Constructed ASN.1 types with a recursive definition (as in PKCS7) could exceed stack given excessive recursion. No such structures within SSL/TLS come from untrusted sources so this is considered safe

Fixed in OpenSSL 1.1.0h (Affected 1.1.0-1.1.0g).
Fixed in OpenSSL 1.0.2o (Affected 1.0.2b-1.0.2n).