In circumstances where an application holds important data client-side in tokens (cookies, URLs, data files, and so forth) that data can be manipulated. If client or server-side application components reinterpret that data as authentication tokens or data (such as store item pricing or wallet information) then even opaquely manipulating that data may bear fruit for an Attacker. In this pattern an attacker undermines the assumption that client side tokens have been adequately protected from tampering through use of encryption or obfuscation.

How does a Manipulating Opaque Client-based Data Tokens attack work?

It typically unfolds over 4 phases. It begins with: [Enumerate information passed to client side] The attacker identifies the parameters used as part of tokens to take business or security decisions

How do you prevent CAPEC-39?

One solution to this problem is to protect encrypted data with a CRC of some sort. If knowing who last manipulated the data is important, then using a cryptographic "message authentication code" (or hMAC) is prescribed. However, this guidance is not a panacea. In particular, any value created by (and therefore encrypted by) the client, which itself is a "malicious" value, all the protective cryptography in the world can't make the value 'correct' again. Put simply, if the client has control over the whole process of generating and encoding the value, then simply protecting its integrity doesn't help.

What weaknesses does CAPEC-39 target?

CAPEC-39 exploits 9 CWE weaknesses, including CWE-233 (Improper Handling of Parameters), CWE-285 (Improper Authorization), CWE-302 (Authentication Bypass by Assumed-Immutable Data), CWE-315 (Cleartext Storage of Sensitive Information in a Cookie).

How severe is CAPEC-39?

MITRE rates CAPEC-39 as Medium severity with high likelihood of attack.

CAPEC-39: Manipulating Opaque Client-based Data Tokens

Examples

With certain price watching websites, that aggregate products available prices, the user can buy items through whichever vendors has product availability, the best price, or other differentiator. Once a user selects an item, the site must broker the purchase of that item with the vendor. Because vendors sell the same product through different channel partners at different prices, token exchange between price watching sites and selling vendors will often contain pricing information. With some price watching sites, manipulating URL-data (which is encrypted) even opaquely yields different prices charged by the fulfilling vendor. If the manipulated price turns out higher, the Attacker can cancel purchase. If the Attacker succeeded in manipulating the token and creating a lower price, they proceed.

Upon successful authentication user is granted an encrypted authentication cookie by the server and it is stored on the client. One piece of information stored in the authentication cookie reflects the access level of the user (e.g. "u" for user). The authentication cookie is encrypted using the Electronic Code Book (ECB) mode, that naively encrypts each of the plaintext blocks to each of the ciphertext blocks separately. An attacker knows the structure of the cookie and can figure out what bits (encrypted) store the information relating to the access level of the user. An attacker modifies the authentication cookie and effectively substitutes "u" for "a" by flipping some of the corresponding bits of ciphertext (trial and error). Once the correct "flip" is found, when the system is accessed, the attacker is granted administrative privileges in the system. Note that in this case an attacker did not have to figure out the exact encryption algorithm or find the secret key, but merely exploit the weakness inherent in using the ECB encryption mode.

Archangel Weblog 0.90.02 allows remote attackers to bypass authentication by setting the ba_admin cookie to 1. See also: CVE-2006-0944

CAPEC-39: Manipulating Opaque Client-based Data Tokens

Overview

How the attack works

What the attacker needs

Prerequisites

Skills required

Resources required

Consequences

How to mitigate it

Examples

Frequently asked questions

References

Defend against CAPEC-39

Overview

How the attack works

What the attacker needs

Prerequisites

Skills required

Resources required

Consequences

How to mitigate it

Examples

Related weaknesses

Related attack patterns

Parent

Child

Frequently asked questions

References

Defend against CAPEC-39