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| Home > CAPEC List > CAPEC-39: Manipulating Opaque Client-based Data Tokens (Release 1.7.1) |
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 DescriptionSummary 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 Explore
Experiment
Attack PrerequisitesAn 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 ExploitLikelihood: Very High Methods of Attack
Examples-InstancesDescription 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. Description 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. Description Archangel Weblog 0.90.02 allows remote attackers to bypass authentication by setting the ba_admin cookie to 1. Related Vulnerabilities CVE-2006-0944 Attacker Skills or Knowledge RequiredSkill or Knowledge Level: Medium If the client site token is obfuscated. Skill or Knowledge Level: High If the client site token is encrypted. Resources RequiredThe 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. Probing TechniquesDescription Tamper with the client side data token and observe the effects it has on interaction with the system. Description This attack is in and of itself a trial-and-error-based probing technique. Solutions and MitigationsOne 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 panecea. In particular, any value created by (and therefore encrypted by) the client, which itself is a "malicous" 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. Attack Motivation-Consequences
Related Weaknesses
Related Attack Patterns
Relevant Security RequirementsSensitive information stored client side must be integrity checked upon return before use Related Security Principles
Related Guidelines
Purposes
CIA Impact
Technical Context
Content History
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Page Last Updated:
May 04, 2012
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