Since aeb1a281ca ("gtls: fix OCSP stapling management"), the function
parses the stapled OCSP response and reads the certificate status via
gnutls_ocsp_resp_get_single(), but never calls gnutls_ocsp_resp_verify()
or gnutls_ocsp_resp_verify_direct(). A response with a forged or
corrupted signature is accepted without question.
Fix by calling gnutls_ocsp_resp_verify() against the trust list obtained
from the session credentials immediately after gnutls_ocsp_resp_import().
This handles both directly-signed responses and delegated OCSP responders
without requiring the issuer certificate to be present in the peer chain.
The missing check only affects the CURLOPT_SSL_VERIFYSTATUS code path
when CURLOPT_SSL_VERIFYPEER is disabled. With peer verification enabled,
gnutls_certificate_verify_peers2() independently catches the invalid
response via GNUTLS_CERT_INVALID_OCSP_STATUS before
gtls_verify_ocsp_status() is reached. As a result, no attack is possible
that is not already trivially achievable without OCSP stapling when peer
verification is off. This is a correctness and consistency fix, not a
security vulnerability.
Reported-by: Joshua Rogers
Closes#21677
be6c4ee7fa