CVE-2026-40103 is a medium-severity (CVSS 4.3) CWE-836 vulnerability in code.vikunja.io/api. A fix is available for code.vikunja.io/api — see the affected versions and patch details below.
Vikunja's Scoped API tokens with projects.background permission can delete project backgrounds
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-2026-40103.
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
CVE-2026-40103 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 377,636 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
Real-World Exposure
code.vikunja.io/apiReal-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
Vikunja's scoped API token enforcement for custom project background routes is method-confused. A token with only projects.background can successfully delete a project background, while a token with only projects.background_delete is rejected.
This is a scoped-token authorization bypass.
Details
I verified this locally on commit c5450fb55f5192508638cbb3a6956438452a712e.
Relevant code paths:
pkg/models/api_routes.gopkg/routes/routes.gopkg/modules/background/handler/background.go
Route registration exposes separate permissions for the same path:
GET /api/v1/projects/:project/background->projects.backgroundDELETE /api/v1/projects/:project/background->projects.background_delete
At enforcement time, CanDoAPIRoute() falls back to the parent group and reconstructs the child permission from the path segments only. For the DELETE request, that becomes background, so the matcher accepts any token containing projects.background without re-checking the HTTP method or matching the stored route detail.
This matters because RemoveProjectBackground() is a real destructive operation:
- It checks project update rights.
- It deletes the background file if present.
- It clears the project's
BackgroundFileID.
PoC
- Log in as a user who can update a project that already has a background.
- Create an API token with only:
{"projects":["background"]} - Send:
DELETE /api/v1/projects/<project_id>/backgroundAuthorization: Bearer <token> - Observe that the request succeeds and the project background is removed.
For comparison:
- Create an API token with only:
{"projects":["background_delete"]} - Repeat the same DELETE request.
- Observe that the request is rejected with
401 Unauthorized.
I confirmed this locally with three validations:
/api/v1/routesadvertises bothbackgroundandbackground_delete.- The matcher unit test proves
CanDoAPIRoute()accepts DELETE forbackground. - The webtest proves a real API token with only
backgroundsuccessfully deletes the background.
Impact
Scoped API tokens can exceed their intended capability. A token intended for project background access can delete project backgrounds, which weakens the trust model for automation and third-party integrations that rely on narrowly scoped tokens.
The attacker needs a valid API token created by a user who has update rights on the target project, but the token itself only needs the weaker projects.background permission.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐹Go | code.vikunja.io/api | all versions | 2.3.0go get code.vikunja.io/api@v2.3.0 |
Detection & mitigation playbook
Open-source dependencyDetect
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 2.3.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-40103 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2026-40103 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2026-40103. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2026-40103 in your dependencies?
O3 Security finds CVE-2026-40103 across Go dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.