That malicious application was able to spoof a legitimate, Microsoft-signed digital certificate, which the malware used to install entire copies of itself on targeted computers. The malware's authors accomplished that feat by first cracking a Microsoft digital certificate, created using the MD5 cryptographic hash function, to essentially give themselves a Microsoft Windows "god mode."
"Microsoft was using the certificate in three places: Windows Update server, for licensing, and for code-signing," said Jeff Hudson, CEO of enterprise key and certificate management company Venafi. "This allowed the attacker to use an MD5 collision attack to create what some in the press have called a digital certificate 'evil twin.'"
[ Read about financial malware that attempted to collect information on readers, possibly to target their bank accounts: Banking Trojan Harvests Newspaper Readers' Credentials. ]
But the problem wasn't with MD5, per se, which researchers first cracked in 2008 using 2,000 Sony PlayStation 3s, and which was then cracked in 2010 by German researcher Thomas Roth using on-demand computing power from Amazon Elastic Compute Cloud (EC2). Rather, the problem was with Microsoft, which in 2008 via its TechNet site began urging MD5 users to instead adopt the more secure SHA1 encryption algorithm.
In other words, Microsoft failed to heed its own MD5-related warnings. Of course, Microsoft is hardly alone in its use of MD5. According to Venafi's scans of customers' networks, about one-quarter of the 2,000 largest businesses in the world use MD5-signed certificates. Meanwhile, 17% of all digital certificates in use at the so-called "global 2,000" are signed with MD5. "The numbers are worse for governments, but we're not even going there," said Hudson.
In the wake of Flame, Microsoft nuked the bad certificate and updated Windows to help prevent future such attacks. But the attack illustrates that all businesses need to replace their MD5-signed certificates, especially since, as Hudson noted, digital certificates are used by businesses to secure any data that's sensitive--"involving identity, access management, authentication, and keeping secrets."
To help, Tuesday Venafi released a free tool, MD5 Certificate Assessor, which can be used to catalog all digital certificates in use on a corporate network, highlight which certificates are MD5-signed, detail each certificate's issuing certificate authority (CA), and identify any encryption keys that are out of compliance. Eliminating MD5 from enterprises is a priority because while Flame may have primarily targeted Iran, Lebanon, and Syria, its attack techniques stand to be aped by others.
"Cybercriminals are exceptionally creative, financially organized, and highly motivated to steal confidential information. Organizations focused on reducing security risk need to do all they can to close as many open doors and to change as many locks as they can," said Eric Ogren, principal analyst with Ogren Group, in a statement. "Free tools such as this one being provided by Venafi to track down weak certificates could provide an advantage in staying a step ahead of the attackers."
What should businesses be using instead of MD5-signed certificates? Ideally, select the SHA1 or SHA2 hash algorithm instead. "SHA1 is right on the cusp of being able to be broken; people have come up with theoretical attacks, but they would take a tremendous amount of computing power to execute," Hudson said.
But when using either SHA1 or SHA2, be careful to employ a strong--as in long--key. "NIST [The National Institute of Standards and Technology] has told everyone that 1,024 is crackable now, theoretically," said Hudson. In its key management recommendations, NIST says that 1,024-bit keys should be dropped in favor of 2,048-bit keys, which the organization estimates won't be vulnerable to cracking until 2030. But some security experts even recommend protecting root passwords or accounts with 4,096-bit encryption, Hudson said, "because if you crack the root ... you get everything."
Currently, however, many businesses don't use digital certificates correctly. "The technology is fine, but where it breaks down is [when] people use known-bad stuff like MD5, and it's been known-bad for a while. That's why managing it is important. But first you have to find it," said Hudson.
"Some of the things you find out there are incredible--512-bit keys, self-signed certificates, and that's the worst," he said. "Many times in development, developers put a certificate in there to test it, and then it ends up in the production version, and it's not signed by a CA. That's as bad a practice as there is."
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