CWE-134 使用外部控制的格式字符串

Use of Externally-Controlled Format String

结构: Simple

Abstraction: Base

状态: Draft

被利用可能性: High

基本描述

The software uses a function that accepts a format string as an argument, but the format string originates from an external source.

扩展描述

When an attacker can modify an externally-controlled format string, this can lead to buffer overflows, denial of service, or data representation problems.

It should be noted that in some circumstances, such as internationalization, the set of format strings is externally controlled by design. If the source of these format strings is trusted (e.g. only contained in library files that are only modifiable by the system administrator), then the external control might not itself pose a vulnerability.

相关缺陷

• cwe_Nature: ChildOf cwe_CWE_ID: 668 cwe_View_ID: 1000 cwe_Ordinal: Primary

• cwe_Nature: ChildOf cwe_CWE_ID: 668 cwe_View_ID: 1003 cwe_Ordinal: Primary

• cwe_Nature: ChildOf cwe_CWE_ID: 668 cwe_View_ID: 699 cwe_Ordinal: Primary

• cwe_Nature: CanPrecede cwe_CWE_ID: 123 cwe_View_ID: 1000

• cwe_Nature: ChildOf cwe_CWE_ID: 20 cwe_View_ID: 700 cwe_Ordinal: Primary

适用平台

Language: [{'cwe_Name': 'C', 'cwe_Prevalence': 'Often'}, {'cwe_Name': 'C++', 'cwe_Prevalence': 'Often'}, {'cwe_Name': 'Perl', 'cwe_Prevalence': 'Rarely'}]

常见的影响

检测方法

DM-1 Automated Static Analysis

Black Box

Automated Static Analysis - Binary or Bytecode

Manual Static Analysis - Binary or Bytecode

Dynamic Analysis with Automated Results Interpretation

Dynamic Analysis with Manual Results Interpretation

Manual Static Analysis - Source Code

Automated Static Analysis - Source Code

Architecture or Design Review

可能的缓解方案

Requirements

策略:

Choose a language that is not subject to this flaw.

Implementation

策略:

Ensure that all format string functions are passed a static string which cannot be controlled by the user and that the proper number of arguments are always sent to that function as well. If at all possible, use functions that do not support the %n operator in format strings. [REF-116] [REF-117]

Build and Compilation

策略:

Heed the warnings of compilers and linkers, since they may alert you to improper usage.

示例代码

例

The following program prints a string provided as an argument.

bad C

The example is exploitable, because of the call to printf() in the printWrapper() function. Note: The stack buffer was added to make exploitation more simple.

例

The following code copies a command line argument into a buffer using snprintf().

bad C

This code allows an attacker to view the contents of the stack and write to the stack using a command line argument containing a sequence of formatting directives. The attacker can read from the stack by providing more formatting directives, such as %x, than the function takes as arguments to be formatted. (In this example, the function takes no arguments to be formatted.) By using the %n formatting directive, the attacker can write to the stack, causing snprintf() to write the number of bytes output thus far to the specified argument (rather than reading a value from the argument, which is the intended behavior). A sophisticated version of this attack will use four staggered writes to completely control the value of a pointer on the stack.

例

Certain implementations make more advanced attacks even easier by providing format directives that control the location in memory to read from or write to. An example of these directives is shown in the following code, written for glibc:

bad C

This code produces the following output: 5 9 5 5 It is also possible to use half-writes (%hn) to accurately control arbitrary DWORDS in memory, which greatly reduces the complexity needed to execute an attack that would otherwise require four staggered writes, such as the one mentioned in the first example.

分析过的案例

Notes

Applicable Platform

Other

Research Gap
Format string issues are under-studied for languages other than C. Memory or disk consumption, control flow or variable alteration, and data corruption may result from format string exploitation in applications written in other languages such as Perl, PHP, Python, etc.

分类映射

相关攻击模式

• CAPEC-135
• CAPEC-67

引用

• REF-116 Format String Vulnerabilities in Perl Programs

• REF-117 Programming Language Format String Vulnerabilities

• REF-118 Format String Attacks

• REF-7 Writing Secure Code

• REF-44 24 Deadly Sins of Software Security

• REF-62 The Art of Software Security Assessment

• REF-962 Automated Source Code Security Measure (ASCSM)

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