CVE-2026-49478

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

Workarounds

None, upgrade to v1.8.6

Three security vulnerabilities were identified in the OIDC Discovery client:

1. 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.

2. 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.

3. 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.

Untrusted input controls the target URL of a server-initiated request, which may reach internal services not otherwise accessible from outside. Typical impact: access to internal metadata services, internal APIs, or cloud credentials.

CVE-2026-49478 has a CVSS score of 8.7 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (1.8.6); upgrading removes the vulnerable code path.

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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).

1. What is CVE-2026-49478? CVE-2026-49478 is a high-severity server-side request forgery (SSRF) vulnerability in github.com/sigstore/fulcio (go), affecting versions <= 1.8.5. It is fixed in 1.8.6. Untrusted input controls the target URL of a server-initiated request, which may reach internal services not otherwise accessible from outside.
2. How severe is CVE-2026-49478? CVE-2026-49478 has a CVSS score of 8.7 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of github.com/sigstore/fulcio are affected by CVE-2026-49478? github.com/sigstore/fulcio (go) versions <= 1.8.5 is affected.
4. Is there a fix for CVE-2026-49478? Yes. CVE-2026-49478 is fixed in 1.8.6. Upgrade to this version or later.
5. Is CVE-2026-49478 exploitable, and should I be worried? Whether CVE-2026-49478 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-49478 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-49478? Upgrade github.com/sigstore/fulcio to 1.8.6 or later.

Other vulnerabilities in github.com/sigstore/fulcio