The X509_verify_cert function in crypto/x509/x509_vfy.c in OpenSSL 1.0.1n, 1.0.1o, 1.0.2b, and 1.0.2c does not properly process X.509 Basic Constraints cA values during identification of alternative certificate chains, which allows remote attackers to spoof a Certification Authority role and trigger unintended certificate verifications via a valid leaf certificate.
mdeslaur> introduced by the following commit in 1.0.2b and 1.0.1n: mdeslaur> https://git.openssl.org/?p=openssl.git;a=commit;h=6281abc79623419eae6a64768c478272d5d3a426