CVE-2026-91973

Vikunja before 2.6.0 contains an authentication bypass vulnerability in CalDAV BasicAuth endpoints that lack rate limiting protection. Remote unauthenticated attackers can issue unbounded credential-guessing requests against /dav, /.well-known, and /feeds routes to bypass the instance's anti-brute-force controls and compromise password-only accounts.

Vikunja before 2.6.0 contains an authentication bypass vulnerability in CalDAV BasicAuth endpoints that lack rate limiting protection. Remote unauthenticated attackers can issue unbounded credential-guessing requests against /dav, /.well-known, and /feeds routes to bypass the instance's anti-brute-force controls and compromise password-only accounts.

Duplicate Advisory

This advisory has been withdrawn because it is a duplicate of GHSA-m469-88xx-8rx2. This link is maintained to preserve external references.

Original Description

Vikunja before 2.6.0 contains an authentication bypass vulnerability in CalDAV BasicAuth endpoints that lack rate limiting protection. Remote unauthenticated attackers can issue unbounded credential-guessing requests against /dav, /.well-known, and /feeds routes to bypass the instance's anti-brute-force controls and compromise password-only accounts.

Summary

The /dav, /.well-known, and /feeds groups are registered on the root Echo instance with only BasicAuth and no rate limiter. CalDAV BasicAuth accepts the plain account password, so password guessing over /dav is unbounded and never returns 429, while /api/v1/login is throttled from the tenth attempt. The only anti-brute-force control on the instance is therefore bypassable.

Details

pkg/routes/routes.go (~lines 238-249) registers /.well-known, /dav, and /feeds with middleware.BasicAuth(...) and nothing else; registerCalDavRoutes adds no limiter. pkg/routes/caldav/auth.go (~lines 88-93) falls through to user.CheckUserCredentials with the plain account password when no CalDAV token matches. In contrast, /register, /login, etc. are wrapped by unauthRateLimit() — an unconditional 10/min/IP pre-auth floor that ignores ratelimit.enabled (default false).

TOTP-enabled accounts and bot users are correctly refused, so this targets password-only accounts.

PoC (verified at runtime against v2.5.0)

POST /api/v1/login  x25 wrong passwords  -> 429 from attempt 2 (throttled)
PROPFIND /dav/principals/{user}/  x60 wrong passwords  -> 401 x60, 429 x0
GET /feeds/notifications.atom  x30 wrong passwords  -> 401 x30, 429 x0
PROPFIND /dav/... with correct password -> 207 (proves the 401s are real auth failures)

Impact

The anti-brute-force floor guarding /login is entirely absent on /dav, /feeds, and /.well-known, giving an unbounded credential-guessing surface against account passwords. (bcrypt caps throughput to a few guesses/second, but nothing caps the number of attempts.) Reported as an authentication-control bypass, not a DoS.

Fix

Apply the unconditional pre-auth rate-limit floor to the /dav, /.well-known, and /feeds groups.

Vikunja before 2.6.0 contains an authentication bypass vulnerability in CalDAV BasicAuth endpoints that lack rate limiting protection. Remote unauthenticated attackers can issue unbounded credential-guessing requests against /dav, /.well-known, and /feeds routes to bypass the instance's anti-brute-force controls and compromise password-only accounts.

Vikunja before 2.6.0 contains an authentication bypass vulnerability in CalDAV BasicAuth endpoints that lack rate limiting protection. Remote unauthenticated attackers can issue unbounded credential-guessing requests against /dav, /.well-known, and /feeds routes to bypass the instance's anti-brute-force controls and compromise password-only accounts.

Duplicate Advisory

This advisory has been withdrawn because it is a duplicate of GHSA-m469-88xx-8rx2. This link is maintained to preserve external references.

Original Description

Vikunja before 2.6.0 contains an authentication bypass vulnerability in CalDAV BasicAuth endpoints that lack rate limiting protection. Remote unauthenticated attackers can issue unbounded credential-guessing requests against /dav, /.well-known, and /feeds routes to bypass the instance's anti-brute-force controls and compromise password-only accounts.

Summary

The /dav, /.well-known, and /feeds groups are registered on the root Echo instance with only BasicAuth and no rate limiter. CalDAV BasicAuth accepts the plain account password, so password guessing over /dav is unbounded and never returns 429, while /api/v1/login is throttled from the tenth attempt. The only anti-brute-force control on the instance is therefore bypassable.

Details

pkg/routes/routes.go (~lines 238-249) registers /.well-known, /dav, and /feeds with middleware.BasicAuth(...) and nothing else; registerCalDavRoutes adds no limiter. pkg/routes/caldav/auth.go (~lines 88-93) falls through to user.CheckUserCredentials with the plain account password when no CalDAV token matches. In contrast, /register, /login, etc. are wrapped by unauthRateLimit() — an unconditional 10/min/IP pre-auth floor that ignores ratelimit.enabled (default false).

TOTP-enabled accounts and bot users are correctly refused, so this targets password-only accounts.

PoC (verified at runtime against v2.5.0)

POST /api/v1/login  x25 wrong passwords  -> 429 from attempt 2 (throttled)
PROPFIND /dav/principals/{user}/  x60 wrong passwords  -> 401 x60, 429 x0
GET /feeds/notifications.atom  x30 wrong passwords  -> 401 x30, 429 x0
PROPFIND /dav/... with correct password -> 207 (proves the 401s are real auth failures)

Impact

The anti-brute-force floor guarding /login is entirely absent on /dav, /feeds, and /.well-known, giving an unbounded credential-guessing surface against account passwords. (bcrypt caps throughput to a few guesses/second, but nothing caps the number of attempts.) Reported as an authentication-control bypass, not a DoS.

Fix

Apply the unconditional pre-auth rate-limit floor to the /dav, /.well-known, and /feeds groups.

Vikunja: CalDAV and feeds BasicAuth endpoints have no rate limit, bypassing the anti-brute-force floor on account passwords in code.vikunja.io/api.

NOTE: The source advisory for this report contains additional versions that could not be automatically mapped to standard Go module versions.

(If this is causing false-positive reports from vulnerability scanners, please suggest an edit to the report.)

The additional affected modules and versions are: code.vikunja.io/api before v2.6.0.