There is a dash vulnerability on Debian, please following the steps below to check your system:

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1. Log into your server as root
2. Execute the following command:

env x='() ; echo vulnerable’ bash -c “echo this is a test”

3. If the output is “this is a test”, that means that your bash needs to be updated. Please refer to your specific OS below and update your system and bash to latest version.

and

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1. Log into your server as root
2. Execute the following command:

export dummy='() ; echo “exploited”‘

1. Enter the following command afterwards:
   bash

2. If the output is “exploited”, that means that your bash needs to be updated.

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On my Debian, I ran apt-get update && apt-get -y upgrade to update my system to latest version, then found my blog inaccessible, it gave me 502 Bad Gateway error.

I finally figured it out in http://jvdc.me/fix-502-bad-gateway-error-on-nginx-server-after-upgrading-php/.