DenyHosts, Fail2ban and BlockHosts are vulnerable to remote log injection
that can lead to arbitrarily injection of IP addresses in /etc/hosts.deny. To
make it more "interesting", not only IP addresses can be added, but
also the wild card "all", causing it to block the whole Internet out of the
box (bypassing white lists) -- see DenyHosts exploit example.
The following paper discuss these issues and contain the available
patches for them:
Snippet from the article:
The purpose of this article is to point out some vulnerabilities that
I found on open source log analysis tools aimed to stop brute force
scans against SSH and ftp services. Since these tools also perform
active response (automatically blocking the offending IP address),
they would be good examples. However, any tool that parse logs can be
We will show three 0-day denial-of-service attacks caused by remote
log injection on BlockHosts, DenyHosts and fail2ban.
This paper talks about remote log injection, where an external
attacker can modify a log, based on the input it provides to an
application (in our case OpenSSH and vsftpd). By modifying the way the
application logs, we are able to attack these log analysis tools. We
are not talking about local log modification or "syslog injection".
Links to these tools:
Link to the article:
Daniel B. Cid
dcid ( at ) ossec.net