Pierluigi Paganini September 29, 2014

The SHA-1 cryptographic hash algorithm has been known vulnerable, Collision attacks against it are too affordable and attacks will get cheaper soon.

Many websites today are using digital certificates signed using algorithms based on the hash algorithm called SHA-1. Hashing algorithms are used to ensure the integrity of the certificate in the signing processes, a flawed algorithm could allow an attacker to forge fraudulent certificates.

In the past collision attacks against the MD5 hash algorithm allowed threat actors to obtain fraudulent certificates, security experts want to avoid similar problems for SHA-1.

Principal vendors are working to phase out support for the SHA-1 hash algorithm which is vulnerable to collision attacks that will be soon possible, in 2012 experts demonstrated how breaking SHA1 is becoming feasible, in November 2013, Microsoft announced that they will phase out SHA1 certificates after 2016.

“. Collision attacks against SHA-1 are too affordable for us to consider it safe for the public web PKI. We can only expect that attacks will get cheaper.” states Google in an official statement on the topic. SHA-1’s use on the Internet has been deprecated since 2011, when the CA/Browser Forum, an industry group of leading web browsers and certificate authorities (CAs) working together to establish basic security requirements for SSL certificates, published their Baseline Requirements for SSL. These Requirements recommended that all CAs transition away from SHA-1 as soon as possible, and followed similar events in other industries and sectors, such as NIST deprecating SHA-1 for government use in 2010.

To have an idea of the impact for the deprecation of SHA-1 let’s analyze the data provided by the SSL Pulse which confirms that only 14,8 % sites use SHA256 certificates in September 2014.

“A collision attack is therefore well within the range of what an organized crime syndicate can practically budget by 2018, and a university research project by 2021.” said the expert Bruce Schneier.

The decision of Microsoft is shared among other IT giants, like Mozilla and Google, the popular search engine, for example, will penalize websites that use SHA1 certificates that expire during 2016 and after.

Certification Authorities (CAs) and Web site administrators have to upgrade their digital certificates to use signature algorithms different from SHA-1, valid alternatives could be SHA-256, SHA-384, or SHA-512.

“Sites with end-entity certificates that expire between 1 January 2016 and 31 December 2016 (inclusive), and which include a SHA-1-based signature as part of the certificate chain, will be treated as “secure, but with minor errors”.  Sites with end-entity certificates that expire on or after 1 January 2017, and which include a SHA-1-based signature as part of the certificate chain, will be treated as “affirmatively insecure”. Sub resources from such domain will be treated as “active mixed content”. The current visual display for “affirmatively insecure” is a lock with a red X, and a red strike-through text treatment in the URL scheme.” states the blog post published by Google titled “Gradually sunsetting SHA-1“.

Mozilla has aligned its position on the acceptance for SHA-1 based certificates,  the company should not be issued after January 1, 2016, or trusted after January 1, 2017. The section 8 of the Mozilla’s CA Certificate Maintenance Policy states:

“We consider the following algorithms and key sizes to be acceptable and supported in Mozilla products: SHA-1 (until a practical collision attack against SHA-1 certificates is imminent) …” NIST Guidance recommended that SHA-1 certificates should not be trusted beyond 2014. However, there are still many Web sites that are using SSL certificates with SHA-1 based signatures, so we agree with the positions of Microsoft and Google that SHA-1 certificates should not be issued after January 1, 2016, or trusted after January 1, 2017. In particular, CAs should not be issuing new SHA-1 certificates for SSL and Code Signing, and should be migrating their customers off of SHA-1 intermediate and end-entity certificates. If a CA still needs to issue SHA-1 certificates for compatibility reasons, then those SHA-1 certificates should expire before January 2017. More information is available in Mozilla’s list of Potentially Problematic CA Practices.”

To be aligned with suggestions provided by the NIST and IT giants I recommend you to:

• Inventory your existing digital certificates, in complex environments, this task could be done automatically using specific tools that scan the entire company network.
• Replace SHA1 certificates that expire after 2015; A good policy could be to start from the most valuable websites for the company business and those website using certificates that expire after 2016.
• Assess those server platforms which might not be able to support SHA256, or superior, certificates (e.g. Windows Server 2003). These systems might require an upgrade or a patching activity.

Pierluigi Paganini

(Security Affairs – SHA-1, digital certificates)