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CVE-2026-91973 – Authentication Bypass via Unrestricted Brute Force – Vikunja before 2.6.0

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Summary

CVE-2026-91973 is an authentication bypass vulnerability in Vikunja, the open-source task management platform, caused by improper restriction of excessive authentication attempts (CWE-307). All versions of Vikunja before 2.6.0 expose CalDAV and feed endpoints that accept BasicAuth credentials without any rate limiting, allowing unauthenticated remote attackers to brute-force account passwords. The flaw carries a CVSS v4.0 base score of 8.7 (High).

Technical details

  • Root cause: the /dav, /.well-known, and /feeds endpoint groups are registered on the root Echo web server instance with only BasicAuth enforcement and no attached rate limiter.
  • Vikunja’s primary login flow (/api/v1/login and /register) enforces a pre-authentication throttle of 10 attempts per minute per IP, but this control is not applied to the CalDAV/feeds routes.
  • The CalDAV authentication handler falls through to plain password validation when no valid token is presented, meaning full password guessing is possible against these routes.
  • Attack vector: network, no authentication or user interaction required, low attack complexity — an attacker only needs to send repeated BasicAuth requests to the affected endpoints.
  • Impact: unbounded credential-guessing enables compromise of password-only accounts (confidentiality impact); TOTP-enabled and bot accounts are not affected by this specific bypass.

Affected software

  • Vikunja (go-vikunja/vikunja) versions 2.5.0 and earlier, and all versions prior to 2.6.0.

Severity

  • CVSS v4.0 Base Score: 8.7 (High) — CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N
  • CVSS v3.1 Base Score: 7.5 (High) — CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N

Mitigation and recommended actions

  • Immediate: Upgrade Vikunja to version 2.6.0 or later, which applies the unconditional pre-authentication rate-limit floor to the /dav, /.well-known, and /feeds endpoint groups.
  • If unable to patch immediately: enforce strong, unique passwords for all accounts, enable two-factor authentication (TOTP) where possible since TOTP-enabled accounts are not exposed to this bypass, and consider placing a reverse proxy or WAF in front of the instance to apply independent rate limiting to CalDAV and feed endpoints until the upgrade can be completed.

References

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