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

What CVEs are caused by CWE-170?

43 recorded CVEs are attributed to CWE-170, including CVE-2021-1418, CVE-2021-1417, CVE-2021-1411.

Is CWE-170 part of the OWASP Top 10?

CWE-170 maps to OWASP Top Ten 2004: Denial of Service (A9) in the OWASP security taxonomy.

How do you prevent CWE-170?

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.

How is CWE-170 detected?

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.)

What are the consequences of CWE-170?

Exploiting CWE-170 can lead to: Read Memory, Execute Unauthorized Code or Commands, DoS: Crash, Exit, or Restart, DoS: Resource Consumption (CPU), DoS: Resource Consumption (Memory), Modify Memory.

Is CWE-170 actively exploited?

43 recorded CVEs are caused by CWE-170; none are currently in CISA's KEV catalog of actively exploited flaws.

CWE-170: Improper Null Termination

Code examples

Illustrative examples from MITRE showing how the weakness appears in code.

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.

Vulnerable example

#define MAXLEN 1024

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().

Vulnerable example

char buf[MAXPATH];

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:

Vulnerable example

#include <stdio.h>

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.

CWE-170: Improper Null Termination

Overview

Real-world CVEs

Common consequences

How it happens

When it is introduced

Applies to

How to prevent it

How to detect it

Automated Static Analysis

Code examples

Illustrative examples

Terminology & mappings

Mapped taxonomies

Frequently asked questions

References

Stay ahead of CWE-170

Overview

Real-world CVEs

Common consequences

How it happens

When it is introduced

Applies to

How to prevent it

How to detect it

Automated Static Analysis

Code examples

Illustrative examples

Related weaknesses

Parent

Peer

Can precede

Can also be

Related

Terminology & mappings

Mapped taxonomies

Frequently asked questions

References

Stay ahead of CWE-170