COMMON WEAKNESS ENUMERATIONIncomplete

CWE-170

Improper Null Termination

The product does not terminate or incorrectly terminates a string or array with a null character or equivalent terminator.

Abstraction

Base

Structure

Simple

Likelihood of Exploit

Medium

Understand the problem

What is the problem?

The product does not terminate or incorrectly terminates a string or array with a null character or equivalent terminator.

Extended description

Null termination errors frequently occur in two different ways. An off-by-one error could cause a null to be written out of bounds, leading to an overflow. Or, a program could use a strncpy() function call incorrectly, which prevents a null terminator from being added at all. Other scenarios are possible.

Root cause

How and when does the problem arise?

Lifecycle phase

Implementation

Risk

What does successful exploitation lead to?

Read MemoryExecute Unauthorized Code or Commands

Affected security scopes: Confidentiality, Integrity, Availability

The case of an omitted null character is the most dangerous of the possible issues. This will almost certainly result in information disclosure, and possibly a buffer overflow condition, which may be exploited to execute arbitrary code.
DoS: Crash, Exit, or RestartRead MemoryDoS: Resource Consumption (CPU)DoS: Resource Consumption (Memory)

Affected security scopes: Confidentiality, Integrity, Availability

If a null character is omitted from a string, then most string-copying functions will read data until they locate a null character, even outside of the intended boundaries of the string. This could: cause a crash due to a segmentation fault cause sensitive adjacent memory to be copied and sent to an outsider trigger a buffer overflow when the copy is being written to a fixed-size buffer.
Modify MemoryDoS: Crash, Exit, or Restart

Affected security scopes: Integrity, Availability

Misplaced null characters may result in any number of security problems. The biggest issue is a subset of buffer overflow, and write-what-where conditions, where data corruption occurs from the writing of a null character over valid data, or even instructions. A randomly placed null character may put the system into an undefined state, and therefore make it prone to crashing. A misplaced null character may corrupt other data in memory.
Alter Execution LogicExecute Unauthorized Code or Commands

Affected security scopes: Integrity, Confidentiality, Availability, Access Control, Other

Should the null character corrupt the process flow, or affect a flag controlling access, it may lead to logical errors which allow for the execution of arbitrary code.

Defence

How is it prevented and fixed?

Requirements

Security requirement

Detail
Use a language that is not susceptible to these issues. However, be careful of null byte interaction errors (CWE-626) with lower-level constructs that may be written in a language that is susceptible.

Implementation

Implementation-phase defence

Detail
Ensure that all string functions used are understood fully as to how they append null characters. Also, be wary of off-by-one errors when appending nulls to the end of strings.

Implementation

Implementation-phase defence

Detail
If performance constraints permit, special code can be added that validates null-termination of string buffers, this is a rather naive and error-prone solution.

Implementation

Implementation-phase defence

Detail
Switch to bounded string manipulation functions. Inspect buffer lengths involved in the buffer overrun trace reported with the defect.

Implementation

Implementation-phase defence

Detail
Add code that fills buffers with nulls (however, the length of buffers still needs to be inspected, to ensure that the non null-terminated string is not written at the physical end of the buffer).

Verification

How is it detected?

Automated Static Analysis

High
Automated static analysis, commonly referred to as Static Application Security Testing (SAST), can find some instances of this weakness by analyzing source code (or binary/compiled code) without having to execute it. Typically, this is done by building a model of data flow and control flow, then searching for potentially-vulnerable patterns that connect "sources" (origins of input) with "sinks" (destinations where the data interacts with external components, a lower layer such as the OS, etc.)