CVE-2026-40242 — backend
HIGHCVE-2026-40242 is a high-severity (CVSS 7.2) Server-Side Request Forgery (SSRF) vulnerability in github.com/getarcaneapp/arcane/backend. A fix is available for github.com/getarcaneapp/arcane/backend — see the affected versions and patch details below.
Arcane Unauthenticated SSRF with Conditional Response Reflection in Template Fetch Endpoint
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-2026-40242.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
Where this sits among everything scored
Real-World Exposure
github.com/getarcaneapp/arcane/backendReal-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 /api/templates/fetch endpoint accepts a caller-supplied url parameter and performs a server-side HTTP GET request to that URL without authentication and without URL scheme or host validation. The server's response is returned directly to the caller. type. This constitutes an unauthenticated SSRF vulnerability affecting any publicly reachable Arcane instance.
Details
- No allowlist or denylist of destination hosts/CIDRs
- No requirement for the caller to be authenticated
Response handling produces four distinct outcomes observable by the caller:
- Valid JSON targets return a fully reflected response body if the returned fields fit the expected internal struct
- Non-JSON HTTP 200 responses produce an error leaking the first byte of the response (
"Invalid JSON response: invalid character '<'...") - Non-200 responses leak the HTTP status code
- TCP-level failures distinguish between closed ports (
"connection refused") and filtered ones ("i/o timeout")
PoC
Send an unauthenticated GET request to /api/templates/fetch, passing the target URL as the url query parameter.
Impact
- Unauthenticated port scanning of internal networks
- Access to internal HTTP services not exposed to the public internet (service discovery endpoints, internal dashboards, Kubernetes API)
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐹Go | github.com/getarcaneapp/arcane/backend | all versions | 1.17.3go get github.com/getarcaneapp/arcane/backend@v1.17.3 |
Affected Products
arcanegetarcaneDetection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/getarcaneapp/arcane/backend, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update github.com/getarcaneapp/arcane/backend to 1.17.3 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-40242 is resolved across your whole dependency graph.
Workarounds
Restrict outbound requests from the affected component to an allowlist of hosts, block access to link-local and internal address ranges at the network layer, and require authentication on internal services so a forged request cannot reach them unauthenticated.
How to detect CVE-2026-40242
A community-maintained Nuclei template exists for this CVE. You can scan for it directly:
nuclei -id cve-2026-40242 -u https://target- Template
- Arcane <= 1.17.2 - Server-Side Request Forgery
- Severity
- high
Template by ProjectDiscovery nuclei-templates (0x_Akoko), MIT licensed. View the full template. Scan only systems you are authorised to test.
Frequently Asked Questions
Is CVE-2026-40242 in your dependencies?
Find it across Go, including transitive dependencies.