CAPEC Details
Name Format String Injection
Likelyhood of attack Typical severity
High High
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.
  • Spider web sites for all available links
  • List parameters, external variables, configuration files variables, etc. that are possibly used by the application.
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.
  • Inject probe payload which contains formatting characters (%s, %d, %n, etc.) through input parameters.
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.
  • Insert various formatting characters to read or write the memory, e.g. overwrite return address, etc.
Solutions 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 ID Description
CWE-20 Improper Input Validation
CWE-74 Improper Neutralization of Special Elements in Output Used by a Downstream Component ('Injection')
CWE-133 String Errors
CWE-134 Use of Externally-Controlled Format String
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: OWASP Attacks
Entry ID Entry Name
Link Format string attack