COMMON WEAKNESS ENUMERATIONDraft

CWE-134

Use of Externally-Controlled Format String

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

Abstraction

Base

Structure

Simple

Likelihood of Exploit

High

Understand the problem

What is the problem?

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

Root cause

How and when does the problem arise?

Lifecycle phase

Implementation

The programmer rarely intends for a format string to be externally-controlled at all. This weakness is frequently introduced in code that constructs log messages, where a constant format string is omitted.

Lifecycle phase

Implementation

In cases such as localization and internationalization, the language-specific message repositories could be an avenue for exploitation, but the format string issue would be resultant, since attacker control of those repositories would also allow modification of message length, format, and content.

Risk

What does successful exploitation lead to?

Read Memory

Affected security scopes: Confidentiality

Format string problems allow for information disclosure which can severely simplify exploitation of the program.
Modify MemoryExecute Unauthorized Code or Commands

Affected security scopes: Integrity, Confidentiality, Availability

Format string problems can result in the execution of arbitrary code, buffer overflows, denial of service, or incorrect data representation.

Defence

How is it prevented and fixed?

Requirements

Security requirement

Detail
Choose a language that is not subject to this flaw.

Implementation

Implementation-phase defence

Detail
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

Build and Compilation aşaması savunması

Detail
Run compilers and linkers with high warning levels, since they may detect incorrect usage.

Verification

How is it detected?

Automated Static Analysis

This weakness can often be detected using automated static analysis tools. Many modern tools use data flow analysis or constraint-based techniques to minimize the number of false positives.

Black Box

Limited
Since format strings often occur in rarely-occurring erroneous conditions (e.g. for error message logging), they can be difficult to detect using black box methods. It is highly likely that many latent issues exist in executables that do not have associated source code (or equivalent source.

Automated Static Analysis - Binary or Bytecode

High
According to SOAR [REF-1479], the following detection techniques may be useful: ``` Highly cost effective: ``` Bytecode Weakness Analysis - including disassembler + source code weakness analysis Binary Weakness Analysis - including disassembler + source code weakness analysis ``` Cost effective for partial coverage: ``` Binary / Bytecode simple extractor - strings, ELF readers, etc.

Manual Static Analysis - Binary or Bytecode

SOAR Partial
According to SOAR [REF-1479], the following detection techniques may be useful: ``` Cost effective for partial coverage: ``` Binary / Bytecode disassembler - then use manual analysis for vulnerabilities & anomalies

Dynamic Analysis with Automated Results Interpretation

SOAR Partial
According to SOAR [REF-1479], the following detection techniques may be useful: ``` Cost effective for partial coverage: ``` Web Application Scanner Web Services Scanner Database Scanners

Dynamic Analysis with Manual Results Interpretation

SOAR Partial
According to SOAR [REF-1479], the following detection techniques may be useful: ``` Cost effective for partial coverage: ``` Fuzz Tester Framework-based Fuzzer

Manual Static Analysis - Source Code

High
According to SOAR [REF-1479], the following detection techniques may be useful: ``` Highly cost effective: ``` Manual Source Code Review (not inspections) ``` Cost effective for partial coverage: ``` Focused Manual Spotcheck - Focused manual analysis of source

Automated Static Analysis - Source Code

High
According to SOAR [REF-1479], the following detection techniques may be useful: ``` Highly cost effective: ``` Source code Weakness Analyzer Context-configured Source Code Weakness Analyzer ``` Cost effective for partial coverage: ``` Warning Flags

Architecture or Design Review

High
According to SOAR [REF-1479], the following detection techniques may be useful: ``` Highly cost effective: ``` Formal Methods / Correct-By-Construction ``` Cost effective for partial coverage: ``` Inspection (IEEE 1028 standard) (can apply to requirements, design, source code, etc.)

MITRE diagram

Weakness relationships and flow

CWE-134 MITRE diagramı