CERT warns of more SSH flaws

A number of newly discovered vulnerabilities in implementations of the Secure Shell transport layer communications protocol could enable attackers to launch denial of service attacks or run malicious code on machines running SSH client or server software provided by a number of software vendors.

The advisory was issued Monday by the CERT Coordination Center and labeled CA-2002-36.

All vulnerabilities affect both SSH client and server software, since both use the transport layer protocol, according to CERT.

The advisory describes multiple vulnerabilities in SSH implementations that include “buffer overflows,” in which a program or a process used by a program is forced to store more data in a buffer (a temporary data storage area) than it was intended to hold.

Buffer overflows can cause software applications to crash or allow attackers to place and run their own code on a remote system.

Problems were also identified with the way that many SSH transport layer protocol implementations handle data elements with incorrect length specifiers, lists of data containing empty elements, and strings of characters containing “null” or empty characters.

These flaws, also, could enable remote attackers to crash the SSH client or server application – a denial-of-service attack – or possibly place and execute code on the machine running the vulnerable software.

Because SSH servers run with system or root-level privileges on both Windows and UNIX systems, attackers exploiting SSH server vulnerabilities would be able to take advantage of those elevated privileges when carrying out their attack. In most cases, however, attackers exploiting the vulnerabilities on SSH clients would only inherit the permission level of the user who started the client application, CERT said.

Responses to the CERT advisory by SSH software vendors indicated that products produced by some leading vendors, including Cisco and NetScreen, did not contain the transport layer protocol vulnerabilities.

Among the SSH products containing the vulnerabilities were some versions of SecureShell by Pragma Systems of Austin, Texas, SecureNetTerm by Intersoft International and SSH products by F-Secure of Helsinki, Finland, according to CERT.

Those vendors issued statements with information on obtaining software upgrades or patches that close the security holes, as did other companies with vulnerable products.

As of Tuesday, however, the vulnerability of SSH products by many other software vendors was still unknown.

CERT recommended applying the appropriate patch or software upgrade provided by your software vendor to remove the SSH vulnerabilities.

In the absence of a software fix, customers can use firewalls or packet filtering systems to limit access to SSH servers, while limiting SSH clients to connections with trusted SSH servers by IP address, according to CERT.