DenyHosts: Denial of service

Background

DenyHosts is designed to monitor SSH servers for repeated failed login attempts.

Description

Tavis Ormandy of the Gentoo Linux Security Audit Team discovered that DenyHosts used an incomplete regular expression to parse failed login attempts.

Impact

A remote unauthenticated attacker can add arbitrary hosts to the blacklist by attempting to login with a specially crafted username. An attacker may use this to prevent legitimate users from accessing a host remotely.

Workaround

There is no known workaround at this time.

Resolution

All DenyHosts users should upgrade to the latest version:

 # emerge --sync
 # emerge --ask --oneshot --verbose ">=app-admin/denyhosts-2.6"