8.7

CVSS Score

3.1

-

CVSS Score

Basic Information

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CVE-2026-49478

CVE-2026-49478: Fulcio has OIDC Discovery Redirect Following Allows SSRF and JWKS Substitution for Meta-Issuer Paths, with Kubernetes Service-Account Token Leakage

Impact

Three security vulnerabilities were identified in the OIDC Discovery client:

Blind Server-Side Request Forgery (SSRF) via Cross-Host Redirects: Fulcio uses an HTTP client to fetch OIDC discovery metadata (/.well-known/openid-configuration). Prior to this fix, if a configured issuer returned an HTTP redirect to a different host, the client followed it by default. This allowed a compromised or malicious issuer to redirect Fulcio's discovery requests to internal-only systems, resulting in blind SSRF.
JWKS Substitution and Cache Poisoning: Because cross-host redirects were permitted during OIDC discovery, an attacker could manipulate the discovery flow to return a malicious jwks_uri pointing to an attacker-controlled host. When Fulcio successfully initialized the provider and cached the resulting verifier in the verifier cache, it poisoned the cache with the attacker's verification keys. The attacker could then present signatures validated against the poisoned keys.
Kubernetes ServiceAccount Token Leakage: Fulcio mounts an in-cluster Kubernetes ServiceAccount token to authenticate OIDC discovery requests sent to the local control plane API server (https://kubernetes.default.svc).
- Cross-Host Redirects & JWKS: The token was previously attached globally by the transport, leaking it to third-party hosts if the issuer performed a redirect or if the jwks_uri pointed to a different domain.
- Wildcard MetaIssuers: If a wildcard MetaIssuer of type kubernetes (e.g., matching external EKS/GKE endpoints) was matched, and a local Kubernetes issuer was present in the config, the transport loaded and attached the local in-cluster ServiceAccount token to outbound requests sent to the external host.

Patches

The following mitigations have been applied:

Blocked Cross-Host Redirects: A custom callback is configured on all OIDC discovery HTTP clients to reject redirects that attempt to cross the original issuer's host boundary.
Restricted Token Injection: Updated the transport to only attach the ServiceAccount token when the outgoing request's host exactly matches the configured host of the issuer.
Restricted Local Token Loading: Constrained the loader to only load and wrap the transport with the local ServiceAccount token when the target issuer URL exactly matches the private local API server (https://kubernetes.default.svc).

Workarounds

None, upgrade to v1.8.6

(GitHub Advisory)

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CVE-2026-49478

CVE-2026-49478:

8.7

CVSS Score

3.1

-

CVSS Score

Basic Information

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Technical Details

Package Name	Ecosystem	Vulnerable Versions	First Patched Version
github.com/sigstore/fulcio	go	<= 1.8.5	1.8.6

Technical Details

Vulnerability Intelligence
Miggo AI

Root Cause Analysis

The vulnerability consists of three related issues in Fulcio's OIDC discovery client. The root cause is that the HTTP client used for discovery would follow cross-host redirects by default. This could be exploited in several ways:

Server-Side Request Forgery (SSRF): A malicious OIDC issuer could redirect Fulcio's discovery request to an internal, non-public service. The function github.com/sigstore/fulcio/pkg/config.httpClientForIssuer was responsible for creating the client that exhibited this vulnerable behavior.
Kubernetes ServiceAccount Token Leakage: Fulcio uses a custom HTTP transport, bearerTokenTransport, to attach a Kubernetes ServiceAccount token to requests for authentication. The RoundTrip method of this transport, github.com/sigstore/fulcio/pkg/config.bearerTokenTransport.RoundTrip, attached the token to any request, regardless of the destination host. When a redirect occurred to a malicious external server, this sensitive token was leaked.
JWKS Substitution: The redirect could also be used to point to a malicious jwks_uri on an attacker-controlled server. This would cause Fulcio to fetch and cache the attacker's JSON Web Key Set (JWKS), effectively poisoning the verifier cache and allowing the attacker to forge valid signatures.

The entry point for this vulnerable workflow is the github.com/sigstore/fulcio/pkg/config.FulcioConfig.GetVerifier function, which triggers the OIDC discovery. The patch addresses these issues by adding a CheckRedirect callback to the HTTP client to block cross-host redirects and by modifying the bearerTokenTransport to only attach the token when the request's host matches the intended issuer's host.

Vulnerable functions

github.com/sigstore/fulcio/pkg/config.httpClientForIssuer

pkg/config/config.go

This function was responsible for creating the HTTP client used for OIDC discovery. The original implementation returned a standard http.Client, which by default follows HTTP redirects. This allowed a malicious OIDC issuer to redirect Fulcio to an arbitrary internal or external URL, resulting in a Server-Side Request Forgery (SSRF) vulnerability. The patch wraps the client with a 'noRedirectClient' function that prevents redirects to different hosts.

github.com/sigstore/fulcio/pkg/config.bearerTokenTransport.RoundTrip

pkg/config/config.go

This method is part of a custom http.RoundTripper that attaches a Kubernetes ServiceAccount token to outgoing requests. The vulnerable version attached the token to all requests unconditionally. When combined with the SSRF vulnerability (from following redirects) or a malicious 'jwks_uri', this resulted in the sensitive ServiceAccount token being leaked to unintended third-party hosts.

github.com/sigstore/fulcio/pkg/config.FulcioConfig.GetVerifier

pkg/config/config.go

This function initiates the OIDC discovery process to obtain a verifier for a given issuer. It uses the vulnerable HTTP client created by `httpClientForIssuer`. An attacker could exploit the vulnerability by providing a malicious issuer URL that triggers a redirect. This function serves as the high-level entry point for the entire vulnerable workflow, making it a key runtime indicator during exploitation.

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Reasoning

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