Date: Wed, 06 Feb 2013 19:23:18 -0700 From: Kurt Seifried <kseifried@...hat.com> To: oss-security@...ts.openwall.com Subject: Re: CVE id request: openssh? -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 On 02/06/2013 02:20 PM, Nico Golde wrote: > Hello, years ago CVE-2006-1206 was raised for a denial of service > attack against dropbear based on exhausting the maximum number of > connections. Back in 2010 I played around with this in openssh to > find out if similar attacks work against that. Since then I never > really knew what to do with this, but every now and then I remember > it and after this bugged me for a while, I finally brought up the > topic to the openssh developers. > > The attached program demonstrates a similar attack against a > default openssh installation. The program simply connects to an ssh > server and waits for the socket to be closed, thus determining the > LoginGraceTime setting of the server. Next, it opens up connections > to the server, keeping them open until no further connection is > allowed and thus determining the MaxStartUps setting (of course, > this may not be always accurate depending on the currently active > sessions etc, but this is a minor detail). > > The code continues to sleep for logingracetime seconds and spawns > maxstartup connections again. As a result, unless you are very > lucky and you hit the time window between the connection respawn, a > user can not login anymore. > > While this is a standard problem for any network service that > limits the number of connections, I think in openssh's case this is > supported by very historically very long LoginGraceTime default > settings (2 minutes) and a lack of random early drop usage for > MaxStartups. > > While you could argue that this is not per-se an openssh security > issue, the default settings aid here to a trivial denial of service > attack against ssh installations by all linux distributions I've > seen. > > The result for a user who tries to login is this: > ssh_exchange_identification: Connection closed by remote host > > The openssh maintainers actually agree here and it resulted in the > following changes: > http://www.openbsd.org/cgi-bin/cvsweb/src/usr.bin/ssh/servconf.c?r1=1.234#rev1.234 > > http://www.openbsd.org/cgi-bin/cvsweb/src/usr.bin/ssh/sshd_config.5?r1=1.156#rev1.156 > http://www.openbsd.org/cgi-bin/cvsweb/src/usr.bin/ssh/sshd_config?r1=1.89#rev1.89 > > I personally don't mind whether this get's a CVE id or not,but > considering that dropbear got one in the past,I thought I'd bring > this up. > > Kind regards Nico Please use CVE-2010-5107 for this issue. - -- Kurt Seifried Red Hat Security Response Team (SRT) PGP: 0x5E267993 A90B F995 7350 148F 66BF 7554 160D 4553 5E26 7993 -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.13 (GNU/Linux) iQIcBAEBAgAGBQJRExAWAAoJEBYNRVNeJnmTkjIP/1OZL0I3yaXM/f7QUQbC9TcF yVKK8s6FsXgUcIMigtvm1CwHLWU1QVDXr+Q6SgytPqo/SF6r8+xWTOOLslPgKL39 oUEAE+0kIZ5900q3bsbLeJ7vLT0YXbPeFtd4tCE8WhFLKnX8zpbYx17xPtwowO0C cFXLYbkl8XS6ZFOynxaSxexXLJCrhtJMqSqfJBDFd/tjRU8jM0WHne85+wGIPiI6 vQWNbV59aAn3GAmKk2j+lET2D+3JHwHS/QkCRvkxiEuhka+Gx+nmdqQ5ms0hdeIi 4h65F+ppOfeQ6gkS+fnTPvkajPo7RQGwQ5GPGkaLX3i54q9aCIc5JCfXv7L3r1uA J9Ix+4zlTdLPcTy2m2aU5m4G9yk2cv7OgwQvilZTGQF9Ro1acIYSm019WNSvr47N 9ItUQHfUsEqrY89Lnd/fS/gviCjW9cYTPaJCcPfWO38j+L7mD15UgrQXGyzwXrY0 RbYqWOGJ83aAGzFm8Xa24wo7g5spk1zlCYQoKiFPKq8yAXMb258SDkgDPrXPgY0o +HQ7NkE4pAK2x9qvkeZ/LLHvwPYGiSjJdvivnCQMNtZPqkbHdyF4ULOu93sw6PkO ++Ih1RmeyKyTiVB60UkiCIMHNMvCGk6Zp4OJxpMmPPhq2K/usWXEGqTDrKa+LgL6 4bajkNh3HLA5ZdC+Wq0g =tGUF -----END PGP SIGNATURE-----
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