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Home > CWE List > CWE-158: Improper Neutralization of Null Byte or NUL Character (4.18)
ID
CWE Glossary Definition

CWE-158: Improper Neutralization of Null Byte or NUL Character

Weakness ID: 158
Vulnerability Mapping: ALLOWED This CWE ID may be used to map to real-world vulnerabilities
Abstraction: Variant Variant - a weakness that is linked to a certain type of product, typically involving a specific language or technology. More specific than a Base weakness. Variant level weaknesses typically describe issues in terms of 3 to 5 of the following dimensions: behavior, property, technology, language, and resource.
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Description
The product receives input from an upstream component, but it does not neutralize or incorrectly neutralizes NUL characters or null bytes when they are sent to a downstream component.
Extended Description
As data is parsed, an injected NUL character or null byte may cause the product to believe the input is terminated earlier than it actually is, or otherwise cause the input to be misinterpreted. This could then be used to inject potentially dangerous input that occurs after the null byte or otherwise bypass validation routines and other protection mechanisms.
Common Consequences
Section HelpThis table specifies different individual consequences 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 how likely the specific consequence is expected to be seen relative to the other 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.
Impact Details

Unexpected State

Scope: Integrity

Potential Mitigations
Phase(s) Mitigation
Developers should anticipate that null characters or null bytes will be injected/removed/manipulated in the input vectors of their product. Use an appropriate combination of denylists and allowlists to ensure only valid, expected and appropriate input is processed by the system.

Implementation

Strategy: Input Validation

Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does.

When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, "boat" may be syntactically valid because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as "red" or "blue."

Do not rely exclusively on looking for malicious or malformed inputs. This is likely to miss at least one undesirable input, especially if the code's environment changes. This can give attackers enough room to bypass the intended validation. However, denylists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright.

Implementation

Strategy: Input Validation

Inputs should be decoded and canonicalized to the application's current internal representation before being validated (CWE-180). Make sure that the application does not decode the same input twice (CWE-174). Such errors could be used to bypass allowlist validation schemes by introducing dangerous inputs after they have been checked.
Relationships
Section Help This table shows the weaknesses and high level categories that are related to this weakness. These relationships are defined as ChildOf, ParentOf, MemberOf and give insight to similar items that may exist at higher and lower levels of abstraction. In addition, relationships such as PeerOf and CanAlsoBe are defined to show similar weaknesses that the user may want to explore.
Relevant to the view "Research Concepts" (View-1000)
Nature Type ID Name
ChildOf Class Class - a weakness that is described in a very abstract fashion, typically independent of any specific language or technology. More specific than a Pillar Weakness, but more general than a Base Weakness. Class level weaknesses typically describe issues in terms of 1 or 2 of the following dimensions: behavior, property, and resource. 138 Improper Neutralization of Special Elements
Modes Of Introduction
Section HelpThe different Modes of Introduction provide information about how and when this weakness may be introduced. The Phase identifies a point in the life cycle at which introduction may occur, while the Note provides a typical scenario related to introduction during the given phase.
Phase Note
Implementation
Applicable Platforms
Section HelpThis listing shows possible areas for which the given weakness could appear. These may be for specific named Languages, Operating Systems, Architectures, Paradigms, Technologies, or a class of such platforms. The platform is listed along with how frequently the given weakness appears for that instance.
Languages

Class: Not Language-Specific (Undetermined Prevalence)

Selected Observed Examples

Note: this is a curated list of examples for users to understand the variety of ways in which this weakness can be introduced. It is not a complete list of all CVEs that are related to this CWE entry.

Reference Description
NUL byte in theme name causes directory traversal impact to be worse
Source code disclosure using trailing null.
Source code disclosure using trailing null.
Trailing null allows file include.
Null character in MIME header allows detection bypass.
Web server allows remote attackers to view the source code for CGI programs via a null character (%00) at the end of a URL.
Web server earlier allows allows remote attackers to bypass access restrictions, list directory contents, and read source code by inserting a null character (%00) in the URL.
Logging system allows an attacker to cause a denial of service (hang) by causing null bytes to be placed in log messages.
Web server allows source code for executable programs to be read via a null character (%00) at the end of a request.
Protection mechanism for limiting file access can be bypassed using a null character (%00) at the end of the directory name.
Application server allows remote attackers to read JSP source code via an encoded null byte in an HTTP GET request, which causes the server to send the .JSP file unparsed.
XSS protection mechanism only checks for sequences with an alphabetical character following a (<), so a non-alphabetical or null character (%00) following a < may be processed.
Decoding function in proxy allows regular expression bypass in ACLs via URLs with null characters.
Null byte bypasses PHP regexp check (interaction error).
Null byte bypasses PHP regexp check (interaction error).
Memberships
Section HelpThis MemberOf Relationships table shows additional CWE Categories and Views that reference this weakness as a member. This information is often useful in understanding where a weakness fits within the context of external information sources.
Nature Type ID Name
MemberOf CategoryCategory - a CWE entry that contains a set of other entries that share a common characteristic. 990 SFP Secondary Cluster: Tainted Input to Command
MemberOf CategoryCategory - a CWE entry that contains a set of other entries that share a common characteristic. 1407 Comprehensive Categorization: Improper Neutralization
Vulnerability Mapping Notes
Usage ALLOWED
(this CWE ID may be used to map to real-world vulnerabilities)
Reason Acceptable-Use

Rationale

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.

Comments

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.
Notes

Relationship

This can be a factor in multiple interpretation errors, other interaction errors, filename equivalence, etc.
Taxonomy Mappings
Mapped Taxonomy Name Node ID Fit Mapped Node Name
PLOVER Null Character / Null Byte
WASC 28 Null Byte Injection
Software Fault Patterns SFP24 Tainted input to command
References
[REF-62] Mark Dowd, John McDonald and Justin Schuh. "The Art of Software Security Assessment". Chapter 8, "NUL Character Injection", Page 411. 1st Edition. Addison Wesley. 2006.
Content History
Submissions
Submission Date Submitter Organization
2006年07月19日
(CWE Draft 3, 2006年07月19日) 		PLOVER
Modifications
Modification Date Modifier Organization
2023年06月29日 CWE Content Team MITRE
updated Mapping_Notes
2023年04月27日 CWE Content Team MITRE
updated Relationships
2023年01月31日 CWE Content Team MITRE
updated Description, Potential_Mitigations
2020年06月25日 CWE Content Team MITRE
updated Observed_Examples, Potential_Mitigations
2020年02月24日 CWE Content Team MITRE
updated Potential_Mitigations, Relationships
2017年11月08日 CWE Content Team MITRE
updated Applicable_Platforms
2017年05月03日 CWE Content Team MITRE
updated Potential_Mitigations
2014年07月30日 CWE Content Team MITRE
updated Relationships, Taxonomy_Mappings
2012年10月30日 CWE Content Team MITRE
updated Potential_Mitigations
2012年05月11日 CWE Content Team MITRE
updated Observed_Examples, References, Relationships
2011年06月27日 CWE Content Team MITRE
updated Common_Consequences
2011年06月01日 CWE Content Team MITRE
updated Common_Consequences
2011年03月29日 CWE Content Team MITRE
updated Potential_Mitigations
2010年04月05日 CWE Content Team MITRE
updated Description, Name
2010年02月16日 CWE Content Team MITRE
updated Taxonomy_Mappings
2009年07月27日 CWE Content Team MITRE
updated Potential_Mitigations
2008年11月24日 CWE Content Team MITRE
updated Observed_Examples
2008年10月14日 CWE Content Team MITRE
updated Description
2008年09月08日 CWE Content Team MITRE
updated Relationships, Relationship_Notes, Taxonomy_Mappings
2008年07月01日 Eric Dalci Cigital
updated Potential_Mitigations
Previous Entry Names
Change Date Previous Entry Name
2008年01月30日 Null Character / Null Byte
2008年04月11日 Failure to Remove Null Character / Null Byte
2010年04月05日 Failure to Sanitize Null Byte or NUL Character
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Page Last Updated: September 09, 2025

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