A cryptographic library used in all Windows versions is affected by a recently disclosed vulnerability in SSL/TLS implementations that allows man-in-the-middle attackers to force clients and servers to use weak encryption. Internet Explorer and other programs using the library are affected.

The FREAK (Factoring Attack on RSA-EXPORT Keys) vulnerability stems from a decision made in the 1990s to limit the strength of RSA encryption keys to 512 bits in SSL (Secure Sockets Layer) implementations intended for export in order to meet U.S. government rules on exports of encryption systems.

Those “export” cipher suites are no longer used today, but a team of researchers recently discovered that many servers still support them and some SSL/TLS clients, including Web browsers, can be forced to accept them because of bugs in the crypto libraries they rely on.

Attackers in a position to intercept SSL or TLS (Transport Layer Security) connections between such vulnerable clients and servers can downgrade the encryption and then crack the 512-bit keys using several hours of computing on cloud services such as Amazon’s EC3.

Tests run earlier this week, when the vulnerability was publicly disclosed, showed that around 36 percent of all HTTPS-enabled websites with browser-trusted certificates were potentially vulnerable. Vulnerable clients included software that relied on OpenSSL or Apple’s Secure Transport, like the Chrome, Safari, Opera, the Android and the BlackBerry stock browsers.

On Thursday, Microsoft released a security advisory warning that Secure Channel (Schannel), a crypto library included in all supported versions of Windows, is also vulnerable. This means that Internet Explorer and programs that rely on Schannel are also affected.

The Microsoft security advisory describes a workaround that involves disabling RSA key exchange ciphers by using the Group Policy Object Editor. However, this could result in connections being refused by servers that don’t support any of the remaining cipher suites. The workaround can’t be applied to Windows Server 2003, which is affected, because the cipher management architecture on the platform doesn’t allow for enabling or disabling individual ciphers.

Computer scientists at the University of Michigan have set up a website that can check if a user’s browser is vulnerable or not. The site also contains information about the current state of patching efforts for different browsers and SSL/TLS libraries, as well as a list of the most popular affected HTTPS websites.

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