Vulnerability Note VU#665372

SSH connections using RC4 and password authentication can be replayed

Overview

This vulnerability may allow an attacker to replay a captured SSH1 session.

I. Description

Preconditions:

Client requests RC4 and the server grants request
Client uses password authentication

When an SSH1 session using the RC4 cipher is established, the client and server agree upon a session key (SK), which is generated using a server key (VK) that is renewed on a regular basis (one hour by default). The server uses the first 16 bytes of SK as an RC4 key (SK1) and the client uses the last 16 bytes (SK2). The server also generates a session ID (SID) that uniquely identifies the current session. During the session, the server XORs the SID with SK1 and uses the resulting key (SK1') to encrypt its traffic. However, the client does not perform this XOR operation, so it simply uses SK2 for encrypting traffic to the server.

If the victim (client) has requested password authentication for this session, the attacker can record the victim's conversation and then replay it as long as the server key (VK) has not yet expired. Under default configurations, this presents the attacker with a one hour window in which to replay the conversation.

The reason this vulnerability exists is twofold. First, the method used to generate the RC4 keys is faulty. Second, when password authentication is used, the server ignores session ID information, preventing it from detecting that the SID of the replayed connection does not match the SID that the server generated when the attacker began the conversation.

It is important to note that there are significant preconditions and limitations present in this vulnerability. The preconditions are that the victim must request the use of both RC4 and password authentication, and the server must allow both. The limitation is that attackers cannot actually decrypt the original conversation; they can only replay what they've recorded.

II. Impact

Attackers can replay a client's session to the server within a limited window of time.

III. Solution

SSH suggests that customers upgrade to SSH2.

If an upgrade is not possible, users should disable RC4 and password authentication.

Systems Affected

Vendor	Status	Date Notified	Date Updated
OpenSSH	Not Vulnerable	29-Oct-2001
SSH Communications Security	Vulnerable	6-Feb-2001

References

http://www.ssh.com/products/ssh/cert/

Credit

The CERT/CC thanks Antti Huima, Tuomas Aura, and Janne Salmi for their analysis and Tatu Ylonen for bringing this vulnerability to our attention.

This document was written by Jeffrey P. Lanza.

Other Information

Date Public:	2001-01-18
Date First Published:	2001-01-18
Date Last Updated:	2002-03-05
CERT Advisory:
CVE-ID(s):
NVD-ID(s):
US-CERT Technical Alerts:
Metric:	1.69
Document Revision:	10

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