CVE-2026-42183: Argo Workflows: SSO RBAC Delegation Nil Pointer Dereference DoS (gatekeeper.go)

Summary A nil pointer dereference in server/auth/gatekeeper.go rbacAuthorization() causes a panic (denial of service) for SSO users whose claims match a namespace-level RBAC rule but not an SSO-namespace rule, when SSODELEGATERBACTONAMESPACE=true.

Details When getServiceAccount(claims, ssoNamespace) returns nil (no matching rule), the error is suppressed and loginAccount remains nil. If RBAC delegation finds a matching namespaceAccount, line 304 calls precedence(loginAccount) which unconditionally accesses serviceAccount.Annotations — nil pointer dereference.

Affected code (v4.0.4):

go // gatekeeper.go:304 } else if precedence(namespaceAccount) > precedence(loginAccount) { // loginAccount is nil here -> precedence(nil) -> PANIC

// gatekeeper.go:232-234 func precedence(serviceAccount corev1.ServiceAccount) int { i, := strconv.Atoi(serviceAccount.Annotations[common.AnnotationKeyRBACRulePrecedence]) return i }

PoC Live-tested 2026-04-17: kind cluster, Argo Workflows v4.0.4, Dex v2.43.1 OIDC provider.

1. Deploy Argo Workflows with --auth-mode=sso --auth-mode=client, SSO pointing to Dex, RBAC enabled. 2. Set SSODELEGATERBACTONAMESPACE=true on the argo-server deployment. 3. Create an RBAC ServiceAccount with workflows.argoproj.io/rbac-rule: "true" annotation in a target namespace (e.g., target-ns). 4. Do not create a matching RBAC rule in the SSO namespace (argo). 5. Authenticate via the Dex SSO flow. 6. Request GET /api/v1/workflows/target-ns with the SSO session cookie. 7. Server returns HTTP 500: {"code":13,"message":"runtime error: invalid memory address or nil pointer dereference"} 8. Server logs: Recovered from panic with stack trace at gatekeeper.go:233 (precedence()) called from gatekeeper.go:304.

Every subsequent API request from affected SSO users triggers the same panic.

Impact Permanent denial of service for any SSO user whose claims don't match SSO-namespace RBAC but do match a target namespace rule. Realistic in multi-tenant deployments with per-namespace RBAC. The gRPC recovery interceptor catches the panic so the server process survives, but the affected user gets HTTP 500 on every request.

Suggested Fix Add nil check: if loginAccount == nil || precedence(namespaceAccount) > precedence(loginAccount)

AI Disclosure This advisory was prepared with AI assistance (Claude Code, Anthropic).