SSH getting a security tune-up from NIST and IETF
Secure Shell, among the most common protocols for safely accessing a remote computer across the internet, is getting a fresh security examination from the US government and an internet standards body.
SSH emerged in the 1990s, became more popular than legacy protocols such as Telnet in the 2000s, and now is starting to show its age. The current version called SSH-2 has been on the losing side of notable hacks and is vulnerable to brute-force attacks, which may start happening more often with a Microsoft implementation due soon.
The newest concern is mismanagement of SSH keys, according to a report (PDF) issued October 2015 by the National Institute of Standards and Technology (NIST). “We are in a position where some organizations have 50 times more SSH keys than they have people,” said Tatu Ylönen, who coauthored the report and invented SSH in 1995.
“We are in a position where some organizations have 50 times more SSH keys than they have people.” Tatu Ylönen
Trouble begins when employees create new keys without their IT staff being aware and continues when people leave companies but their SSH keys remain, Ylönen said. Hackers exploit unmanaged or forgotten keys, so the more prominent the protocol becomes, the greater its risk, he explained. Ylönen’s own company, SSH Communications Security, sells software to address this for very large organizations, but for most businesses, “There haven’t been any controls, no policies, and the keys haven’t been removed,” he cautioned.
The alarm from Ylönen is not a new one, but NIST hoisting the flag is an eye-opener. The report gives 13 suggestions for safely managing SSH keys, mostly focusing on issues such as key lifespans and privileges. Auditing, testing, and deploying are also covered in detail.
Many IT departments may not realize that key management tools are already available to them in the very popular OpenSSH implementation. Damien Miller, who maintains the implementation, cited examples such as support for public keys managed by administrators rather than users, keys held on tokens, hooks that allow command access to fetch keys, and certificates for lifespans.
“Broadly speaking, our medium-term priorities area to continue getting rid of bad/legacy crypto as fast as we can without causing excessive user pain and continue … refactoring and modernization of OpenSSH’s internals. As we’re refactoring, we’re writing unit and fuzz tests as we go to improve correctness and security,” Miller added.
Ed Skoudis, a security instructor at training company SANS Institute, said these improvements make sense. “The biggest weakness [of SSH] is the management and storage of the keys,” he said. “That’s why I think what NIST has done is really good.”
Another way to keep SSH secure is to prevent people from using obsolete cryptography algorithms, said officials at the Internet Engineering Task Force (IETF), which manages the actual protocol. There aren’t any plans to upgrade SSH-2, although security experts on Nov. 18, 2015 proposed a new working group to formally deprecate outdated methods. This will apply to several protocols, not just SSH, they said. The new group’s proposed name is CURDLE: “CURves, Deprecating and a Little more Encryption.”
“As with many modern protocols, the SSH protocol has been designed to support algorithm agility, that is, the ability to update the cryptographic algorithms used, without other changes being needed to the protocol,” CURDLE leader Stephen Farrell said. “There are currently discussions ongoing… as to how best to incorporate new crypto into the SSH protocol. So far that hasn’t shown a need for a new SSH working group but should one be needed, we’d form one. Instead we may form a working group that handles adding, e.g., the new elliptic curves specifications from the [Crypto Forum Research Group] across a number of protocols at once.”
SANS’ Skoudis added that network communications between Microsoft Windows and other operating systems are the last bastion of non-SSH connections. There are products for achieving SSH connections in that situation, but Microsoft said last summer that it plans to build SSH into its PowerShell system administration application. That is scheduled to ship by the middle of 2016 based on an OpenSSH port, but will also have Redmond’s proprietary cryptology interfaces rather than standard open-source implementations of the Secure Sockets Layer — a move that drew criticism on a Microsoft blog post.
Regardless, Skoudis said, it’s better to have a Microsoft flavor of SSH inside Windows than no SSH at all. Companies can’t assume that people will apply a third-party SSH tool. “That’s the last piece of the puzzle,” he said.
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