Fix Is In for OpenSSH Flaw

A vulnerability in a popular, free implementation of the Secure Shell protocols that prompted a warning from the suites developers has been quickly capped.

The vulnerability in OpenSSH versions 2.9.9 through 3.3 was the result of an input validation error that enabled an integer overflow and privilege escalation, according to developers. OpenSSH, a free set of network connectivity tools developed by the OpenBSD Project, is frequently used in place of telnet, rlogin and ftp access and comes bundled with OpenBSD and many other Unix operating systems, including the recently released Solaris 9.

The vulnerability was first disclosed on the OpenSSH Web site Tuesday, with a warning that users should enable privilege separation features and prepare to upgrade to OpenSSH 3.4 on Monday, July 1. The security threat was detailed by Internet Security Systems researchers on Wednesday morning, however, prompting an early release on the new SSH suite.

According to the ISS advisory, the vulnerability exists within the “challenge-response” authentication mechanism in the OpenSSH daemon or sshd.

“This mechanism, part of the SSH2 protocol, verifies a users identity by generating a challenge and forcing the user to supply a number of responses. It is possible for a remote attacker to send a specially-crafted reply that triggers an overflow,” ISS researchers wrote. “This can result in a remote denial of service attack on the OpenSSH daemon or a complete remote compromise. The OpenSSH daemon runs with superuser privilege, so remote attackers can gain superuser access by exploiting this vulnerability.”

ISS researchers said they are aware of active development efforts to exploit the vulnerability.

The OpenSSH advisory and patch is at www.openssh.org/txt/preauth.adv.

The initial vulnerability disclosure came just days after the release of the Version 3.3 of the SSH package.

“We believe we have the information contained. It is after all in 27,000 lines of code,” developer Theo de Raadt, founder of the OpenBSD and OpenSSH projects said late Tuesday. “If it does leak out, or a parallel discovery of it happens, we will be ready with an immediate patch.”

Even before the latest vulnerability was disclosed, OpenSSH developers have consistently suggested that users employ the tools privilege separation feature. The feature safeguards against any corruption in the sshd, which could lead to root compromise, according to OpenSSH developers.

OpenSSH encrypts all traffic, including passwords, to thwart eavesdropping, connection hijacking and other network-level attacks, according to developers. In addition, OpenSSH provides secure tunneling capabilities and a variety of authentication methods.

In addition to OpenBSD and FreeBSD, OpenSSH works with dozens of operating systems including most flavors of Linux; NetBSD; Computone; Stallion; MacOS X Version 10.1; HP Procurve Switch 4108GL and 2524/2512; and IBM AIX.