Vikunja: Every /api/v2 pre-auth endpoint is unthrottled on a stock install while its /api/v1 twin is rate limited

Why it is Amber

• CVSS 8.7 high
• Remotely exploitable without logging in
• On your watchlist: Go

Affected packages

References

• https://github.com/advisories/GHSA-6rvj-qwjf-3m4q
• https://github.com/go-vikunja/vikunja/security/advisories/GHSA-6rvj-qwjf-3m4q
• https://nvd.nist.gov/vuln/detail/CVE-2026-91972
• https://github.com/go-vikunja/vikunja/releases/tag/v2.6.0
• https://www.vulncheck.com/advisories/vikunja-before-2.6.0-authentication-bypass-via-unthrottled-api
• https://github.com/advisories/GHSA-6rvj-qwjf-3m4q

Original description

### Summary `registerAPIRoutesV2` never applies the unconditional pre-auth rate-limit floor (`unauthRateLimit()`) to the v2 public routes — it passes that limiter only to `/api/v2/ws` — and otherwise relies on `setupRateLimit`, which registers nothing when `ratelimit.enabled` is false (the default). So on a stock install every v2 pre-auth endpoint (login, register, password-reset token, oauth token) is unthrottled, while its v1 twin is throttled. ### Details `unauthRateLimit()` -> `perMinuteIPRateLimit("noauth", RateLimitNoAuthRoutesLimit)` (`pkg/routes/rate_limit.go`, ~lines 100-118) is an unconditional per-IP floor (default 10/60s) that deliberately ignores `RateLimitEnabled`, which is why v1's pre-auth routes are throttled even with the global limiter off. v1 applies it: `ur := a.Group(""); ur.Use(unauthRateLimit())` (`pkg/routes/routes.go` ~line 459). `registerAPIRoutesV2` (~lines 405-431) passes the `unauthRateLimit()` instance only to `/api/v2/ws`; its auth routes get only `setupRateLimit(a, ...)`, which is config-gated and registers nothing by default. ### PoC (verified at runtime against v2.5.0) ``` POST /api/v1/login x25 -> 429 from attempt 5 POST /api/v2/login x25 -> 403 x25, 429 x0 POST /api/v1/user/password/token -> 429 (throttled) POST /api/v2/user/password/token x20 -> 404 x20, 429 x0 ``` Request bodies are byte-identical across versions (shared `user.Login` / `user.PasswordTokenRequest`). Both v2 endpoints reach their handlers (403/404, not route-404), so the comparison is valid. ### Impact The pre-auth rate-limit floor — the instance's only default anti-brute-force / anti-abuse control — is absent on all v2 public endpoints. Enables unbounded credential guessing, account-enumeration probing, and password-reset flooding on a default install. Reported as an authentication-control bypass, not a DoS. ### Fix Apply `unauthRateLimit()` to the v2 public route group, matching v1.