CWE-170: Improper Null Termination
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Edit Custom FilterThe product does not terminate or incorrectly terminates a string or array with a null character or equivalent terminator.
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.
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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
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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.
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Relevant to the view "Research Concepts" (CWE-1000)
Relevant to the view "Software Development" (CWE-699)
Relevant to the view "Seven Pernicious Kingdoms" (CWE-700)
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Languages C (Undetermined Prevalence) C++ (Undetermined Prevalence) 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: C
#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: C
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: C
#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.
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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.
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