CWE-170 不恰当的空终结符

Improper Null Termination

结构: Simple

Abstraction: Base

状态: Incomplete

被利用可能性: Medium

基本描述

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

扩展描述

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.

相关缺陷

• cwe_Nature: ChildOf cwe_CWE_ID: 707 cwe_View_ID: 1000 cwe_Ordinal: Primary

• cwe_Nature: ChildOf cwe_CWE_ID: 138 cwe_View_ID: 699 cwe_Ordinal: Primary

• cwe_Nature: CanPrecede cwe_CWE_ID: 120 cwe_View_ID: 1000

• cwe_Nature: CanPrecede cwe_CWE_ID: 126 cwe_View_ID: 1000

• cwe_Nature: CanAlsoBe cwe_CWE_ID: 147 cwe_View_ID: 1000

• cwe_Nature: PeerOf cwe_CWE_ID: 464 cwe_View_ID: 1000

• cwe_Nature: PeerOf cwe_CWE_ID: 463 cwe_View_ID: 1000

• cwe_Nature: ChildOf cwe_CWE_ID: 20 cwe_View_ID: 700 cwe_Ordinal: Primary

适用平台

Language: [{'cwe_Name': 'C', 'cwe_Prevalence': 'Undetermined'}, {'cwe_Name': 'C++', 'cwe_Prevalence': 'Undetermined'}]

常见的影响

可能的缓解方案

Requirements

策略:

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

策略:

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

策略:

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

策略:

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

Implementation

策略:

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).

示例代码

例

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 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.

例

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 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.

例

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 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.

分析过的案例

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

Maintenance As currently described, this entry is more like a category than a weakness.

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