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Common Weakness Enumeration

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ID

CWE-170: Improper Null Termination

Weakness ID: 170
Vulnerability Mapping: ALLOWEDThis CWE ID may be used to map to real-world vulnerabilities
Abstraction: BaseBase - a weakness that is still mostly independent of a resource or technology, but with sufficient details to provide specific methods for detection and prevention. Base level weaknesses typically describe issues in terms of 2 or 3 of the following dimensions: behavior, property, technology, language, and resource.
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+ Description
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.
+ Relationships
Section HelpThis table shows the weaknesses and high level categories that are related to this weakness. These relationships are defined as ChildOf, ParentOf, MemberOf and give insight to similar items that may exist at higher and lower levels of abstraction. In addition, relationships such as PeerOf and CanAlsoBe are defined to show similar weaknesses that the user may want to explore.
+ Relevant to the view "Research Concepts" (CWE-1000)
NatureTypeIDName
ChildOfPillarPillar - a weakness that is the most abstract type of weakness and represents a theme for all class/base/variant weaknesses related to it. A Pillar is different from a Category as a Pillar is still technically a type of weakness that describes a mistake, while a Category represents a common characteristic used to group related things.707Improper Neutralization
PeerOfBaseBase - a weakness that is still mostly independent of a resource or technology, but with sufficient details to provide specific methods for detection and prevention. Base level weaknesses typically describe issues in terms of 2 or 3 of the following dimensions: behavior, property, technology, language, and resource.463Deletion of Data Structure Sentinel
PeerOfBaseBase - a weakness that is still mostly independent of a resource or technology, but with sufficient details to provide specific methods for detection and prevention. Base level weaknesses typically describe issues in terms of 2 or 3 of the following dimensions: behavior, property, technology, language, and resource.464Addition of Data Structure Sentinel
CanAlsoBeVariantVariant - a weakness that is linked to a certain type of product, typically involving a specific language or technology. More specific than a Base weakness. Variant level weaknesses typically describe issues in terms of 3 to 5 of the following dimensions: behavior, property, technology, language, and resource.147Improper Neutralization of Input Terminators
CanFollowBaseBase - a weakness that is still mostly independent of a resource or technology, but with sufficient details to provide specific methods for detection and prevention. Base level weaknesses typically describe issues in terms of 2 or 3 of the following dimensions: behavior, property, technology, language, and resource.193Off-by-one Error
CanFollowPillarPillar - a weakness that is the most abstract type of weakness and represents a theme for all class/base/variant weaknesses related to it. A Pillar is different from a Category as a Pillar is still technically a type of weakness that describes a mistake, while a Category represents a common characteristic used to group related things.682Incorrect Calculation
CanPrecedeBaseBase - a weakness that is still mostly independent of a resource or technology, but with sufficient details to provide specific methods for detection and prevention. Base level weaknesses typically describe issues in terms of 2 or 3 of the following dimensions: behavior, property, technology, language, and resource.120Buffer Copy without Checking Size of Input ('Classic Buffer Overflow')
CanPrecedeVariantVariant - a weakness that is linked to a certain type of product, typically involving a specific language or technology. More specific than a Base weakness. Variant level weaknesses typically describe issues in terms of 3 to 5 of the following dimensions: behavior, property, technology, language, and resource.126Buffer Over-read
Section HelpThis table shows the weaknesses and high level categories that are related to this weakness. These relationships are defined as ChildOf, ParentOf, MemberOf and give insight to similar items that may exist at higher and lower levels of abstraction. In addition, relationships such as PeerOf and CanAlsoBe are defined to show similar weaknesses that the user may want to explore.
+ Relevant to the view "Software Development" (CWE-699)
NatureTypeIDName
MemberOfCategoryCategory - a CWE entry that contains a set of other entries that share a common characteristic.137Data Neutralization Issues
Section HelpThis table shows the weaknesses and high level categories that are related to this weakness. These relationships are defined as ChildOf, ParentOf, MemberOf and give insight to similar items that may exist at higher and lower levels of abstraction. In addition, relationships such as PeerOf and CanAlsoBe are defined to show similar weaknesses that the user may want to explore.
+ Relevant to the view "Seven Pernicious Kingdoms" (CWE-700)
NatureTypeIDName
ChildOfClassClass - a weakness that is described in a very abstract fashion, typically independent of any specific language or technology. More specific than a Pillar Weakness, but more general than a Base Weakness. Class level weaknesses typically describe issues in terms of 1 or 2 of the following dimensions: behavior, property, and resource.20Improper Input Validation
+ Modes Of Introduction
Section HelpThe different Modes of Introduction provide information about how and when this weakness may be introduced. The Phase identifies a point in the life cycle at which introduction may occur, while the Note provides a typical scenario related to introduction during the given phase.
PhaseNote
Implementation
+ Applicable Platforms
Section HelpThis listing shows possible areas for which the given weakness could appear. These may be for specific named Languages, Operating Systems, Architectures, Paradigms, Technologies, or a class of such platforms. The platform is listed along with how frequently the given weakness appears for that instance.

Languages

C (Undetermined Prevalence)

C++ (Undetermined Prevalence)

+ Common Consequences
Section HelpThis table specifies different individual consequences associated with the weakness. The Scope identifies the application security area that is violated, while the Impact describes the negative technical impact that arises if an adversary succeeds in exploiting this weakness. The Likelihood provides information about how likely the specific consequence is expected to be seen relative to the other consequences in the list. For example, there may be high likelihood that a weakness will be exploited to achieve a certain impact, but a low likelihood that it will be exploited to achieve a different impact.
ScopeImpactLikelihood
Confidentiality
Integrity
Availability

Technical Impact: Read Memory; Execute 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.
Confidentiality
Integrity
Availability

Technical Impact: DoS: Crash, Exit, or Restart; Read Memory; DoS: 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.
Integrity
Availability

Technical Impact: Modify Memory; DoS: 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.
Integrity
Confidentiality
Availability
Access Control
Other

Technical Impact: Alter Execution Logic; Execute 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.
+ Likelihood Of Exploit
Medium
+ Demonstrative Examples

Example 1

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.

(bad code)
Example Language:
#define MAXLEN 1024
...
char *pathbuf[MAXLEN];
...
read(cfgfile,inputbuf,MAXLEN); //does not null terminate
strcpy(pathbuf,inputbuf); //requires null terminated input
...

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.

Example 2

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().

(bad code)
Example Language:
char buf[MAXPATH];
...
readlink(pathname, buf, MAXPATH);
int length = strlen(buf);
...

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.

Example 3

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:

(bad code)
Example Language:
#include <stdio.h>
#include <string.h>

int main() {

char longString[] = "String signifying nothing";
char shortString[16];

strncpy(shortString, longString, 16);
printf("The last character in shortString is: %c (%1$x)\n", shortString[15]);
return (0);
}

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
ReferenceDescription
Attacker does not null-terminate argv[] when invoking another program.
Interrupted step causes resultant lack of null termination.
Fault causes resultant lack of null termination, leading to buffer expansion.
Multiple vulnerabilities related to improper null termination.
Product does not null terminate a message buffer after snprintf-like call, leading to overflow.
Chain: product does not handle when an input string is not NULL terminated (CWE-170), leading to buffer over-read (CWE-125) or heap-based buffer overflow (CWE-122).
+ Potential Mitigations

Phase: Requirements

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.

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).
+ Weakness Ordinalities
OrdinalityDescription
Resultant
(where the weakness is typically related to the presence of some other weaknesses)
+ Detection Methods

Automated Static Analysis

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.)

Effectiveness: High

+ Memberships
Section HelpThis MemberOf Relationships table shows additional CWE Categories and Views that reference this weakness as a member. This information is often useful in understanding where a weakness fits within the context of external information sources.
NatureTypeIDName
MemberOfCategoryCategory - a CWE entry that contains a set of other entries that share a common characteristic.730OWASP Top Ten 2004 Category A9 - Denial of Service
MemberOfCategoryCategory - a CWE entry that contains a set of other entries that share a common characteristic.741CERT C Secure Coding Standard (2008) Chapter 8 - Characters and Strings (STR)
MemberOfCategoryCategory - a CWE entry that contains a set of other entries that share a common characteristic.748CERT C Secure Coding Standard (2008) Appendix - POSIX (POS)
MemberOfCategoryCategory - a CWE entry that contains a set of other entries that share a common characteristic.875CERT C++ Secure Coding Section 07 - Characters and Strings (STR)
MemberOfViewView - a subset of CWE entries that provides a way of examining CWE content. The two main view structures are Slices (flat lists) and Graphs (containing relationships between entries).884CWE Cross-section
MemberOfCategoryCategory - a CWE entry that contains a set of other entries that share a common characteristic.973SFP Secondary Cluster: Improper NULL Termination
MemberOfCategoryCategory - a CWE entry that contains a set of other entries that share a common characteristic.1161SEI CERT C Coding Standard - Guidelines 07. Characters and Strings (STR)
MemberOfCategoryCategory - a CWE entry that contains a set of other entries that share a common characteristic.1171SEI CERT C Coding Standard - Guidelines 50. POSIX (POS)
MemberOfCategoryCategory - a CWE entry that contains a set of other entries that share a common characteristic.1306CISQ Quality Measures - Reliability
MemberOfViewView - a subset of CWE entries that provides a way of examining CWE content. The two main view structures are Slices (flat lists) and Graphs (containing relationships between entries).1340CISQ Data Protection Measures
MemberOfCategoryCategory - a CWE entry that contains a set of other entries that share a common characteristic.1407Comprehensive Categorization: Improper Neutralization
+ Vulnerability Mapping Notes

Usage: ALLOWED

(this CWE ID could be used to map to real-world vulnerabilities)

Reason: Acceptable-Use

Rationale:

This CWE entry is at the Base level of abstraction, which is a preferred level of abstraction for mapping to the root causes of vulnerabilities.

Comments:

Carefully read both the name and description to ensure that this mapping is an appropriate fit. Do not try to 'force' a mapping to a lower-level Base/Variant simply to comply with this preferred level of abstraction.
+ Notes

Relationship

Factors: 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.

Relationship

Overlaps 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.

Maintenance

As currently described, this entry is more like a category than a weakness.
+ Taxonomy Mappings
Mapped Taxonomy NameNode IDFitMapped Node Name
PLOVERImproper Null Termination
7 Pernicious KingdomsString Termination Error
CLASPMiscalculated null termination
OWASP Top Ten 2004A9CWE More SpecificDenial of Service
CERT C Secure CodingPOS30-CCWE More AbstractUse the readlink() function properly
CERT C Secure CodingSTR03-CDo not inadvertently truncate a null-terminated byte string
CERT C Secure CodingSTR32-CExactDo not pass a non-null-terminated character sequence to a library function that expects a string
Software Fault PatternsSFP11Improper Null Termination
+ Content History
+ Submissions
Submission DateSubmitterOrganization
2006-07-19
(CWE Draft 3, 2006-07-19)
PLOVER
+ Modifications
Modification DateModifierOrganization
2008-07-01Eric DalciCigital
updated Time_of_Introduction
2008-08-01KDM Analytics
added/updated white box definitions
2008-09-08CWE Content TeamMITRE
updated Applicable_Platforms, Causal_Nature, Common_Consequences, Description, Likelihood_of_Exploit, Maintenance_Notes, Relationships, Other_Notes, Relationship_Notes, Taxonomy_Mappings, Weakness_Ordinalities
2008-11-24CWE Content TeamMITRE
updated Relationships, Taxonomy_Mappings
2009-03-10CWE Content TeamMITRE
updated Common_Consequences
2009-05-27CWE Content TeamMITRE
updated Demonstrative_Examples
2009-07-17KDM Analytics
Improved the White_Box_Definition
2009-07-27CWE Content TeamMITRE
updated Common_Consequences, Other_Notes, Potential_Mitigations, White_Box_Definitions
2009-10-29CWE Content TeamMITRE
updated Description
2011-03-29CWE Content TeamMITRE
updated Common_Consequences
2011-06-01CWE Content TeamMITRE
updated Common_Consequences
2011-09-13CWE Content TeamMITRE
updated Relationships, Taxonomy_Mappings
2012-05-11CWE Content TeamMITRE
updated Relationships
2014-06-23CWE Content TeamMITRE
updated Observed_Examples
2014-07-30CWE Content TeamMITRE
updated Relationships, Taxonomy_Mappings
2017-11-08CWE Content TeamMITRE
updated Causal_Nature, Observed_Examples, Relationships, Taxonomy_Mappings, White_Box_Definitions
2018-03-27CWE Content TeamMITRE
updated Demonstrative_Examples
2019-01-03CWE Content TeamMITRE
updated Relationships
2020-02-24CWE Content TeamMITRE
updated Relationships
2020-08-20CWE Content TeamMITRE
updated Relationships
2020-12-10CWE Content TeamMITRE
updated Relationships
2023-01-31CWE Content TeamMITRE
updated Description
2023-04-27CWE Content TeamMITRE
updated Detection_Factors, Relationships
2023-06-29CWE Content TeamMITRE
updated Mapping_Notes
Page Last Updated: February 29, 2024