FreeBSD Security Advisory (FreeBSD-SA-03:15.openssh.asc)

Published: 2008-09-04 18:41:11

CVSS Base Vector:

Technical Details:
OpenSSH is a free version of the SSH protocol suite of network connectivity tools. OpenSSH encrypts all traffic (including passwords) to effectively eliminate eavesdropping, connection hijacking, and other network-level attacks. Additionally, OpenSSH provides a myriad of secure tunneling capabilities, as well as a variety of authentication methods. The SSH protocol exists in two versions, hereafter named simply `ssh1' and `ssh2'. The ssh1 protocol is a legacy protocol for which there exists no formal specification, while the ssh2 protocol is the product of the IETF SECSH working group and is defined by a series of IETF draft standards. The ssh2 protocol supports a wide range of authentication mechanisms, including a generic challenge / response mechanism, called `keyboard-interactive' or `kbdint', which can be adapted to serve any authentication scheme in which the server and client exchange a arbitrarily long series of challenges and responses. In particular, this mechanism is used in OpenSSH to support PAM authentication. The ssh1 protocol, on the other hand, supports a much narrower range of authentication mechanisms. Its challenge / response mechanisms, called `TIS', allows for only one challenge from the server and one response from the client. OpenSSH contains interface code which allows kbdint authentication back-ends to be used for ssh1 TIS authentication, provided they only emit one challenge and expect only one response. Finally, recent versions of OpenSSH implement a mechanism called `privilege separation' in which the task of communicating with the client is delegated to an unprivileged child process, while the privileged parent process performs the actual authentication and double-checks every important decision taken by its unprivileged child. 1) Insufficient checking in the ssh1 challenge / response interface code, combined with a peculiarity of the PAM kbdint back-end, causes OpenSSH to ignore a negative result from PAM (but not from any other kbdint back-end). 2) A variable used by the PAM conversation function to store challenges and the associated client responses is incorrectly interpreted as an array of pointers to structures instead of a pointer to an array of structures. 3) When challenge / response authentication is used with protocol version 1, and a legitimate user interrupts challenge / response authentication but successfully authenticates through some other mechanism (such as password authentication), the server fails to reclaim resources allocated by the challenge / response mechanism, including the child process used for PAM authentication. When a certain number of leaked processes is reached, the master server process will refuse subsequent client connections.

Upgrade your system to the appropriate stable release or security branch dated after the correction date

The remote host is missing an update to the system as announced in the referenced advisory FreeBSD-SA-03:15.openssh.asc

Detection Type:
Linux Distribution Package

Solution Type:
Vendor Patch

CVSS Score
FreeBSD Local Security Checks

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