A security issue has been found in subversion < 1.9.7. A Subversion client sometimes connects to URLs provided by the repository. This happens in two primary cases: during 'checkout', 'export', 'update', and 'switch', when the tree being downloaded contains svn:externals properties; and when using 'svnsync sync' with one URL argument.
A maliciously constructed svn+ssh:// URL would cause Subversion clients to run an arbitrary shell command. Such a URL could be generated by a malicious server, by a malicious user committing to a honest server (to attack another user of that server's repositories), or by a proxy server.
The vulnerability affects all clients, including those that use file://, http://, and plain (untunneled) svn://.