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The product does not properly handle null bytes or NUL characters when passing data between different representations or components.
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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.
4 recorded CVEs are caused by CWE-626 (Null Byte Interaction Error (Poison Null Byte)). The highest-severity and most recent are shown first. 1 new CWE-626 CVE has been recorded so far in 2026.
What can happen when CWE-626 is exploited.
Unexpected State
Affects: Integrity
Typically introduced during these phases of the software lifecycle.
Languages
Practical mitigations for CWE-626, grouped by where in the lifecycle they apply.
Remove null bytes from all incoming strings.
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.)
Real CVEs that MITRE cites as examples of this weakness.
Common questions about CWE-626.
The product does not properly handle null bytes or NUL characters when passing data between different representations or components.
4 recorded CVEs are attributed to CWE-626, including CVE-2019-17137, CVE-2020-10773, CVE-2026-42040.
Remove null bytes from all incoming strings.
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.)
Exploiting CWE-626 can lead to: Unexpected State.
4 recorded CVEs are caused by CWE-626; none are currently in CISA's KEV catalog of actively exploited flaws.
Weakness data is sourced from the MITRE CWE catalog (v4.20). CVE associations are aggregated and kept current by RadicalNotion.AI.
Get alerted the moment a new CWE-626 vulnerability affects your stack, with AI-written analysis, severity context, and remediation guidance.