Bash shell has a serious vulnerability that can be exploited by
an remote attacker in some cases.
To check if you are using bash can be done with entering the following at the shell:
echo $0
or
echo $SHELL
If it prints bash, then you are using bash.
Debian Wheezy is using dash by default since version 6.10, so it should not be vulnerable.
If your version of bash is vulnerable can be tested by entering the following in bash:
env X='() { (a)=>\' bash -c "echo echo vuln"; [[ "$(cat echo)" == "vuln" ]] && echo "still vulnerable :("