CAPEC-461 Details

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.

Ausführungsablauf

[Find a vulnerable web service] The adversary finds a web service that uses a vulnerable authentication scheme, where an authentication token is concatenated with the parameters of a request and then hashed

• Read application documentation to learn about authentication schemes being used
• Observe web service traffic to look for vulnerable authentication schemes

[Attempt adding padding to parameters] An adversary tests if they can simply add padding to the parameters of a request such that the request is technically changed, with the hash remaining the same

• Exploit the hash function extension / padding weakness with only padding to test the weakness

[Add malicious parameters to request] Add malicious parameters to a captured request in addition to what is already present. Do this by exploiting the padding weakness of the hash function and send the request to the web service so that it believes it is authenticated and acts on the extra parameters.

• Exploit the hash function extension / padding weakness by adding malicious parameters to a web service request such that it is still deemed authentic

Voraussetzungen

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

Erforderliche Kenntnisse

Medium level of cryptography knowledge, specifically how iterative hash functions work. This is needed to select proper padding.

Erforderliche Ressourcen

Gegenmaßnahmen

Design: Use a secure message authentication code (MAC) function such as an HMAC-SHA1

Verwandte Schwachstellen

Referenzen

REF-398

Flickr's API Signature Forgery Vulnerability
Thai Duong, Juliano Rizzo.
http://netifera.com/research/flickr_api_signature_forgery.pdf

Einreichung

Änderungen