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Crypto weakness leaves online banking apps open to attack

Padding Oracle 'affects every ASP.NET web application'

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Flaws in the way web applications handle encrypted session cookies might leave online banking accounts open to attack.

The security risk stems from a cryptographic weakness in web applications developed using Microsoft's ASP.Net framework. ASP.Net uses the US government-approved AES encryption algorithm to secure the cookies generated by applications during online banking sessions and the like.

However implementation flaws in how ASP.NET handles errors when the encrypted data in a cookie has been modified give clues to a potential attacker that would allow him to narrow down the possible range of the keys used in an online banking session. Attacks based on this weakness might allow a hacker to decrypt sniffed cookies or forge authentications tickets, among other attacks.

Researchers Thai Duong and Juliano Rizzo have developed a Padding Oracle Exploit Tool to demonstrate the feasibility of the attack, an extension of their previous research on similar flaws in JavaServer Faces and other Web frameworks.

"The most significant new discovery is a universal Padding Oracle affecting every ASP.NET web application," Rizzo explains. "In short, you can decrypt cookies, view states, form authentication tickets, membership password, user data, and anything else encrypted using the framework's API.

"The vulnerabilities exploited affect the framework used by 25 per cent of the internet websites. The impact of the attack depends on the applications installed on the server, from information disclosure to total system compromise," he adds.

More details of the security weakness are due to be outlined at a presentation during the Ekoparty conference in Argentina this week.

Rizzo told threatpost that the attack might be exploited to allow a "moderately skilled attacker" to break into a website in an hour or less.

"The first stage of the attack takes a few thousand requests, but once it succeeds and the attacker gets the secret keys, it's totally stealthy. The cryptographic knowledge required is very basic," Rizzo said. ®

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