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Improper Null Termination

Incomplete Base
Structure: Simple
Description

This weakness occurs when software fails to properly end a string or array with the required null character or equivalent terminator.

Extended Description

Improper null termination typically stems from two common coding mistakes. The first is an off-by-one error, where a null character is written just outside the allocated memory boundary, which can corrupt adjacent data or trigger a buffer overflow. The second frequent cause is the incorrect use of functions like `strncpy()`, which does not automatically append a null terminator if the source string is too long, leaving the destination array without a valid end marker. Without this crucial terminator, string-handling functions will continue reading memory until they encounter a null byte by chance, leading to information disclosure, crashes, or unpredictable behavior. Developers should always ensure buffers are sized to hold the content plus the terminator and prefer safer, bounded string functions that guarantee proper termination.
Common Consequences 4
Scope: ConfidentialityIntegrityAvailability

Impact: Read MemoryExecute Unauthorized Code or Commands

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.

Scope: ConfidentialityIntegrityAvailability

Impact: DoS: Crash, Exit, or RestartRead MemoryDoS: Resource Consumption (CPU)DoS: Resource Consumption (Memory)

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.

Scope: IntegrityAvailability

Impact: Modify MemoryDoS: Crash, Exit, or Restart

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.

Scope: IntegrityConfidentialityAvailabilityAccess ControlOther

Impact: Alter Execution LogicExecute Unauthorized Code or Commands

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.
Detection Methods 1
Automated Static AnalysisHigh
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.)
Potential Mitigations 5
Phase: Requirements
Use a language that is not susceptible to these issues. However, be careful of null byte interaction errors (Null Byte Interaction Error (Poison Null Byte)) with lower-level constructs that may be written in a language that is susceptible.
Phase: Implementation
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.
Phase: Implementation
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.
Phase: Implementation
Switch to bounded string manipulation functions. Inspect buffer lengths involved in the buffer overrun trace reported with the defect.
Phase: Implementation
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).
Demonstrative Examples 3
The following code reads from cfgfile and copies the input into inputbuf using strcpy(). The code mistakenly assumes that inputbuf will always contain a NULL terminator.

Code Example:

Bad
C
c
The code above will behave correctly if the data read from cfgfile is null terminated on disk as expected. But if an attacker is able to modify this input so that it does not contain the expected NULL character, the call to strcpy() will continue copying from memory until it encounters an arbitrary NULL character. This will likely overflow the destination buffer and, if the attacker can control the contents of memory immediately following inputbuf, can leave the application susceptible to a buffer overflow attack.
In the following code, readlink() expands the name of a symbolic link stored in pathname and puts the absolute path into buf. The length of the resulting value is then calculated using strlen().

Code Example:

Bad
C
c
The code above will not always behave correctly as readlink() does not append a NULL byte to buf. Readlink() will stop copying characters once the maximum size of buf has been reached to avoid overflowing the buffer, this will leave the value buf not NULL terminated. In this situation, strlen() will continue traversing memory until it encounters an arbitrary NULL character further on down the stack, resulting in a length value that is much larger than the size of string. Readlink() does return the number of bytes copied, but when this return value is the same as stated buf size (in this case MAXPATH), it is impossible to know whether the pathname is precisely that many bytes long, or whether readlink() has truncated the name to avoid overrunning the buffer. In testing, vulnerabilities like this one might not be caught because the unused contents of buf and the memory immediately following it may be NULL, thereby causing strlen() to appear as if it is behaving correctly.
While the following example is not exploitable, it provides a good example of how nulls can be omitted or misplaced, even when "safe" functions are used:

Code Example:

Bad
C
c
The above code gives the following output: "The last character in shortString is: n (6e)". So, the shortString array does not end in a NULL character, even though the "safe" string function strncpy() was used. The reason is that strncpy() does not impliciitly add a NULL character at the end of the string when the source is equal in length or longer than the provided size.
Observed Examples 6
CVE-2000-0312Attacker does not null-terminate argv[] when invoking another program.
CVE-2003-0777Interrupted step causes resultant lack of null termination.
CVE-2004-1072Fault causes resultant lack of null termination, leading to buffer expansion.
CVE-2001-1389Multiple vulnerabilities related to improper null termination.
CVE-2003-0143Product does not null terminate a message buffer after snprintf-like call, leading to overflow.
CVE-2009-2523Chain: product does not handle when an input string is not NULL terminated (Improper Null Termination), leading to buffer over-read (Out-of-bounds Read) or heap-based buffer overflow (Heap-based Buffer Overflow).
Likelihood of Exploit

Medium

Applicable Platforms
Languages:
C : UndeterminedC++ : Undetermined
Modes of Introduction
Implementation
Related Weaknesses
ChildOf: Improper Neutralization (CWE-707)
CanPrecede: Buffer Copy without Checking Size of Input ('Classic Buffer Overflow') (CWE-120)
CanPrecede: Buffer Over-read (CWE-126)
CanAlsoBe: Improper Neutralization of Input Terminators (CWE-147)
PeerOf: Addition of Data Structure Sentinel (CWE-464)
PeerOf: Deletion of Data Structure Sentinel (CWE-463)
ChildOf: Improper Input Validation (CWE-20)
Taxonomy Mapping
  • PLOVER
  • 7 Pernicious Kingdoms
  • CLASP
  • OWASP Top Ten 2004
  • CERT C Secure Coding
  • CERT C Secure Coding
  • CERT C Secure Coding
  • Software Fault Patterns
Notes
RelationshipFactors: this is usually resultant from other weaknesses such as off-by-one errors, but it can be primary to boundary condition violations such as buffer overflows. In buffer overflows, it can act as an expander for assumed-immutable data.
RelationshipOverlaps missing input terminator.
Applicable Platform Conceptually, this does not just apply to the C language; any language or representation that involves a terminator could have this type of problem.
MaintenanceAs currently described, this entry is more like a category than a weakness.