CAPEC-39: Manipulating Opaque Client-based Data Tokens
Manipulating Opaque Client-based Data Tokens
Definition in a New
Attack Pattern ID: 39
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.
Attack Execution Flow
Enumerate information passed to client side:
The attacker identifies the parameters used as part of tokens to take business or security decisions
Attack Step Techniques
Attack Step Technique Description
Use WebScarab to reveal hidden fields while browsing.
Use a sniffer to capture packets
env-ClientServer env-Peer2Peer env-CommProtocol
View source of web page to find hidden fields
Examine URL to see if any opaque tokens are in it
Disassemble or decompile client-side application
Use debugging tools such as File Monitor, Registry Monitor, Debuggers, etc.
Detailed error message describing problem with token, received from server
Security Control Description
Unexpected/invalid token/parameter value in application logs on server
Reset session upon receipt of unexpected/invalid token/parameter value
An attacker already has some access to the system or can steal the client based data tokens from another user who has access to the system.
For an Attacker to viably execute this attack, some data (later interpreted by the application) must be held client-side in a way that can be manipulated without detection. This means that the data or tokens are not CRCd as part of their value or through a separate meta-data store elsewhere.
Typical Likelihood of Exploit
Likelihood: Very High
Methods of Attack
Modification of Resources
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, he/she proceeds.
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.
The Attacker needs no special hardware-based resources in order to conduct this attack. Software plugins, such as Tamper Data for Firefox, may help in manipulating URL- or cookie-based data.
Tamper with the client side data token and observe the effects it has on interaction with the system.
This attack is in and of itself a trial-and-error-based probing technique.
Solutions and Mitigations
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.
Make sure to protect client side authentication tokens for confidentiality (encryption) and integrity (signed hash)
Make sure that all session tokens use a good source of randomness
Perform validation on the server side to make sure that client side data tokens are consistent with what is expected.
Sensitive information stored client side must be integrity checked upon return before use
Related Security Principles
Reluctance to Trust
Never Assuming that your Secrets are Safe
Never Use Unvalidated Input as Part of a Directive to any Internal Component
Confidentiality Impact: High
Integrity Impact: High
Availability Impact: Low
CAPEC Content Team
The MITRE Corporation
CAPEC Content Team
The MITRE Corporation
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Page Last Updated or Reviewed:
December 07, 2015
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