By exploiting the vulnerability in the OpenSSL library, an attacker could bypass certain checks on certificates, such as the Certificate Authority (CA) flag check, enabling a certificate issued by a valid leaf certificate to be wrongly verified as issued by the valid CA.
The attack could lead to wrongful verification of spoofed certificates at systems using OpenSSL.
The vulnerability is fixed in OpenSSL 1.0.1p and 1.0.2d. Users with systems such as HTTPS protected websites or SSL-VPN gateways using OpenSSL to encrypt network traffic should check with their product vendors if the vulnerable OpenSSL versions are used and if so, upgrade to the fixed versions or follow the recommendations provided by the product vendors to mitigate the risk.
https://www.openssl.org/news/secadv_20150709.txt
https://www.hkcert.org/my_url/en/alert/15071001
https://www.us-cert.gov/ncas/current-activity/2015/07/09/OpenSSL-Releases-Security-Advisory
http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2015-1793