Grav: Authenticated ReDoS via regex_replace in Twig Sandbox
The advisory, in context
Summary
The regex_replace filter and function are allowlisted in Grav's Twig content sandbox. When Twig processing in page content is enabled security.twig_content.process_enabled: true, authenticated page editors can supply a catastrophically backtracking PCRE pattern, causing unbounded CPU consumption and denying service to the entire web server process.
Details
The Twig sandbox allowlists, defined in system/config/security.yaml, explicitly include regex_replace in both the filter and function permission lists:
Source: system/config/security.yaml
twig_sandbox:
allowed_filters:
# ...
- regex_replace # user-controlled pattern allowed in sandbox
allowed_functions:
# ...
- regex_replace # same
The underlying implementation passes the caller-controlled $pattern directly into PHP's preg_replace() without any pattern complexity validation:
Source: system/src/Grav/Common/Twig/Extension/GravExtension.php:1317-1319
public function regexReplace($subject, $pattern, $replace, $limit = -1)
{
return preg_replace($pattern, $replace, $subject, $limit);
}
When twig_content.process_enabled is true, page body content is sandboxed but can use any allowlisted filter. An editor who embeds a catastrophic backtracking pattern causes the PCRE engine to enter exponential time complexity, consuming 100% CPU until the PHP process is killed or the request times out.
Conditions required:
security.twig_content.process_enabled: true(opt-in,falseby default on fresh 2.0 installs)security.twig_sandbox.enabled: true(default) - the function is reachable under sandbox- Attacker must have page edit access (authenticated contributor / editor role)
PoC
Configuration prerequisite - enable Twig in content:
# user/config/security.yaml
twig_content:
process_enabled: true
Payload — embed in any Grav page body with process: { twig: true } in frontmatter:
---
title: Test
process:
twig: true
---
{{ 'aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaab'|regex_replace('/^(a+)+$/', '') }}
Or as a function call in a page where the editor has Twig access:
{{ regex_replace('aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaab', '/^(a+)+$/', '') }}
Result: The PHP-FPM worker (or CLI server process) enters catastrophic PCRE backtracking. On a 2 GHz host, a 32-character string with the above pattern will exhaust one CPU core for seconds to minutes. With a slightly longer string, the time grows exponentially.
Impact
Vulnerability type: Regular Expression Denial of Service - ReDoS
Who is impacted: Server availability. Any Grav installation where:
- An editor-role account exists (or has been compromised), AND
- The operator has enabled
twig_content.process_enabled: true
An attacker with page-edit access can render the site unresponsive for all visitors by publishing a page with a catastrophic regex. On single-worker PHP configurations this is a complete outage. On multi-worker setups, multiple concurrent page renders of the malicious page can saturate all workers.
Affected packages
Start with the recorded fix for your release branch. Check your installed version and test compatibility before upgrading. Fixing this advisory does not rule out other vulnerabilities in the release.
Earlier releases before 2.0.4
Fixed in 2.0.4
338 explicitly affected versions
0.8.0, 0.9.0, 0.9.1, 0.9.10, 0.9.11, 0.9.12, 0.9.13, 0.9.14, 0.9.15, 0.9.16, 0.9.17, 0.9.18, 0.9.19, 0.9.2, 0.9.20, 0.9.21, 0.9.22, 0.9.23, 0.9.24, 0.9.25, 0.9.26, 0.9.27, 0.9.28, 0.9.29, 0.9.3, 0.9.30, 0.9.31, 0.9.32, 0.9.33, 0.9.34, 0.9.35, 0.9.36, 0.9.37, 0.9.38, 0.9.39, 0.9.4, 0.9.40, 0.9.41, 0.9.42, 0.9.43, 0.9.44, 0.9.45, 0.9.5, 0.9.6, 0.9.7, 0.9.8, 0.9.9, 1.0.0, 1.0.0-rc.1, 1.0.0-rc.2, 1.0.0-rc.3, 1.0.0-rc.4, 1.0.0-rc.5, 1.0.0-rc.6, 1.0.1, 1.0.10, 1.0.2, 1.0.3, 1.0.4, 1.0.5, 1.0.6, 1.0.7, 1.0.8, 1.0.9, 1.1.0, 1.1.0-beta.1, 1.1.0-beta.2, 1.1.0-beta.3, 1.1.0-beta.4, 1.1.0-beta.5, 1.1.0-rc.1, 1.1.0-rc.2, 1.1.0-rc.3, 1.1.1, 1.1.10, 1.1.11, 1.1.12, 1.1.13, 1.1.14, 1.1.15, 1.1.16, 1.1.17, 1.1.2, 1.1.3, 1.1.4, 1.1.5, 1.1.6, 1.1.7, 1.1.8, 1.1.9, 1.1.9-rc.1, 1.1.9-rc.2, 1.1.9-rc.3, 1.2.0, 1.2.0-rc.1, 1.2.0-rc.2, 1.2.0-rc.3, 1.2.1, 1.2.2, 1.2.3, 1.2.4, 1.3.0, 1.3.0-rc.1, 1.3.0-rc.2, 1.3.0-rc.3, 1.3.0-rc.4, 1.3.0-rc.5, 1.3.1, 1.3.10, 1.3.2, 1.3.3, 1.3.4, 1.3.5, 1.3.6, 1.3.7, 1.3.8, 1.3.9, 1.4.0, 1.4.0-beta.1, 1.4.0-beta.2, 1.4.0-beta.3, 1.4.0-rc.1, 1.4.0-rc.2, 1.4.1, 1.4.2, 1.4.3, 1.4.4, 1.4.5, 1.4.6, 1.4.7, 1.4.8, 1.5.0, 1.5.0-beta.1, 1.5.0-beta.2, 1.5.0-rc.1, 1.5.1, 1.5.10, 1.5.2, 1.5.3, 1.5.4, 1.5.5, 1.5.6, 1.5.7, 1.5.8, 1.5.9, 1.6.0, 1.6.0-beta.1, 1.6.0-beta.2, 1.6.0-beta.3, 1.6.0-beta.4, 1.6.0-beta.5, 1.6.0-beta.6, 1.6.0-beta.7, 1.6.0-beta.8, 1.6.0-rc.1, 1.6.0-rc.2, 1.6.0-rc.3, 1.6.0-rc.4, 1.6.1, 1.6.10, 1.6.11, 1.6.12, 1.6.13, 1.6.14, 1.6.15, 1.6.16, 1.6.17, 1.6.18, 1.6.19, 1.6.2, 1.6.20, 1.6.21, 1.6.22, 1.6.23, 1.6.24, 1.6.25, 1.6.26, 1.6.27, 1.6.28, 1.6.29, 1.6.3, 1.6.30, 1.6.31, 1.6.4, 1.6.5, 1.6.6, 1.6.7, 1.6.8, 1.6.9, 1.7.0, 1.7.0-beta.1, 1.7.0-beta.10, 1.7.0-beta.2, 1.7.0-beta.3, 1.7.0-beta.4, 1.7.0-beta.5, 1.7.0-beta.6, 1.7.0-beta.7, 1.7.0-beta.8, 1.7.0-beta.9, 1.7.0-rc.1, 1.7.0-rc.10, 1.7.0-rc.11, 1.7.0-rc.12, 1.7.0-rc.13, 1.7.0-rc.14, 1.7.0-rc.15, 1.7.0-rc.16, 1.7.0-rc.17, 1.7.0-rc.18, 1.7.0-rc.19, 1.7.0-rc.2, 1.7.0-rc.20, 1.7.0-rc.3, 1.7.0-rc.4, 1.7.0-rc.5, 1.7.0-rc.6, 1.7.0-rc.7, 1.7.0-rc.8, 1.7.0-rc.9, 1.7.1, 1.7.10, 1.7.12, 1.7.13, 1.7.14, 1.7.15, 1.7.16, 1.7.17, 1.7.18, 1.7.19, 1.7.20, 1.7.21, 1.7.22, 1.7.23, 1.7.24, 1.7.25, 1.7.26, 1.7.26.1, 1.7.27, 1.7.27.1, 1.7.28, 1.7.29, 1.7.29.1, 1.7.3, 1.7.30, 1.7.31, 1.7.32, 1.7.33, 1.7.34, 1.7.35, 1.7.36, 1.7.37, 1.7.37.1, 1.7.38, 1.7.39, 1.7.39.1, 1.7.39.2, 1.7.39.3, 1.7.39.4, 1.7.4, 1.7.40, 1.7.41, 1.7.41.1, 1.7.41.2, 1.7.42, 1.7.42.1, 1.7.42.2, 1.7.42.3, 1.7.43, 1.7.44, 1.7.45, 1.7.46, 1.7.47, 1.7.48, 1.7.49, 1.7.49.1, 1.7.49.2, 1.7.49.3, 1.7.49.4, 1.7.49.5, 1.7.5, 1.7.51, 1.7.52, 1.7.53, 1.7.53.1, 1.7.53.2, 1.7.53.3, 1.7.6, 1.7.7, 1.7.8, 1.7.9, 1.8.0-beta.1, 1.8.0-beta.10, 1.8.0-beta.11, 1.8.0-beta.12, 1.8.0-beta.13, 1.8.0-beta.14, 1.8.0-beta.15, 1.8.0-beta.16, 1.8.0-beta.17, 1.8.0-beta.18, 1.8.0-beta.19, 1.8.0-beta.2, 1.8.0-beta.20, 1.8.0-beta.21, 1.8.0-beta.22, 1.8.0-beta.23, 1.8.0-beta.24, 1.8.0-beta.25, 1.8.0-beta.26, 1.8.0-beta.27, 1.8.0-beta.28, 1.8.0-beta.29, 1.8.0-beta.3, 1.8.0-beta.4, 1.8.0-beta.5, 1.8.0-beta.6, 1.8.0-beta.7, 1.8.0-beta.8, 1.8.0-beta.9, 2.0.0, 2.0.0-beta.1, 2.0.0-beta.2, 2.0.0-beta.3, 2.0.0-beta.4, 2.0.0-rc.1, 2.0.0-rc.10, 2.0.0-rc.2, 2.0.0-rc.3, 2.0.0-rc.4, 2.0.0-rc.5, 2.0.0-rc.6, 2.0.0-rc.7, 2.0.0-rc.8, 2.0.0-rc.9, 2.0.1, 2.0.2, 2.0.3
View affected-version evidence
{
"ranges": [
{
"events": [
{
"introduced": "0"
},
{
"fixed": "2.0.4"
}
],
"type": "ECOSYSTEM"
}
],
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"0.9.0",
"0.9.1",
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}