This Metasploit module exploits a logic error in OpenSSL by impersonating the server and sending a specially-crafted chain of certificates, resulting in certain checks on untrusted certificates to be bypassed on the client, allowing it to use a valid leaf certificate as a CA certificate to sign a fake certificate. The SSL/TLS session is then proxied to the server allowing the session to continue normally and application data transmitted between the peers to be saved. The valid leaf certificate must not contain the keyUsage extension or it must have at least the keyCertSign bit set (see X509_check_issued function in crypto/x509v3/v3_purp.c); otherwise; X509_verify_cert fails with X509_V_ERR_UNABLE_TO_GET_ISSUER_CERT_LOCALLY. This Metasploit module requires an active man-in-the-middle attack.
0be0198fd35b0f082fb3872672e7f1dbe40db0a2ae2abc971e5936c264d03b3bFreeBSD Security Advisory - During certificate verification, OpenSSL will attempt to find an alternative certificate chain if the first attempt to build such a chain fails, unless the application explicitly specifies X509_V_FLAG_NO_ALT_CHAINS. An error in the implementation of this logic could erroneously mark certificate as trusted when they should not. An attacker could cause certain checks on untrusted certificates, such as the CA (certificate authority) flag, to be bypassed, which would enable them to use a valid leaf certificate to act as a CA and issue an invalid certificate.
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