The most popular full disk encryption solution for Linux is LUKS (Linux Unified Key Setup), which provides an easy to use encryption layer for block devices. By default, newly generated LUKS devices are set up with 256-bit AES in CBC mode. Since there is no integrity protection/checksum, it is obviously possible to destroy parts of plaintext files by changing the corresponding ciphertext blocks. Nevertheless many users expect the encryption to make sure that an attacker can only change the plaintext to an unpredictable random value. The CBC mode used by default in LUKS however allows some more targeted manipulation of the plaintext file given that the attacker knows the original plaintext. This article demonstrates how this can be used to inject a full remote code execution backdoor into an encrypted installation of Ubuntu 12.04 created by the alternate installer (the default installer of Ubuntu 12.04 doesn't allow setting up full disk encryption).
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