CAPEC-135 Metadata
Likelihood of Attack
High
Typical Severity
High
Overview
Summary
An adversary includes formatting characters in a string input field on the target application. Most applications assume that users will provide static text and may respond unpredictably to the presence of formatting character. For example, in certain functions of the C programming languages such as printf, the formatting character %s will print the contents of a memory location expecting this location to identify a string and the formatting character %n prints the number of DWORD written in the memory. An adversary can use this to read or write to memory locations or files, or simply to manipulate the value of the resulting text in unexpected ways. Reading or writing memory may result in program crashes and writing memory could result in the execution of arbitrary code if the adversary can write to the program stack.
Prerequisites
The target application must accept a strings as user input, fail to sanitize string formatting characters in the user input, and process this string using functions that interpret string formatting characters.
Execution Flow
| Step | Phase | Description | Techniques |
|---|---|---|---|
| 1 | Explore | [Survey application] The adversary takes an inventory of the entry points of the application. |
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| 2 | Experiment | [Determine user-controllable input susceptible to format string injection] Determine the user-controllable input susceptible to format string injection. For each user-controllable input that the adversary suspects is vulnerable to format string injection, attempt to inject formatting characters such as %n, %s, etc.. The goal is to manipulate the string creation using these formatting characters. |
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| 3 | Exploit | [Try to exploit the Format String Injection vulnerability] After determining that a given input is vulnerable to format string injection, hypothesize what the underlying usage looks like and the associated constraints. |
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Potential Solutions / Mitigations
Limit the usage of formatting string functions. Strong input validation - All user-controllable input must be validated and filtered for illegal formatting characters.
Related Weaknesses (CWE)
Related CAPECs
| CAPEC ID | Description |
|---|---|
| CAPEC-137 | An adversary manipulates the content of request parameters for the purpose of undermining the security of the target. Some parameter encodings use text characters as separators. For example, parameters in a HTTP GET message are encoded as name-value pairs separated by an ampersand (&). If an attacker can supply text strings that are used to fill in these parameters, then they can inject special characters used in the encoding scheme to add or modify parameters. For example, if user input is fed directly into an HTTP GET request and the user provides the value "myInput&new_param=myValue", then the input parameter is set to myInput, but a new parameter (new_param) is also added with a value of myValue. This can significantly change the meaning of the query that is processed by the server. Any encoding scheme where parameters are identified and separated by text characters is potentially vulnerable to this attack - the HTTP GET encoding used above is just one example. |
Taxonomy Mappings
Taxonomy: OWASP Attacks
| Entry ID | Entry Name |
|---|---|
| Link | Format string attack |
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