CWE-120: Buffer Copy without Checking Size of Input ('Classic Buffer Overflow')

Common consequences

• IntegrityConfidentialityAvailability

  Modify Memory; Execute Unauthorized Code or Commands

  Buffer overflows often can be used to execute arbitrary code, which is usually outside the scope of the product's implicit security policy. This can often be used to subvert any other security service.

• Availability

  Modify Memory; DoS: Crash, Exit, or Restart; DoS: Resource Consumption (CPU)

  Buffer overflows generally lead to crashes. Other attacks leading to lack of availability are possible, including putting the product into an infinite loop.

Potential mitigations

• RequirementsLanguage Selection

  Use a language that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid. For example, many languages that perform their own memory management, such as Java and Perl, are not subject to buffer overflows. Other languages, such as Ada and C#, typically provide overflow protection, but the protection can be disabled by the programmer. Be wary that a language's interface to native code may still be subject to overflows, even if the language itself is theoretically safe.

• Architecture and DesignLibraries or Frameworks

  Use a vetted library or framework that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid. Examples include the Safe C String Library (SafeStr) by Messier and Viega, and the Strsafe.h library from Microsoft. These libraries provide safer versions of overflow-prone string-handling functions.

• OperationBuild and CompilationEnvironment HardeningEffectiveness: Defense in Depth

  Use automatic buffer overflow detection mechanisms that are offered by certain compilers or compiler extensions. Examples include: the Microsoft Visual Studio /GS flag, Fedora/Red Hat FORTIFY_SOURCE GCC flag, StackGuard, and ProPolice, which provide various mechanisms including canary-based detection and range/index checking. D3-SFCV (Stack Frame Canary Validation) from D3FEND discusses canary-based detection in detail.

• Implementation

  Consider adhering to the following rules when allocating and managing an application's memory: Double check that your buffer is as large as you specify. When using functions that accept a number of bytes to copy, such as strncpy(), be aware that if the destination buffer size is equal to the source buffer size, it may not NULL-terminate the string. Check buffer boundaries if accessing the buffer in a loop and make sure there is no danger of writing past the allocated space. If necessary, truncate all input strings to a reasonable length before passing them to the copy and concatenation...

• ImplementationInput Validation

  Assume all input is malicious. Use an accept known good input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does. When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, boat may be syntactically valid...

• Architecture and Design

  For any security checks that are performed on the client side, ensure that these checks are duplicated on the server side, in order to avoid CWE-602. Attackers can bypass the client-side checks by modifying values after the checks have been performed, or by changing the client to remove the client-side checks entirely. Then, these modified values would be submitted to the server.

• OperationBuild and CompilationEnvironment HardeningEffectiveness: Defense in Depth

  Run or compile the software using features or extensions that randomly arrange the positions of a program's executable and libraries in memory. Because this makes the addresses unpredictable, it can prevent an attacker from reliably jumping to exploitable code. Examples include Address Space Layout Randomization (ASLR) and Position-Independent Executables (PIE). Imported modules may be similarly realigned if their default memory addresses conflict with other modules, in a process known as rebasing (for Windows) and prelinking (for Linux) using randomly generated addresses. ASLR for libraries...

• OperationEnvironment HardeningEffectiveness: Defense in Depth

  Use a CPU and operating system that offers Data Execution Protection (using hardware NX or XD bits) or the equivalent techniques that simulate this feature in software, such as PaX. These techniques ensure that any instruction executed is exclusively at a memory address that is part of the code segment. For more information on these techniques see D3-PSEP (Process Segment Execution Prevention) from D3FEND.

MITRE lists 5 more mitigations on the official entry.

Related weaknesses

CAPEC attack patterns that exploit it

• CAPEC-8Buffer Overflow in an API Call
• CAPEC-9Buffer Overflow in Local Command-Line Utilities
• CAPEC-10Buffer Overflow via Environment Variables
• CAPEC-14Client-side Injection-induced Buffer Overflow
• CAPEC-24Filter Failure through Buffer Overflow
• CAPEC-42MIME Conversion
• CAPEC-44Overflow Binary Resource File
• CAPEC-45Buffer Overflow via Symbolic Links
• CAPEC-46Overflow Variables and Tags
• CAPEC-47Buffer Overflow via Parameter Expansion
• CAPEC-67String Format Overflow in syslog()
• CAPEC-92Forced Integer Overflow
• CAPEC-100Overflow Buffers