Common Attack Pattern Enumeration and Classification
A Community Resource for Identifying and Understanding Attacks
An adversary, aware of an application's location (and possibly authorized to use the application), probes an application's structure and evaluates its robustness by submitting requests and examining responses. Often, this is accomplished by sending variants of expected queries in the hope that these modified queries might return information beyond what the expected set of queries would provide.
Blind SQL injection is an example of this technique, applied to successful exploit.
Attacker sends bad data at various servlets in a J2EE system, records returned exception stack traces, and maps application functionality.
In addition, this technique allows attackers to correlate those servlets used with the underlying open source packages (and potentially version numbers) that provide them.
Skill or Knowledge Level: Medium
Although fuzzing parameters is not difficult, and often possible with automated fuzzers, 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.
The Attacker needs the ability to probe application functionality and provide it erroneous directives or data without triggering intrusion detection schemes or making enough of an impact on application logging that steps are taken against the attacker.
The Attack does not need special hardware, software, skills, or access.
Repeated errors generated by the same piece of code are an indication, although it requires careful monitoring of the application and its associated error logs, if any.
To defeat correlation, the attacker may try changing the origin IP addresses or client browser identification strings or start a new session from where he left off; any technique aimed at defeating the use of certain identification parameters for correlation goes a small way in obfuscating the attack.
Application designers can 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 cataloged 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.
Application designers can 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.
Content, based on application context, crafted to elicit error conditions from the application
The impact of activation is an error condition that, hopefully for the attacker, reveals sufficient information to further map the application.
Custom error pages must be used to handle exceptions such that they do not reveal any information about the architecture of the application or the database.
Employ application-level safeguards to filter data and handle exceptions gracefully.
[R.54.1] [REF-3] "Common Weakness Enumeration (CWE)". CWE-20 - Input Validation. Draft. The MITRE Corporation. 2007. <http://cwe.mitre.org/data/definitions/20.html>.
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