CAPEC Definitions / CAPEC-461
CAPEC-461: Web Services API Signature Forgery Leveraging Hash Function Extension Weakness
An adversary utilizes a hash function extension/padding weakness, to modify the parameters passed to the web service requesting authentication by generating their own call in order to generate a legitimate signature hash (as described in the notes), without knowledge of the secret token sometimes provided by the web service.
Extended Description
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. Because of the iterative design of the hash function, it is possible, from only the hash of a message and its length, for an adversary to conduct signature forgery by computing 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 on other hash functions such as SHA1.
Mitigations
Design: Use a secure message authentication code (MAC) function such as an HMAC-SHA1
Relationships with other CAPECs
CAPEC-115: Authentication Bypass
Prerequisites
Web services check the signature of the API calls
Authentication tokens / secrets are shared between the server and the legitimate client
The API call signature is generated by concatenating the parameter list with the shared secret and hashing the result.
An iterative hash function like MD5 and SHA1 is used.
An attacker is able to intercept or in some other way gain access to the information passed between the legitimate client and the server in order to retrieve the hash value and length of the original message.
The communication channel between the client and the server is not secured via channel security such as TLS
