CVE-2026-58196 ToolHive: SSRF in remote MCP server authentication discovery (host-side, bypasses container isolation)
A malicious remote MCP server can trigger Server-Side Request Forgery in ToolHive, allowing access to internal services and cloud metadata credentials.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- A user has to click or open something
- Fix
- Available
What is CVE-2026-58196?
CVE-2026-58196 is a medium-severity server-side request forgery vulnerability in stacklok toolhive, published on 15 Sep 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Official description
ToolHive is a utility designed to simplify the deployment and management of Model Context Protocol (MCP) servers. Prior to 0.31.0, remote.Handler.Authenticate in pkg/auth/remote/handler.go invokes discovery.DetectAuthenticationFromServer in pkg/auth/discovery/discovery.go, whose host-side HTTP clients trust remote-server-controlled authentication discovery destinations, follow redirects without host or scheme restrictions, and do not consistently block private addresses. A malicious or compromised remote MCP server can place a resource_metadata URL in WWW-Authenticate for ParseWWWAuthenticate to extract, after which FetchResourceMetadata, OIDC issuer discovery, and well-known discovery can issue GET requests to link-local, RFC1918, or other internal services outside the server's container. The user connects to a server that the user intends to use, but the server controls the internal destination; this path does not depend on the separate NAT64 guard omission because the affected clients did not invoke IsPrivateIP, and it differs from the DCR resolver path that already refused redirects. The demonstrated primitive reaches internal-only HTTP services and reachability or error oracles, and can retrieve AWS metadata credentials where IMDSv1 accepts an unauthenticated GET, while IMDSv2 and GCP metadata prerequisites are not satisfied by the demonstrated request. This vulnerability is fixed in 0.31.0.
Weakness: CWE-918
Which versions are affected by CVE-2026-58196?
3 open source packages ship an affected version, including github.com/stacklok/toolhive.
| Package | Affected | Fixed in |
|---|---|---|
github.com/stacklok/toolhive Go | All versions | 0.31.0 |
github.com/stacklok/toolhive Go | All versions | 0.31.0 |
github.com/stacklok/toolhive Go | All versions | 0.31.0 |
How do I fix CVE-2026-58196?
Upgrade github.com/stacklok/toolhive to 0.31.0 or later.
Upgrade, and send outbound requests through an allow-list that blocks internal addresses. More about Server-side request forgery.
Is CVE-2026-58196 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-58196 exists.
Sources
Also known as GHSA-pr64-jmmf-jp54, GO-2026-6002.