CBC Bit-flipping Attack

Overview Given an encrypted value, CBC Bit Flipping can be used to alter the decrypted plain text. If the plain text is used by the system to make decisions (such as the user privilege level), the system can be influenced by CBC bit flipping. Discovery Methodology Toggle bits and bytes of initialization vectors and observe changes to the plain text. Exploitation Alter the initialization vector only in the bits or bytes that affect the target plain text. Small amounts of plain text can be brute forced. For better results, calculate the value of initialization vector needed to produce the plain text desired. Example The view user privilege level page is vulnerable to cipher block chaining (CBC) bit flipping. The goal is to modify the initialization vector (IV) in order to cause the user ID and group ID to both be "000". When this occurs a message will appear. Note that in security level 0 the user ID and group ID are already "100". Only the first character ("1") needs to be modified. Try to leave the "00" alone. First, determine which of the bytes affects the user ID and group ID respectively. Modify each byte until the user ID and group ID are affected. Note the position of the bytes carefully. One byte in the IV will affect the "1" in the user ID and another byte will affect the "1" in the group ID. A byte can only have 255 distinct values. One way to solve this problem is to brute force the answer by trying all 255 bytes until a "0" appears where the "1" is currently shown. A much better way is to XOR the value you input with the value that appears in the User ID or Group ID. This is the respective byte of the cipher text. Next, XOR this byte of cipher text with the byte you want to appear; "0" which is 0X30. Input this result into the IV in the same position that was tampered with to modify the "1" in the User ID or Group ID. The answer is "6bc24fc1aa650b24b4114e93a98f1eba" for security level 0 but not for security level 1. Note the initial user ID in security level 1 is more challenging to change correctly. Videos Warning: Could not reach YouTube via network connection. Failed to embed video. Introduction to CBC Bit Flipping Attack: Visit YouTube Site