reject signing certificates not marked for code signing#648
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reject signing certificates not marked for code signing#648Support-itecz wants to merge 1 commit into
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_verify_certificates treated the digitalSignature key usage bit and a code-signing extended key usage as alternatives, so a certificate whose extended key usage is serverAuth only was accepted as long as digitalSignature was set. A present extended key usage is restrictive per RFC 5280, so a TLS certificate chaining to a trusted root (the default is the certifi bundle) could be used to sign a model that the verifier then accepts. Reject a certificate whose extended key usage lists neither code signing nor anyExtendedKeyUsage, even when the key usage bit is set. Keeping the check in the verifier means every certificate-based verification path is covered without callers validating the chain themselves. Signed-off-by: Itecz Solution <support@itecz.au>
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Summary
_verify_certificatesdecides a signing certificate is usable when it has the digitalSignature key usage bit or a code-signing extended key usage, treating the two as alternatives. An extended key usage is restrictive when present (RFC 5280 4.2.1.12), so a certificate whose only EKU isserverAuth(an ordinary TLS certificate) is still accepted as long as digitalSignature is set. Because the default trust roots are the certifi bundle, any holder of a publicly trusted TLS certificate can produce a signature this verifier accepts.The fix rejects a certificate whose extended key usage lists neither code signing nor anyExtendedKeyUsage, even when the key usage bit is set. Keeping the check inside the verifier covers every certificate-based verification path without callers validating the chain themselves. Added a regression test that mints a
serverAuth-only leaf under a private root and confirms it is now rejected, along with positive cases for a code-signing leaf and a leaf with no EKU.To reproduce before the fix: build a leaf with
ExtendedKeyUsage([serverAuth])andKeyUsage(digital_signature=True)chaining to a root in the verifier's store, then verify a signature made with its key. Verification passes.Checklist