Server-side Template Injection (SSTI) in grav

Vulnerability Description

Grav is a file-based Web-platform built in PHP. Grav is subject to a server side template injection (SSTI) vulnerability. The fix for another SSTI vulnerability 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 (`\\`). The `isDangerousFunction()` check in version 1.7.42 and onwards retuns `false` value instead of `true` when the `\` symbol is found in the `$name`. This vulnerability can be exploited if the attacker has access to: 1. an Administrator account, or 2. a non-administrator, user account that has Admin panel access and Create/Update page permissions. A fix for this vulnerability has been introduced in commit `b4c6210` and is included in release version `1.7.42.2`. Users are advised to upgrade. There are no known workarounds for this vulnerability.

References

• https://github.com/getgrav/grav/security/advisories/GHSA-9436-3gmp-4f53
• https://github.com/getgrav/grav/commit/71bbed12f950de8335006d7f91112263d8504f1b
• https://github.com/getgrav/grav/commit/b4c62101a43051fc7f5349c7d0a5b6085375c1d7