Description

The software 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.

Modes of Introduction:

– Implementation

 

Likelihood of Exploit: Medium

 

Related Weaknesses

CWE-707
CWE-120
CWE-126
CWE-147
CWE-464
CWE-463
CWE-20

 

Consequences

Confidentiality, Integrity, Availability: 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: 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: 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: 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.

 

Potential Mitigations

Phase: Requirements

Description: 

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

Description: 

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

Description: 

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

Description: 

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

Phase: Implementation

Description: 

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).

CVE References

• CVE-2000-0312
  • Attacker does not null-terminate argv[] when invoking another program.

• CVE-2003-0777
  • Interrupted step causes resultant lack of null termination.

• CVE-2004-1072
  • Fault causes resultant lack of null termination, leading to buffer expansion.

• CVE-2001-1389
  • Multiple vulnerabilities related to improper null termination.

• CVE-2003-0143
  • Product does not null terminate a message buffer after snprintf-like call, leading to overflow.

• CVE-2009-2523
  • 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).