All versions of ssh prior to 1.2.12.92 had a security flaw which allowed local users to get access to the secret host key. This is fixed in 1.2.13 and later.
If you run ssh 1.2.13 on Alpha OSF 1.3 or SCO in C2 security mode,
local users can gain root access. This is fixed by applying
ftp://ftp.cs.hut.fi/pub/ssh/ssh-osf1-c2-setluid.patch
or by upgrading to 1.2.14 or later.
Versions of ssh prior to 1.2.17 had problems with authentication agent handling on some machines. There is a chance (a race condition) that a malicious user could steal another user's credentials. This should be fixed in 1.2.17.
The arcfour cipher is used in a way which makes it susceptible in version 1 of the ssh protocol. Therefore, its use has been disabled in 1.2.18 and later.
As with every piece of freely available software, this is difficult to find out. The best current estimates are that at least 1000 insitutions in 40 countries use it. This estimate is based on
The protocols ssh uses are freely available. There are no restrictions if anybody wants to write a version that is available under different conditions and is interoperable with existing ssh installations.
Ssh is also on the Internet Standards Track. This means that a second, independent implementation is required.
You will have to be aware of patent (RSA, IDEA) and export control issues before writing a second implementation.
Most of the credit, of course, goes to Tatu Ylönen for writing ssh and making it available to the public. I have also used parts of his text from the documentation accompanying the ssh source distribution. Thanks also for his corrections for this FAQ.
Also of invaluable help were corrections and additions from members of the ssh mailing list and the Usenet newsgroups, by Mark Martinec, Pedro Melo, Michael Soukas, Adrian Colley, Kenneth J. Hendrickson, Adam Hammer, Olaf Titz, David Mazieres, Axel Boldt and Wayne Schroeder.
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