Software Engineering Institute

Versions of OpenSSH between 2.5.x - 2.9.x may fail to enforce the IP based access control restriction feature. A user may specify from which IP's a key may be used. They may have several entries for several keys. Expected behavior of this feature can be demonstrated as follows. If the authorized_keys2 file contained an entry for a key A that was an RSA key and restricted to 10.0.0.1 via the "from=" line option and key B was a DSA key and restricted to 10.0.0.2, then key B would not be of any use if compromised unless it was used from the machine with an IP address of 10.0.0.2.

Due to the flaw in this feature, when a user specifies two keys of differing types in their ~/.ssh/authorized_keys2, OpenSSH may fail to apply the proper source IP based access control restrictions specified by the "from=" line. For example, assume key A was an RSA key and restricted to 10.0.0.1 via the "from=" line and key B was a DSA key and restricted to 10.0.0.2. Now assume that key B is compromised. One would expect that key B could only be used from 10.0.0.1. However, since key A is specified on the line immediately before the line containing the entry for the compromised key and is of a different type and "from=", then the intruder can access the network from the IP address of key A (10.0.0.1) using the compromised key B.

Likewise a systems administrator could set up a single authorized_keys2 file and direct the individual users ssh clients to this file via a symbolic link. If the systems administrator kept the file world readable, but not writable, then he could control the contents of the file. In this case, a malicious user could use their key in the same method as described above to bypass any IP restrictions that the systems administrator may have placed on them.

An attacker with a compromised key, or authorized users can circumvent the security policies and login from IP addresses that are not permitted to access the system.

This vulnerability is fixed in OpenSSH 2.9.9. Upgrade to a version 2.9.9 or later.