Three security vulnerabilities were identified in Fulcio's OIDC Discovery client prior to version 1.8.6: (1) Blind SSRF via cross-host HTTP redirects during OIDC discovery metadata fetching, allowing attackers to redirect requests to internal-only systems; (2) JWKS substitution and cache poisoning by manipulating the discovery flow to return malicious jwks_uri values, poisoning the verifier cache with attacker keys; (3) Kubernetes ServiceAccount token leakage due to the token being attached globally to outbound requests, including those redirected to or targeting external hosts, especially when wildcard MetaIssuers of type kubernetes were configured. The vulnerabilities were mitigated by blocking cross-host redirects, restricting token injection to exact host matches, and constraining local token loading to the local Kubernetes API server URL. Users must upgrade to version 1.8.6 to remediate these issues.

Successful exploitation can lead to blind server-side request forgery (SSRF) allowing attackers to make internal network requests via Fulcio, poisoning of the JWKS verifier cache enabling acceptance of attacker-signed tokens, and leakage of Kubernetes ServiceAccount tokens to unauthorized external hosts. This can compromise the integrity of token verification and expose sensitive credentials, potentially impacting confidentiality and integrity of systems relying on Fulcio for OIDC authentication.

Upgrade to Fulcio version 1.8.6 or later, which includes fixes that block cross-host redirects during OIDC discovery, restrict ServiceAccount token injection to exact issuer hosts, and limit local token loading to the Kubernetes API server URL. No workarounds are available; applying the official patch is required to mitigate these vulnerabilities.