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The CVE-2025-61727 vulnerability in OpenTofu originates from a bug in the Go programming language’s crypto/x509 library, which OpenTofu uses for TLS certificate verification. When OpenTofu acts as a TLS client, it authenticates certificate chains provided by servers. A flaw causes improper application of excluded DNS name constraints when verifying wildcard Subject Alternative Names (SANs). Specifically, if a certificate authority imposes a constraint excluding a specific subdomain like “test.example.com”, this constraint fails to restrict wildcard SANs in the leaf certificate. Consequently, a leaf certificate can claim a wildcard SAN such as “.example.com” even though “test.example.com” is excluded, creating a contradiction in the chain’s validity for certain hostnames. OpenTofu employs TLS when communicating with registries, downloading packages via HTTPS, and interacting with remote services for state management. In these connections, OpenTofu might accept a certificate chain with conflicting information about its validity for the target hostname. However, all certificates are still individually validated, so exploitation requires an attacker to produce a chain of valid-but-contradictory certificates, possess the corresponding private keys, and orchestrate OpenTofu to connect to the compromised hostname. This complexity makes attacks challenging but feasible under controlled conditions.
Platform: OpenTofu
Version: v1.8 v1.9
Vulnerability: TLS certificate verification
Severity: Medium
date: Not specified
Prediction: Patched v1.10.8
What Undercode Say:
Analytics:
bash: tofu version
bash: go version
code: // Constraint check flaw in crypto/x509
if isWildcard(san) && !excludedConstraintMatch {
// Vulnerability permits wildcard
}
How Exploit:
Obtain constraint-excluded CA. Issue wildcard leaf certificate. Serve malicious TLS. Redirect OpenTofu connections.
Protection from this CVE:
Update OpenTofu v1.10.8. Use Go 1.24.11.
Impact:
Potential MITM attacks. Requires valid certificates. Limited exploitation scope.
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Sources:
Reported By: github.com
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