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Multiple vendors' SSH transport layer protocol implementations contain vulnerabilities in key exchange and initialization

Vulnerability Note VU#389665

Original Release Date: 2002-12-16 | Last Revised: 2003-06-18

Overview

Secure shell (SSH) transport layer protocol implementations from different vendors contain multiple vulnerabilities in code that handles key exchange and initialization. Both SSH servers and clients are affected. A remote attacker could execute arbitrary code with the privileges of the SSH process or cause a denial of service.

Description

From the IETF draft SSH Transport Layer Protocol:

SSH is a protocol for secure remote login and other secure network services over an insecure network.

This document describes the SSH transport layer protocol which typically runs on top of TCP/IP.  The protocol can be used as a basis for a number of secure network services.  It provides strong encryption, server authentication, and integrity protection.  It may also provide compression.

Key exchange method, public key algorithm, symmetric encryption algorithm, message authentication algorithm, and hash algorithm are all negotiated.
Rapid7 has developed a suite of test cases (SSHredder) that examine the connection initialization, key exchange, and negotiation phase (KEX, KEXINIT) of the SSH transport layer protocol. The suite tests the way an SSH transport layer implementation handles invalid or incorrect packet and string lengths, padding and padding length, malformed strings, and invalid algorithms.

The test suite has demonstrated a number of vulnerabilities in different vendors' SSH products. These vulnerabilities include buffer overflows, and they occur before user authentication takes place. Common Vulnerabilities and Exposures (CVE) has assigned the following candidate numbers for several classes of tests performed by SSHredder:

CAN-2002-1357: incorrect length fields, i.e. specified length field does not match the actual length of the input

CAN-2002-1358: lists with empty elements or multiple separators

CAN-2002-1359: "classic" buffer overflows (length field, if present, is consistent with the actual length of buffer)

CAN-2002-1360: null characters in strings (which trigger conflicts between delimiter-based and length-based strings)
Rapid7 has posted an advisory (R7-0009) and the SSHredder test suite.

Impact

The impact will vary for different vulnerabilities, but in some cases remote attackers could execute arbitrary code with the privileges of the SSH process. Both SSH servers and clients are affected. On Windows systems, SSH servers commonly run with SYSTEM privileges. SSH daemons on UNIX systems typically run with root privileges. In the case of SSH clients, any attacker-supplied code would run with the privileges of the user who started the client program. Additional privileges may be afforded to an attacker when the SSH client is configured to run with an effective user ID (setuid/setgid) of root. Attackers could also crash a vulnerable SSH process, causing a denial of service.
While OpenSSH does not appear to be affected, it is worth noting that privilege separation would greatly reduce the impact of arbitrary code execution during the KEXINIT phase.

Solution


Upgrade or Apply Patch

Upgrade or apply a patch as specified by your vendor.


Restrict Access

Until patches or upgrades are available, it may be possible to limit access to vulnerable SSH clients and servers using the built-in facilities of some SSH implementations, firewalls, packet-filters, TCP Wrappers, or other similar technology. Note that this workaround will not prevent exploitation of these vulnerabilities, it will only limit the number of potential sources of attacks.

Do Not Trust DNS

SSH clients can reduce the risk of attacks by only connecting to trusted servers by IP address. Again, this will not prevent attacks, but it will remove the ability of an attacker to redirect a client using DNS cache poisoning or by compromising a DNS server.

Vendor Information

389665
 
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CVSS Metrics

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References

Acknowledgements

The CERT/CC thanks Rapid7 for researching and reporting these vulnerabilities.

This document was written by Art Manion and Shawn V. Hernan.

Other Information

CVE IDs: CVE-2002-1357
CERT Advisory: CA-2002-36
Severity Metric: 11.04
Date Public: 2002-12-16
Date First Published: 2002-12-16
Date Last Updated: 2003-06-18 17:15 UTC
Document Revision: 39

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