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.

Affected: Packagist: getgrav/grav

### Summary The fix for SSTI using `|map`, `|filter` and `|reduce` twigs implemented in the commit [71bbed1](https://github.com/getgrav/grav/commit/71bbed12f950de8335006d7f91112263d8504f1b) 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`.

```php ... 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` ```php /** * @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 reache…