How severe is CVE-2026-91973?

No CVSS score has been assigned to CVE-2026-91973 yet. Review the advisory details and affected package list to assess your exposure.

Which packages are affected by CVE-2026-91973?

CVE-2026-91973 affects the following packages: code.vikunja.io/api (Go). Ecosystems affected: Go.

How do I fix CVE-2026-91973?

Update code.vikunja.io/api to 2.6.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-91973 is resolved across your whole dependency graph.

How do I detect CVE-2026-91973 in my Go dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for code.vikunja.io/api, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2026-91973 if there is no patch (or I can't update yet)?

Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.

Is CVE-2026-91973 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-91973 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2026-91973?

CVE-2026-91973 has an EPSS (Exploit Prediction Scoring System) score of 0.7%, placing it in the 51th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2026-91973?

CVE-2026-91973 is classified as CWE-307 (CWE-307). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2026-91973 published, and has it been updated?

CVE-2026-91973 was published on September 15, 2026 and was last updated on September 18, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🐹 Go

Not in CISA KEV

CVE-2026-91973 — api

Fix: go-vikunja/vikunja#3688

CVE-2026-91973 is a CWE-307 vulnerability in code.vikunja.io/api. A fix is available for code.vikunja.io/api — see the affected versions and patch details below.

Vikunja before 2.6.0 Authentication Bypass via CalDAV BasicAuth

Also known asGHSA-m469-88xx-8rx2

Published

Sep 15, 2026

Updated

Sep 18, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Oct 8, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

Proof-of-concept exploit code exists

CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.
CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.

Exploitation and automatability from CISA’s SSVC triage for CVE-2026-91973.

EPSS Exploitation Probability

via FIRST.org ↗

0.7%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs51th percentile — riskier than 51% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

Real-World Exposure

1 pkg affected

🐹code.vikunja.io/api

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Go packages — download data is not available via public APIs for these ecosystems.

Description

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.

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
🐹Go	`code.vikunja.io/api`	all versions	2.6.0`go get code.vikunja.io/api@v2.6.0`

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for code.vikunja.io/api, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update code.vikunja.io/api to 2.6.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-91973 is resolved across your whole dependency graph.
Workarounds
Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.

Frequently Asked Questions

### 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

O3 Security · Impact-Aware SCA

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