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CAPEC-135: Format String Injection

Attack Pattern ID: 135
Abstraction: Standard
Status: Draft
Presentation Filter:
+ Description
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.
+ Likelihood Of Attack

High

+ Typical Severity

High

+ Relationships

The table(s) below shows the other attack patterns and high level categories that are related to this attack pattern. These relationships are defined as ChildOf, ParentOf, MemberOf and give insight to similar items that may exist at higher and lower levels of abstraction. In addition, relationships such as CanFollow, PeerOf, and CanAlsoBe are defined to show similar attack patterns that the user may want to explore.

+ Relevant to the view "Mechanisms of Attack" (CAPEC-1000)
NatureTypeIDName
ChildOfMeta Attack PatternMeta Attack Pattern - A meta level attack pattern in CAPEC is a decidedly abstract characterization of a specific methodology or technique used in an attack. A meta attack pattern is often void of a specific technology or implementation and is meant to provide an understanding of a high level approach. A meta level attack pattern is a generalization of related group of standard level attack patterns. Meta level attack patterns are particularly useful for architecture and design level threat modeling exercises.137Parameter Injection
+ Execution Flow
Explore
  1. 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.

    Techniques
    Spider web sites for all available links
    List parameters, external variables, configuration files variables, etc. that are possibly used by the application.
Experiment
  1. 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.

    Techniques
    Inject probe payload which contains formatting characters (%s, %d, %n, etc.) through input parameters.
Exploit
  1. 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.

    Techniques
    Insert various formatting characters to read or write the memory, e.g. overwrite return address, etc.
+ 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.
+ Skills Required
[Level: High]
In order to discover format string vulnerabilities it takes only low skill, however, converting this discovery into a working exploit requires advanced knowledge on the part of the adversary.
+ Resources Required
None: No specialized resources are required to execute this type of attack.
+ Consequences

The table below specifies different individual consequences associated with the attack pattern. The Scope identifies the security property that is violated, while the Impact describes the negative technical impact that arises if an adversary succeeds in their attack. The Likelihood provides information about how likely the specific consequence is expected to be seen relative to the other consequences in the list. For example, there may be high likelihood that a pattern will be used to achieve a certain impact, but a low likelihood that it will be exploited to achieve a different impact.

ScopeImpactLikelihood
Integrity
Modify Data
Confidentiality
Read Data
Integrity
Modify Data
Confidentiality
Read Data
Integrity
Modify Data
Confidentiality
Read Data
Access Control
Gain Privileges
Integrity
Execute Unauthorized Commands
Access Control
Bypass Protection Mechanism
+ Mitigations
Limit the usage of formatting string functions.
Strong input validation - All user-controllable input must be validated and filtered for illegal formatting characters.
+ Example Instances
Untrusted search path vulnerability in the add_filename_to_string function in intl/gettext/loadmsgcat.c for Elinks 0.11.1 allows local users to cause Elinks to use an untrusted gettext message catalog (.po file) in a "../po" directory, which can be leveraged to conduct format string attacks. See also: CVE-2007-2027
+ References
[REF-14] Hal Burch and Brendan Saulsbury. "FIO30-C. Exclude user input from format strings". CERT. 2011-05. <https://www.securecoding.cert.org/confluence/display/seccode/FIO30-C.+Exclude+user+input+from+format+strings>.
[REF-15] Robert Auger. "WASC Threat Classification 2.0". WASC-06 - Format String. The Web Application Security Consortium (WASC). <http://projects.webappsec.org/Format-String>.
[REF-16] Fortify. "The OWASP Application Security Desk Reference". Format String. The Open Web Application Security Project (OWASP). 2010. <https://www.owasp.org/index.php/Format_String>.
+ Content History
Submissions
Submission DateSubmitterOrganization
2014-06-23CAPEC Content TeamThe MITRE Corporation
Modifications
Modification DateModifierOrganization
2017-08-04CAPEC Content TeamThe MITRE Corporation
Updated Attack_Phases, Attacker_Skills_or_Knowledge_Required, Description Summary, Resources_Required
2018-07-31CAPEC Content TeamThe MITRE Corporation
Updated Attack_Motivation-Consequences

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Page Last Updated or Reviewed: July 31, 2018