Microsoft has been forced to provide workarounds after security bulletin MS14-066 has caused problems with some users' TLS connections.
Released last week as part of its Patch Tuesday lineup, MS14-066 was meant to resolve a privately reported remote code execution vulnerability in the Microsoft Secure Channel (Schannel) security package, which is used to implement SSL/TLS encryption across all supported versions of Windows and Windows Server. Dubbed "WinShock," the flaw can be exploited by attackers sending specially crafted packets to vulnerable systems; it has been rated as a 10.0 according to the Common Vulnerability Scoring System (CVSS).
The severity of the bug left administrators scrambling to patch affected systems last week, but according to an update issued by Microsoft, systems with TLS 1.2 enabled by default may encounter issues with failed negotiations.
"When this problem occurs, TLS 1.2 connections are dropped, processes hang (stop responding), or services become intermittently unresponsive," read the Microsoft update.
Microsoft has advised enterprises encountering the problem to delete the following encryption ciphers in the registry:
TLS_DHE_RSA_WITH_AES_256_GCM_SHA384 TLS_DHE_RSA_WITH_AES_128_GCM_SHA256 TLS_RSA_WITH_AES_256_GCM_SHA384
Enterprises should also closely follow Microsoft's 11-step guidance for modifying the registry to avoid further issues with an incorrect change, warned Microsoft.
Does WinShock compare to recent encryption vulnerabilities?
The widespread Schannel security flaw is of course only the latest in a string of encryption vulnerabilities that have shaken confidence in Web security.
The Heartbleed OpenSSL vulnerability was the first flaw this year to raise awareness of SSL security issues. Heartbleed exposed private encryption keys and other sensitive information from the millions of websites around the world that depended on OpenSSL to secure Web communications, and the problem has persisted as many of those websites have failed to completely remediate the issue.
And just last month, security researchers at Google uncovered a flaw known as POODLE in major Web browsers that could allow attackers to take advantage of the continued support for the long-outdated SSL 3.0 protocol in Chrome, Firefox and Internet Explorer. Those browser vendors have since issued plans to eliminate SSL 3.0 support.
Josh Feinblum, vice president of information security for Boston-based security vendor Rapid7 LLC, said in a blog post last week that while WinShock "poses serious theoretical risks" to enterprises, it does no quite compare to Heartbleed and some of the other highly publicized vulnerabilities discovered this year.
"Heartbleed, Bashbug, and Sandworm are all security risks that were being actively exploited in the wild upon their publication, and exploitation was relatively trivial. Additionally, sufficient remediation via patching was not readily available at the same time when some of these risks were publicly disclosed," wrote Feinblum. "That is not the case with the SChannel vulnerability. Microsoft customers can take a deep breath before they dive head first into patching, but should make sure patching is treated at the highest priority given the potential risk if/when an exploit is successfully developed."
Johannes Ullrich, head of the Bethesda, Md.-based SANS Technology Institute's Internet Storm Center, said in a blog post that the most likely targets for attackers utilizing WinShock will be SSL services that can be reached from outside an enterprise network, namely Web and email servers. Internal servers could also be affected, added Ullrich, as well as employee laptops used outside an enterprise's perimeter that may utilize SSL/TLS connections for certain software.
"You are doing great if you can get these three groups out of the way by the end of the week," wrote Ullrich.
Ullrich said the worst-case scenario for the Schannel bug is that it could be used to foster a malware outbreak like the Slapper worm, which was a self-propogating worm that exploited buffer-overflow vulnerability in OpenSSL Apache servers in the early 2000s. As a result, he advised organizations to patch this issue above all others in Microsoft's November Patch Tuesday release.
"My guess is that you probably have a week, maybe less, to patch your systems before an exploit is released,"Ullrich wrote before security vendor Immunity Inc. released a proof-of-concept exploit for WinShock. "You got a good inventory of your systems? Then you are in good shape to make this work. For the rest (vast majority?): While you patch, also figure out counter measures and alternative emergency configurations."
OpenSSL recently moved to patch the POODLE bug in its open-source encryption software.
SSL 3.0 co-creator Paul Kocher discussed the protocol in an exclusive interview with SearchSecurity at the 2014 RSA Conference.