Microsoft moves to block MD5 certificates and improve RDP authentication

Windows optional security updates restrict use of MD5-based certificates and improve Remote Desktop Protocol network-level authentication

Microsoft released two optional security updates Tuesday to block digital certificates that use the MD5 hashing algorithm and to improve the network-level authentication for the Remote Desktop Protocol.

These two updates are separate from important security patches also released Tuesday for Internet Explorer, Windows and Microsoft Exchange Server, and are not yet being pushed through the Windows Update mechanism.

The first update, referred to as KB2862973, blocks certificates with MD5 signatures that were signed by Certificate Authority (CA) certificates in the Microsoft root certificate program from being used for server authentication, code signing and time stamping.

The MD5 cryptographic hash function has long been considered insecure for use in SSL certificates and digital signatures. In 2008, a team of security researchers demonstrated a practical attack that involved exploiting a known MD5 weakness to generate a rogue CA certificate trusted by all browsers.

Following that attack, Certificate Authorities accelerated the phasing out of MD5-based certificates and such certificates are no longer being issued today. However, some old MD5 certificates that have yet to expire might still be in use and there are also years-old programs that were digitally signed with such insecure certificates.

"Usage of MD5 hash algorithm in certificates could allow an attacker to spoof content, perform phishing attacks, or perform man-in-the-middle attacks," Microsoft said in a security advisory accompanying the KB2862973 update.

For now, the update has been made available as optional Downloadable Content (DLC) for supported editions of Windows Vista, Windows Server 2008, Windows 7, Windows Server 2008 R2, Windows 8, Windows Server 2012, and Windows RT, but there are plans to start pushing the update through the Windows Update mechanism on Feb. 11, 2014.

"We recommend that customers download and test the update in their environment at the earliest opportunity," William Peteroy, security program manager at the Microsoft Security Response Center, said in a blog post. "This will be especially useful for environments that have little or no inventory of their cryptographic and certificate dependencies."

There are some exceptions to the restriction introduced by this update. For example, Microsoft will still allow binary files that were signed before March 2009 with MD5-based certificates to work.

The company will also allow four specific time stamping certificates from VeriSign CA (now owned by Symantec) to work, as well as all code signing certificates that chain up to a specific certificate from Microsoft and one from GeoTrust, also a Symantec subsidiary.

These exceptions are explained in more detail in a support article for KB2862973.

The second optional update released Tuesday is known as KB2861855 and improves the Network Level Authentication (NLA) method in the Remote Desktop Protocol (RDP).

NLA requires RDP users to be authenticated to a RDP server before a remote desktop connection is established and the logon screen appears.

The KB2861855 update adds multiple layers of defense -- known as defense-in-depth measures -- to the NLA technology in order to prevent attackers from compromising its security, Microsoft said in a security advisory.

This RDP update is available to download from the Microsoft Download Center and Microsoft Update Catalog for all supported editions of Windows Vista, Windows Server 2008, Windows 7, and Windows Server 2008 R2.

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