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Security Advisories: GSA_kwCzR0hTQS05NDM2LTNnbXAtNGY1M84AA0zw
grav Server-side Template Injection (SSTI) mitigation bypass
Summary
The fix for SSTI using |map
, |filter
and |reduce
twigs implemented in the commit 71bbed1 introduces bypass of the denylist due to incorrect return value from isDangerousFunction()
, which allows to execute the payload prepending double backslash (\\
)
Details
The isDangerousFunction()
check in version 1.7.42 and onwards retuns false
value instead of true
when the \
symbol is found in the $name
.
...
if (strpos($name, "\\") !== false) {
return false;
}
if (in_array($name, $commandExecutionFunctions)) {
return true;
}
...
Based on the code where the function is used, it is expected that any dangerous condition would return true
/**
* @param Environment $env
* @param array $array
* @param callable|string $arrow
* @return array|CallbackFilterIterator
* @throws RuntimeError
*/
function mapFunc(Environment $env, $array, $arrow)
{
if (!$arrow instanceof \Closure && !is_string($arrow) || Utils::isDangerousFunction($arrow)) {
throw new RuntimeError('Twig |map("' . $arrow . '") is not allowed.');
}
when |map('\system')
is used in the malicious payload, the single backslash is dropped prior to reaching strpos($name, '\\')
check, thus $name
variable already has no backslash, and the command is blacklisted because it reaches the if (in_array($name, $commandExecutionFunctions)) {
validation step.
However if |map('\\system')
is used (i.e. double backslash), then the strpos($name, "\\") !== false
takes effect, and isDangerousFunction()
returns false
, in which case the RuntimeError
is not generated, and blacklist is bypassed leading to code execution.
Exploit Conditions
This vulnerability can be exploited if the attacker has access to:
- an Administrator account, or
- a non-administrator, user account that has Admin panel access and Create/Update page permissions
Steps to reproduce
- Log in to Grav Admin using an administrator account.
- Navigate to
Accounts > Add
, and ensure that the following permissions are assigned when creating a new low-privileged user:- Login to Admin - Allowed
- Page Update - Allowed
- Log out of Grav Admin
- Login using the account created in step 2.
- Choose
Pages -> Home
- Click the
Advanced
tab and select the checkbox besideTwig
to ensure that Twig processing is enabled for the modified webpage. - Under the
Content
tab, insert the following payload within the editor:
{{ ['id'] | map('\\system') | join() }}
- Click the
Preview
button. Observe that the output of the id shell command is returned in the preview.
Mitigation
diff --git a/system/src/Grav/Common/Utils.php b/system/src/Grav/Common/Utils.php
index 2f121bbe3..7b267cd0f 100644
--- a/system/src/Grav/Common/Utils.php
+++ b/system/src/Grav/Common/Utils.php
@@ -2069,7 +2069,7 @@ abstract class Utils
}
if (strpos($name, "\\") !== false) {
- return false;
+ return true;
}
if (in_array($name, $commandExecutionFunctions)) {
Permalink: https://github.com/advisories/GHSA-9436-3gmp-4f53JSON: https://advisories.ecosyste.ms/api/v1/advisories/GSA_kwCzR0hTQS05NDM2LTNnbXAtNGY1M84AA0zw
Source: GitHub Advisory Database
Origin: Unspecified
Severity: High
Classification: General
Published: about 1 year ago
Updated: 10 months ago
CVSS Score: 7.2
CVSS vector: CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:U/C:H/I:H/A:H
Identifiers: GHSA-9436-3gmp-4f53, CVE-2023-37897
References:
- https://github.com/getgrav/grav/security/advisories/GHSA-9436-3gmp-4f53
- https://nvd.nist.gov/vuln/detail/CVE-2023-37897
- https://github.com/getgrav/grav/commit/71bbed12f950de8335006d7f91112263d8504f1b
- https://github.com/getgrav/grav/commit/b4c62101a43051fc7f5349c7d0a5b6085375c1d7
- https://github.com/advisories/GHSA-9436-3gmp-4f53
Blast Radius: 6.9
Affected Packages
packagist:getgrav/grav
Dependent packages: 2Dependent repositories: 9
Downloads: 73,876 total
Affected Version Ranges: <= 1.7.42.1
Fixed in: 1.7.42.2
All affected versions: 0.8.0, 0.9.0, 0.9.1, 0.9.2, 0.9.3, 0.9.4, 0.9.5, 0.9.6, 0.9.7, 0.9.8, 0.9.9, 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, 0.9.20, 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, 0.9.30, 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, 0.9.40, 0.9.41, 0.9.42, 0.9.43, 0.9.44, 0.9.45, 1.0.0, 1.0.1, 1.0.2, 1.0.3, 1.0.4, 1.0.5, 1.0.6, 1.0.7, 1.0.8, 1.0.9, 1.0.10, 1.1.0, 1.1.1, 1.1.2, 1.1.3, 1.1.4, 1.1.5, 1.1.6, 1.1.7, 1.1.8, 1.1.9, 1.1.10, 1.1.11, 1.1.12, 1.1.13, 1.1.14, 1.1.15, 1.1.16, 1.1.17, 1.2.0, 1.2.1, 1.2.2, 1.2.3, 1.2.4, 1.3.0, 1.3.1, 1.3.2, 1.3.3, 1.3.4, 1.3.5, 1.3.6, 1.3.7, 1.3.8, 1.3.9, 1.3.10, 1.4.0, 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, 1.5.1, 1.5.2, 1.5.3, 1.5.4, 1.5.5, 1.5.6, 1.5.7, 1.5.8, 1.5.9, 1.5.10, 1.6.0, 1.6.1, 1.6.2, 1.6.3, 1.6.4, 1.6.5, 1.6.6, 1.6.7, 1.6.8, 1.6.9, 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, 1.6.20, 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, 1.6.30, 1.6.31, 1.7.0, 1.7.1, 1.7.3, 1.7.4-1.1, 1.7.4-1.2, 1.7.4-2.1
All unaffected versions: 1.7.4, 1.7.5, 1.7.6, 1.7.7, 1.7.8, 1.7.9, 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, 1.7.27, 1.7.28, 1.7.29, 1.7.30, 1.7.31, 1.7.32, 1.7.33, 1.7.34, 1.7.35, 1.7.36, 1.7.37, 1.7.38, 1.7.39, 1.7.40, 1.7.41, 1.7.42, 1.7.43, 1.7.44, 1.7.45, 1.7.46