How severe is CVE-2026-91981?

No CVSS score has been assigned to CVE-2026-91981 yet. Review the advisory details and affected package list to assess your exposure.

Which packages are affected by CVE-2026-91981?

CVE-2026-91981 affects the following packages: code.vikunja.io/api (Go). Ecosystems affected: Go.

How do I fix CVE-2026-91981?

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-2026-91981 is resolved across your whole dependency graph.

How do I detect CVE-2026-91981 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-2026-91981 if there is no patch (or I can't update yet)?

Assume what was exposed is already known: rotate any credential, token or key that the affected component could return, restrict the endpoint to callers that genuinely need it, and strip sensitive fields from responses and error output at the boundary rather than relying on the client not to read them.

Is CVE-2026-91981 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-91981 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-2026-91981?

CVE-2026-91981 has an EPSS (Exploit Prediction Scoring System) score of 0.3%, placing it in the 22th 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-2026-91981?

CVE-2026-91981 is classified as Information Exposure (CWE-200). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2026-91981 published, and has it been updated?

CVE-2026-91981 was published on September 15, 2026 and was last updated on October 21, 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-2026-91981 — api

Fix: go-vikunja/vikunja#3688

CVE-2026-91981 is a Information Exposure 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 User Enumeration via v2 API

Also known asGHSA-vfxw-3x8p-2vjrGO-2026-6752

Published

Sep 15, 2026

Updated

Oct 21, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Oct 22, 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.

Exploitation and automatability from CISA’s SSVC triage for CVE-2026-91981.

EPSS Exploitation Probability

via FIRST.org ↗

0.3%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs22th percentile — riskier than 22% 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

A link-share token, the credential you hand out to let someone view a shared project, can call the v2 user-search endpoints, which it was never meant to reach. The v1 versions of the same endpoints correctly reject link-share tokens; the v2 versions don't. As a result, anyone with a share link can list every member of the shared project (and its parent projects), and can test whether any username exists anywhere on the instance.

Details

Two endpoints are affected:

GET /api/v2/projects/{id}/users/search only checks project.CanRead, which a link share satisfies on its own project. With no parameter it returns the full user list, walking up the parent-project chain. GET /api/v2/users?q=<name> is a global search. It needs a query term, but any term works as an existence oracle: a hit confirms the username exists somewhere on the instance and returns the display name.

Emails are correctly stripped from these responses, so the exposure is limited to identities (usernames and display names), but that's still an instance-wide directory that a share-link holder should never see.

The fix is to resolve the caller with GetFromAuth in both handlers, so link-share tokens are rejected the same way v1 already rejects them.

PoC

Get a link-share token. With any account that owns a project, create a read-only link share and note the hash, then exchange it:

POST /api/v1/shares/<hash>/auth
{"password":""}

The response contains a token, a link-share JWT.

Confirm v1 blocks it (the fixed behavior). With that token:

GET /api/v1/projects/<id>/projectusers
Authorization: Bearer <link_share_token>

→ rejected: the token isn't a user token.

Now the vulnerable v2 endpoints, same token:

GET /api/v2/projects/<id>/users/search
Authorization: Bearer <link_share_token>

→ 200, full list of everyone with access to the project and its parents (id, username, name).

GET /api/v2/users?q=admin
Authorization: Bearer <link_share_token>

→ 200, a non-empty result confirms the username exists on the instance. Iterate q over a wordlist to enumerate accounts.

Impact

Information disclosure. A link share is a low-trust credential meant to expose one project's contents; here it also exposes the membership of that project's parents and lets the holder probe for any username across the whole instance. Since share links are often forwarded or semi-public, this hands an untrusted recipient a map of internal users and a username-validation oracle useful for phishing and credential stuffing. No account is required beyond possessing a share link. Emails are not exposed.

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-2026-91981 is resolved across your whole dependency graph.
Workarounds
Assume what was exposed is already known: rotate any credential, token or key that the affected component could return, restrict the endpoint to callers that genuinely need it, and strip sensitive fields from responses and error output at the boundary rather than relying on the client not to read them.

Frequently Asked Questions

### Summary A link-share token, the credential you hand out to let someone view a shared project, can call the v2 user-search endpoints, which it was never meant to reach. The v1 versions of the same endpoints correctly reject link-share tokens; the v2 versions don't. As a result, anyone with a share link can list every member of the shared project (and its parent projects), and can test whether any username exists anywhere on the instance. ### Details Two endpoints are affected: `GET /api/v2/projects/{id}/users/search` only checks project.CanRead, which a link share satisfies on its own pro

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