COMMON ATTACK PATTERN ENUMERATION AND CLASSIFICATIONLive MITRE record

CAPEC-135

Format String Injection

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.

Abstraction

Standard

Typical severity

High

Likelihood of attack

High

Understand the attack

What is this attack pattern?

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.

Execution flow

How does the attack proceed, step by step?

1

Explore

Survey application

The adversary takes an inventory of the entry points of the application.

Techniques used

  • 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.

Techniques used

  • 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.

Techniques used

  • Insert various formatting characters to read or write the memory, e.g. overwrite return address, etc.

Requirements

What does the attack require?

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

  • 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.

Impact

What does a successful attack lead to?

Modify Data

Affected scopes: Integrity

Read Data

Affected scopes: Confidentiality

Gain Privileges

Affected scopes: Access Control

Execute Unauthorized Commands

Affected scopes: Integrity

Bypass Protection Mechanism

Affected scopes: Access Control

Defence

How is it prevented and mitigated?

1
Limit the usage of formatting string functions.
2
Strong input validation - All user-controllable input must be validated and filtered for illegal formatting characters.

Real world

MITRE example instances

Example instance 1Detail
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