CAPEC-215 Fuzzing for application mapping

An attacker sends random, malformed, or otherwise unexpected messages to a target application and observes the application's log or error messages returned. The attacker does not initially know how a target will respond to individual messages but by attempting a large number of message variants they may find a variant that trigger's desired behavior. In this attack, the purpose of the fuzzing is to observe the application's log and error messages, although fuzzing a target can also sometimes cause the target to enter an unstable state, causing a crash.

By observing logs and error messages, the attacker can learn details about the configuration of the target application and might be able to cause the target to disclose sensitive information. In applications that return a stack trace along with the error, this can enumerate the chain of methods that led up to the point where the error was encountered. This can not only reveal the names of the methods (some of which may have known weaknesses) but possibly also the location of class files and libraries as well as parameter values. In some cases, the stack trace might even disclose sensitive configuration or user information.

1. [Explore] [Observe communication and inputs] The fuzzing adversary observes the target system looking for inputs and communications between modules, subsystems, or systems. 2. [Experiment] [Generate fuzzed inputs] Given a fuzzing tool, a target input or protocol, and limits on time, complexity, and input variety, generate a list of inputs to try. Although fuzzing is random, it is not exhaustive. Parameters like length, composition, and how many variations to try are important to get the most cost-effective impact from the fuzzer. 3. [Experiment] [Observe the outcome] Observe the outputs to the inputs fed into the system by fuzzers and see if there are any log or error messages that might provide information to map the application 4. [Exploit] [Craft exploit payloads] An adversary usually needs to modify the fuzzing parameters according to the observed error messages to get the desired sensitive information for the application. To defeat correlation, the adversary may try changing the origin IP addresses or client browser identification strings or start a new session from where they left off in obfuscating the attack.

• The target application must fail to sanitize incoming messages adequately before processing.

• [Medium] Although fuzzing parameters is not difficult, and often possible with automated fuzzing tools, interpreting the error conditions and modifying the parameters so as to move further in the process of mapping the application requires detailed knowledge of target platform, the languages and packages used as well as software design.

• Confidentiality: Other

• Design: Construct a 'code book' for error messages. When using a code book, application error messages aren't generated in string or stack trace form, but are catalogued and replaced with a unique (often integer-based) value 'coding' for the error. Such a technique will require helpdesk and hosting personnel to use a 'code book' or similar mapping to decode application errors/logs in order to respond to them normally. • Design: wrap application functionality (preferably through the underlying framework) in an output encoding scheme that obscures or cleanses error messages to prevent such attacks. Such a technique is often used in conjunction with the above 'code book' suggestion. • Implementation: Obfuscate server fields of HTTP response. • Implementation: Hide inner ordering of HTTP response header. • Implementation: Customizing HTTP error codes such as 404 or 500. • Implementation: Hide HTTP response header software information filed. • Implementation: Hide cookie's software information filed. • Implementation: Obfuscate database type in Database API's error message.