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How poor crypto housekeeping left OpenID open to abuse

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Slipshod cryptographic housekeeping left some OpenID services far less secure than they ought to be.

OpenID is a shared identity service that enables users to eliminate the need for punters to create separate IDs and logins for websites that support the service. A growing number of around 9,000 websites support the decentralised service, which offers a a URL-based system for single sign-on.

Security researchers discovered the websites run by three OpenID providers - including Sun Microsystems - used SSL certificates with weak crypto keys. Instead of being generated from billions of possibilities, the keys came from a a set of just 32,768 options, due to a flaw in the random number generation routines used by Debian. The bug, which has been dormant on systems for 18 months, was discovered and corrected back in May.

Keys generated by cryptographically flawed systems still needed to be replaced even after the software was upgraded. But recent research by Ben Laurie of Google reveals that 1.5 per cent of certificates he looked at contained weak keys. Three OpenID providers (openid.sun.com, xopenid.net and openid.net.nz) were among the guilty parties.

To exploit the vulnerability, malicious hackers would need to trick surfers into visiting a site impersonating a pukka OpenID provider. But faking digital certificate alone wouldn't do the trick without first misdirecting surfers to these bogus sites. Dan Kaminsky's recent discovery of a DNS cache poisoning flaw made it far more plausible to construct an attack that sent surfers the wrong away around the net's address lookup system, potentially to a bogus Open site posing as the real deal.

The security flaw meant that even cautious users who check SSL certificates were at risk of handing over their OpenID credentials as part of a phishing attack. Such an attack would take a lot of effort to pull off and would only yield OpenID login credentials, which aren't especially useful for hackers and are difficult to monetise.

Going after online banking credentials via a site that makes no attempt to offer up fake SSL certificates is a far more reliable moneyspinner, a factor that leads noted security researcher Richard Clayton to describe the attack as the "modern equivalent of a small earthquake in Chile".

Sun has responded to the issue by generating a new secure key, which reduces the scope for mischief but still leaves potential problems from the old key.

More thoughts on the cryptographically interesting - though not especially life-threatening - flaw can be found in Clayton's posting on Cambridge University's Light the Blue Touchpaper blog here. A security advisory by Laurie and Clayton explaining the issue in greater depth can be found here. ®

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