Anomaly - avconv tool getting corrupted randomly

I am facing a strange problem with avconv which is a tool for processing videos part of libav-tools package.

I am on a local ubuntu server 14.04.1 LTS.

What happens is that the avconv tool is getting corrupted, basically /usr/bin/avconv is getting modified by something.

This is the output the file command gives when the command is not yet corrupted

avconv: ELF 64-bit LSB  executable, x86-64, version 1 (SYSV), dynamically linked (uses shared libs), for GNU/Linux 2.6.24, BuildID[sha1]=66393d9b38a75978d8cd1838e7b163dda04df7b9, stripped

When it's corrupted this is the output of file command:

avconv-corrupted-copy-1: ELF 64-bit
avconv-corrupted-copy-2: ELF 64-bit LSB  no file type, SPARC

There is a beanstalkd worker which uses the avconv command.

Sometimes I can see bash fork bombs (:(){ :|: & };:) in bash history.

It looks like this vm is completely compromised. Do you've any suggestions on how to understand the cause of this problem?

I've reported it on launchpad too.