CVE-2026-49478 is a high-severity (CVSS 8.7) Server-Side Request Forgery (SSRF) vulnerability in github.com/sigstore/fulcio. A fix is available for github.com/sigstore/fulcio — see the affected versions and patch details below.
Fulcio has OIDC Discovery Redirect Following Allows SSRF and JWKS Substitution for Meta-Issuer Paths, with Kubernetes Service-Account Token Leakage
Exploitation Status
No confirmed exploitation observed yet
- A successful exploit gives an attacker total control of the affected component, not partial access.
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-49478.
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/sigstore/fulcioReal-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
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_uripointing 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_uripointed to a different domain. - Wildcard MetaIssuers: If a wildcard
MetaIssuerof typekubernetes(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.
- 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
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
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐹Go | github.com/sigstore/fulcio | all versions | 1.8.6go get github.com/sigstore/fulcio@v1.8.6 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/sigstore/fulcio, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update github.com/sigstore/fulcio to 1.8.6 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-49478 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.
Fixing This On Your OS
If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.
A flaw was found in Fulcio's OIDC Discovery client. Three related vulnerabilities allow exploitation when a configured OIDC issuer is compromised or malicious: (1) Blind SSRF — the discovery client follows cross-host HTTP redirects, allowing a malicious issuer to redirect Fulcio's metadata fetch requests to…
Upgrade to Fulcio v1.8.6 or later. No workaround is available.Source: Red Hat security advisory for CVE-2026-49478 (CC BY 4.0)
| Product | Fixed in | Advisory |
|---|---|---|
| Red Hat Advanced Cluster Security 4.9 | advanced-cluster-security/rhacs-main-rhel8:1785413210 | RHSA-2026:48872 |
| Red Hat Advanced Cluster Security for Kubernetes 4.10 | advanced-cluster-security/rhacs-main-rhel8:1785420959 | RHSA-2026:48913 |
Frequently Asked Questions
Is CVE-2026-49478 in your dependencies?
Find it across Go, including transitive dependencies.