The product does not properly handle null bytes or NUL characters when passing data between different representations or components.

A null byte (NUL character) can have different meanings across representations or languages. For example, it is a string terminator in standard C libraries, but Perl and PHP strings do not treat it as a terminator. When two representations are crossed - such as when Perl or PHP invokes underlying C functionality - this can produce an interaction error with unexpected results. Similar issues have been reported for ASP. Other interpreters written in C might also be affected.
The poison null byte is frequently useful in path traversal attacks by terminating hard-coded extensions that are added to a filename. It can play a role in regular expression processing in PHP.

Primary

Implementation

Integrity
Unexpected State

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.)

Implementation
Remove null bytes from all incoming strings.

CVE-2005-4155
NUL byte bypasses PHP regular expression check
https://www.cve.org/CVERecord?id=CVE-2005-4155

CVE-2005-3153
inserting SQL after a NUL byte bypasses allowlist regexp, enabling SQL injection
https://www.cve.org/CVERecord?id=CVE-2005-3153

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

Current usage of "poison null byte" is typically related to this C/Perl/PHP interaction error, but the original term in 1998 was applied to an off-by-one buffer overflow involving a null byte.
There are not many CVE examples, because the poison NULL byte is a design limitation, which typically is not included in CVE by itself. It is typically used as a facilitator manipulation to widen the scope of potential attacks against other vulnerabilities.

CWE Content Team
MITRE
2007-05-07
Draft 6
2007-05-07

Eric Dalci
Cigital
2008-07-01
updated Time_of_Introduction

CWE Content Team
MITRE
2008-09-08
updated Applicable_Platforms, Description, Relationships, Observed_Example, Other_Notes, Weakness_Ordinalities

CWE Content Team
MITRE
2011-03-29
updated Other_Notes

CWE Content Team
MITRE
2011-06-01
updated Common_Consequences

CWE Content Team
MITRE
2011-06-27
updated Common_Consequences

CWE Content Team
MITRE
2012-05-11
updated Relationships

CWE Content Team
MITRE
2012-10-30
updated Potential_Mitigations

CWE Content Team
MITRE
2014-06-23
updated Description, Other_Notes, Research_Gaps, Terminology_Notes

CWE Content Team
MITRE
2014-07-30
updated Relationships

CWE Content Team
MITRE
2020-02-24
updated Relationships

CWE Content Team
MITRE
2020-06-25
updated Observed_Examples, Relationships

CWE Content Team
MITRE
2023-04-27
updated References, Relationships

CWE Content Team
MITRE
2023-06-29
updated Mapping_Notes

CWE Content Team
MITRE
2025-09-09
4.18
2025-09-09
updated References

CWE Content Team
MITRE
2025-12-11
4.19
2025-12-11
updated Detection_Factors