Weakness · Medium
CWE-158: Improper Neutralization of Null Byte or NUL Character
What it is
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.
- Severity (our rating)
- Medium
- Likelihood of exploit
- Not rated by MITRE
- Introduced during
- Implementation
- Exploited in the wild
- 4 known exploited CVEs
In more detail
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.
What an attacker can do
- Unexpected State (Integrity)
How to prevent it
General
- 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
- 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.
- 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.
Exploited in the wild: 4 CVEs
Vulnerabilities caused by CWE-158 that attackers have used, from CISA's Known Exploited Vulnerabilities catalog, newest first.
- CVE-2026-104286Fortinet FortiMail Path Traversal Vulnerability · added October 1, 2026
- CVE-2025-47812Wing FTP Server Improper Neutralization of Null Byte or NUL Character Vulnerability · added July 14, 2025
- CVE-2022-36804Atlassian Bitbucket Server and Data Center Command Injection Vulnerability · added September 30, 2022
- CVE-2022-1040Sophos Firewall Authentication Bypass Vulnerability · added March 31, 2022
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Start freeSource: MITRE Common Weakness Enumeration, CWE version 4.19.1. CWE is a trademark of The MITRE Corporation.