Common Attack Pattern Enumeration and Classification
A Community Resource for Identifying and Understanding Attacks
When web services require callees to authenticate, they sometimes issue a token / secret to the caller that the caller is to use to sign their web service calls. In one such scheme the caller when constructing a request would concatenate all of the parameters passed to the web service with the provided authentication token and then generate a hash of the concatenated string (e.g., MD5, SHA1, etc.). That hash then forms the signature that is passed to the web service which is used on the server side to verify the origin authenticity and integrity of the message. There is a practical attack against an authentication scheme of this nature that makes use of the hash function extension / padding weakness. Leveraging this weakness, an attacker, who does not know the secret token, is able to modify the parameters passed to the web service by generating their own call and still generate a legitimate signature hash. For instance, consider the message to be passed to the web service is M (this message includes the parameters passed to the web service concatenated with the secret token / key bytes). The message M is hashed and that hash is passed to the web service and is used for authentication. The attacker does not know M, but can see Hash (M) and Length (M). The attacker can then compute Hash (M || Padding (M) || M') for any M'. The attacker does not know the entire message M, specifically the attacker does not know the secret bytes, but that does not matter. The attacker is still able to sign their own message M' and make the called web service verify the integrity of the message without an error. Because of the iterative design of the hash function, it is possible, from only the hash of a message and its length, to compute the hash of longer messages that start with the initial message and include the padding required for the initial message to reach a multiple of 512 bits. It is important to note that the attack not limited to MD5 and will work just as well with another hash function like SHA1.
The table below shows the other attack patterns and high level categories that are related to this attack pattern. These relationships are defined as ChildOf and ParentOf, and give insight to similar items that may exist at higher and lower levels of abstraction. In addition, relationships such as CanFollow, PeerOf, and CanAlsoBe are defined to show similar attack patterns that the user may want to explore.
The table below shows the views that this attack pattern belongs to and top level categories within that view.
Access to a function to produce a hash (e.g., MD5, SHA1) Tools that allow the attacker to intercept a message between the client and the server, specifically the hash that is the signature and the length of the original message concatenated with the secret bytes
A Related Weakness relationship associates a weakness with this attack pattern. Each association implies a weakness that must exist for a given attack to be successful. If multiple weaknesses are associated with the attack pattern, then any of the weaknesses (but not necessarily all) may be present for the attack to be successful. Each related weakness is identified by a CWE identifier.
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