strings short circuits at the first differing byte, so the comparison time leaks how many leading bytes of a guess are correct. This is CWE-208, Observable Timing Discrepancy.

The codebase already knows to avoid this pattern. `hash_equals()` is used for the equivalent purpose in four other places I found: `system/src/Grav/Common/Session.php`, `system/src/Grav/Framework/Cache/Adapter/FileCache.php`, `system/src/Grav/Common/Scheduler/Scheduler.php` (the webhook token check), and `system/src/Grav/Common/Scheduler/JobQueue.php`. `Utils::verifyNonce()` is the one place I found that still uses a plain equality check for a secret comparison.

## Affected product and version

Continue reading

Product: Grav CMS, getgrav/grav Confirmed present in: 2.0.15, commit c2b46866857a93a0aa7048e7ed707ed3ed45dbc3

## Affected code

`system/src/Grav/Common/Utils.php`, lines 1512 to 1521: ```php public static function verifyNonce($nonce, $action) { //Safety check for multiple nonces if (is_array($nonce)) { $nonce = array_shift($nonce); }

//Nonce generated 0-12 hours ago if ($nonce === self::getNonce($action)) { return true; }

//Nonce generated 12-24 hours ago return $nonce === self::getNonce($action, true); } ```

The nonce itself is `md5($tick . '|' . $action . '|' . $username . '|' . session_id() . '|' . Security::getNonceKey())`, computed in the private `generateNonceString()` a few lines above. `Security::getNonceKey()` is an installation level secret. So the value being compared with `===` is a value derived from a secret, which is exactly the case `hash_equals()` exists for.

## Proof of concept, verified, real output

I could not exploit this end to end over a real network from this sandbox, since that requires a live deployment and a timing measurement setup outside a single machine.

What I did verify directly, by running real code, is that the underlying primitive this function relies on, PHP's `===` string comparison, is not constant time in the PHP build actually used here, and that a measurable timing signal is still present at the exact length Grav's nonces have, 32 hex characters, an md5 digest.

Step 1, confirm PHP build: ``` $ php -v PHP 8.3.6 (cli) (built: Jul 16 2026 18:30:41) (NTS) ```

Step 2, benchmark script, measures the median time of `$a === $b` over many trials, once with a long string to establish a clean signal, once at the real 32 byte nonce length: ```php <?php // timing_poc2.php function timeCompare(string $a, string $b, int $iterations): float { $r = null; $start = hrtime(true); for ($i = 0; $i < $iterations; $i++) { $r = ($a === $b); } $end = hrtime(true); return ($end - $start) / $iterations; }

function median(array $arr): float { sort($arr); $n = count($arr); $mid = intdiv($n, 2); return $n % 2 ? $arr[$mid] : ($arr[$mid - 1] + $arr[$mid]) / 2; }

function runExperiment(int $len, int $iterations, int $trials): array { $secret = bin2hex(random_bytes((int)ceil($len / 2))); $secret = substr($secret, 0, $len);

$wrongEarly = $secret; $wrongEarly[0] = ($secret[0] === 'a') ? 'b' : 'a';

$wrongLate = $secret; $last = $len - 1; $wrongLate[$last] = ($secret[$last] === 'a') ? 'b' : 'a';

$earlyTimes = []; $lateTimes = []; timeCompare($wrongEarly, $secret, 20000); timeCompare($wrongLate, $secret, 20000); for ($t = 0; $t < $trials; $t++) { $earlyTimes[] = timeCompare($wrongEarly, $secret, $iterations); $lateTimes[] = timeCompare($wrongLate, $secret, $iterations); } return [median($earlyTimes), median($lateTimes)]; }

echo "=== Length 4096 bytes, establishes the primitive is not constant time ===\n"; [$e, $l] = runExperiment(4096, 20000, 15); printf("Median mismatch at position 0 : %.2f ns/op\n", $e); printf("Median mismatch at last position : %.2f ns/op\n", $l); printf("Ratio (late/early) : %.2fx\n\n", $l / $e);

echo "=== Length 32 bytes, the actual Grav nonce length, md5 hex output ===\n"; [$e2, $l2] = runExperiment(32, 200000, 21); printf("Median mismatch at position 0 : %.2f ns/op\n", $e2); printf("Median mismatch at last position : %.2f ns/op\n", $l2); printf("Ratio (late/early) : %.2fx\n", $l2 / $e2); ```

Step 3, run it three times to confirm the result is reproducible and not noise: ``` $ php timing_poc2.php ```

Actual output, run 1: ``` === Length 4096 bytes, establishes the primitive is not constant time === Median mismatch at position 0 : 13.83 ns/op Median mismatch at last position : 338.58 ns/op Ratio (late/early) : 24.48x

=== Length 32 bytes, the actual Grav nonce length, md5 hex output === Median mismatch at position 0 : 14.13 ns/op Median mismatch at last position : 17.41 ns/op Ratio (late/early) : 1.23x ```

Actual output, run 2: ``` === Length 4096 bytes, establishes the primitive is not constant time === Median mismatch at position 0 : 14.25 ns/op Median mismatch at last position : 333.99 ns/op Ratio (late/early) : 23.44x

=== Length 32 bytes, the actual Grav nonce length, md5 hex output === Median mismatch at position 0 : 13.83 ns/op Median mismatch at last position : 17.24 ns/op Ratio (late/early) : 1.25x ```

Actual output, run 3: ``` === Length 4096 bytes, establishes the primitive is not constant time === Median mismatch at position 0 : 14.20 ns/op Median mismatch at last position : 336.89 ns/op Ratio (late/early) : 23.73x

=== Length 32 bytes, the actual Grav nonce length, md5 hex output === Median mismatch at position 0 : 13.98 ns/op Median mismatch at last position : 17.39 ns/op Ratio (late/early) : 1.24x ```

Interpretation, stated honestly. At 4096 bytes the effect is unambiguous and consistent across three independent runs, a mismatch near the end of the string takes about 23 to 24 times longer to reject than a mismatch at the very first byte, which is direct proof `===` is not constant time in this PHP buil

Affected package ranges reported by GitHub: getgrav/grav: <= 2.0.15

Scores and how they were measured

CVSS base severity

3.7

Version 3.1As of

CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N

Measurement source (opens a new tab)

How bad it is if exploited. Not a prediction of exploitation or proof you are exposed.

EPSS probability

0.2%

Scored Percentile 11.9%

Model and measurement source (opens a new tab)

A model's estimate of exploitation in the next 30 days, not certainty or your own risk.

Related technology

Sources

  1. Primary source

    GitHub-reviewed advisory GHSA-38p6-h87p-r4cg (opens a new tab)

    The public advisory supplies the description, affected package/version ranges, and any listed CVSS metric. GitHub review is not local review.

    Published Checked

  2. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-38p6-h87p-r4cg, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  3. Primary source

    FIRST measured EPSS score (opens a new tab)

    Measured EPSS probability and percentile for CVE-2026-72701, scored 2026-10-26. This is a model estimate, not proof of exposure.

    Published Checked

Cite this record

For factual claims, cite the primary sources above; cite KEVWire for its compilation, dates, and classifications.

APA

KEVWire. (2026, October 26). Grav: Non constant time nonce comparison in Utils::verifyNonce() used for CSRF protection (CVE-2026-72701). https://kevwire.com/record/CVE-2026-72701

MLA

"Grav: Non constant time nonce comparison in Utils::verifyNonce() used for CSRF protection (CVE-2026-72701)." KEVWire, 26 Oct. 2026, kevwire.com/record/CVE-2026-72701.

Chicago

KEVWire. "Grav: Non constant time nonce comparison in Utils::verifyNonce() used for CSRF protection (CVE-2026-72701)." Last modified October 26, 2026. https://kevwire.com/record/CVE-2026-72701.

Evidence summary (plain text)

Grav: Non constant time nonce comparison in Utils::verifyNonce() used for CSRF protection (Sep 17, 2026).
`Grav\Common\Utils::verifyNonce()`, the core function Grav and its plugins use to validate CSRF nonces, compares the submitted nonce to the expected value with PHP's `===` operator instead of `hash_equals()`. `===` on strings short circuits at the first differing byte, so the comparison time leaks how many leading bytes of a guess are correct. This is CWE-208, Observable Timing Discrepancy.

The codebase already knows to avoid this pattern. `hash_equals()` is used for the equivalent purpose in four other places I found: `system/src/Grav/Common/Session.php`, `system/src/Grav/Framework/Cache/Adapter/FileCache.php`, `system/src/Grav/Common/Scheduler/Scheduler.php` (the webhook token check), and `system/src/Grav/Common/Scheduler/JobQueue.php`. `Utils::verifyNonce()` is the one place I found that still uses a plain equality check for a secret comparison.
CVSS 3.7 (version 3.1; evidence as of Oct 26, 2026; https://github.com/advisories/GHSA-38p6-h87p-r4cg).
EPSS 0.2% for the following 30 days, scored Oct 26, 2026 (https://api.first.org/data/v1/epss?cve=CVE-2026-72701&date=2026-10-26).
Sources:
GitHub-reviewed advisory GHSA-38p6-h87p-r4cg: https://github.com/advisories/GHSA-38p6-h87p-r4cg
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc
FIRST measured EPSS score: https://api.first.org/data/v1/epss?cve=CVE-2026-72701&date=2026-10-26
Record ID: CVE-2026-72701.
Record origin: Ingested record.
AI evidence: Not assessed.
Known exploitation: Not assessed.
Zero-day status: Not assessed. KEV membership alone does not establish zero-day status.
About this data

This record was ingested from a data source. Retrieved: Oct 27, 2026, 01:40 UTC. Ingestion is not an independent editorial investigation.

A CISA KEV listing documents known exploitation, not necessarily a zero-day; absence from the catalog does not prove a vulnerability has never been exploited.

Technology tags indicate possible relevance, not verified exposure. Confirm the affected product, version, and configuration against your own inventory; KEVWire does not scan your systems.

A personal reading checklist on this device, not verified remediation.

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