Vulnerability Note VU#178990

Erlang/OTP SSH library uses a weak random number generator

Overview

The Erlang/OTP SSH library's random number generator is not cryptographically strong because it relies on predictable seed material.

I. Description

Geoff Cant's report states:

The Erlang/OTP ssh library implements a number of cryptographic operations that depend on cryptographically strong random numbers. Unfortunately the RNG used by the library is not cryptographically strong, and is further weakened by the use of predictable seed material. The RNG (Wichman-Hill) is not mixed with an entropy source.

The seed used for all ssh connections in the library is the current time (to approximately microsecond resolution). By observing the time a connection from this library is established, the first two components of the three RNG seed can be guessed.The third component can be recovered by brute-force; trying each possible value (1..1000000).

Guessing the exact seed is made easier by the 16 byte random session cookie that the library will send in its plaintext kexinit message. This cookie will be bytes 17-32 of the RNG sequence.

Once the session RNG seed is recovered, an attacker can simply perform the same DH key exchange operation as the SSH library and recover the session secret. Additionally, if the ssh library is used on the server side of the connection and DSA host key is used, the private key can be recovered from the kex_dh messages. The secret signing value k is known from the RNG seed (bytes 170 - 190 of the sequence), so with the public DSA key data in the kex_dh_reply message the private part can be recovered by inverting the signature operation.

II. Impact

An attacker can recover SSH session keys and DSA host keys.

III. Solution

Apply an Update

A patch has been committed for issue "ssh 2.0.5 OTP 9225" to the Erlang/OTP source that remediates the vulnerability. All SSH DSA keys used with the vulnerable library should be changed. Any password or secret sent over a connection that used the vulnerable library should be changed as well.

Erlang/OTP R14B03 is the first official release to address this vulnerability. Users that don't apply the patch should upgrade to R14B03 or later.

Vendor Information

Vendor	Status	Date Notified	Date Updated
Ericsson	Affected		2011-04-22

References

https://github.com/erlang/otp/commit/f228601de45c5b53241b103af6616453c50885a5
http://www.erlang.org/
http://www.erlang.org/download.html

Credit

Thanks to Geoff Cant for reporting this vulnerability.

This document was written by Jared Allar.

Other Information

Date Public:	2011-05-25
Date First Published:	2011-05-25
Date Last Updated:	2011-05-25
CERT Advisory:
CVE-ID(s):	CVE-2011-0766
NVD-ID(s):	CVE-2011-0766
US-CERT Technical Alerts:
Severity Metric:	2.74
Document Revision:	14