This vulnerability, which is found existing in certain versions of GNU Bourne Again Shell (Bash) can allow an attacker to execute commands on an affected system. It allows for remote code execution on servers that run these Linux distributions. Bash is used by most Unix and Linux systems, as well as OS X. This vulnerability was reportedly being exploited in the wild already. Trend Micro spotted samples, which are actual payload of the said exploit code.
All versions of Bash up to and including version 4.3 are vulnerable.