Run static analysis (SAST) on the codebase looking for the unsafe pattern in the data flow.
CWE-626 Variant Draft
Null Byte Interaction Error (Poison Null Byte)
This vulnerability occurs when software incorrectly processes null bytes (NUL characters) as data moves between different systems or programming languages, leading to unexpected and potentially…
Definition
What is CWE-626?
This vulnerability occurs when software incorrectly processes null bytes (NUL characters) as data moves between different systems or programming languages, leading to unexpected and potentially dangerous behavior.
A null byte (\0) has different meanings across programming environments. In languages like C and C++, it marks the definitive end of a string. However, in higher-level languages like PHP, Perl, or Python, a null byte is treated as just another character within the string. The security risk emerges when data from these languages passes into underlying C-based functions or system calls, where the null byte suddenly becomes a terminator. This mismatch can truncate data, bypass filters, and create openings for attackers to manipulate how the application processes information.
Attackers frequently exploit this discrepancy in path traversal attacks. By injecting a null byte into a filename (e.g., 'malicious.exe\0.txt'), they can terminate a hard-coded file extension appended by the application, causing the system to process a dangerous file type. This technique can also interfere with security checks, input validation, and regular expression parsing, making it a critical issue for web applications and software that bridges multiple language layers.
Real-world impact
Real-world CVEs caused by CWE-626
-
NUL byte bypasses PHP regular expression check
-
inserting SQL after a NUL byte bypasses allowlist regexp, enabling SQL injection
How attackers exploit it
Step-by-step attacker path
- 1
Identify a code path that handles untrusted input without validation.
- 2
Craft a payload that exercises the unsafe behavior — injection, traversal, overflow, or logic abuse.
- 3
Deliver the payload through a normal request and observe the application's reaction.
- 4
Iterate until the response leaks data, executes attacker code, or escalates privileges.
Vulnerable code example
Vulnerable pseudo
MITRE has not published a code example for this CWE. The pattern below is illustrative — see Resources for canonical references.
// Example pattern — see MITRE for the canonical references.
function handleRequest(input) {
// Untrusted input flows directly into the sensitive sink.
return executeUnsafe(input);
} Secure code example
Secure pseudo
// Validate, sanitize, or use a safe API before reaching the sink.
function handleRequest(input) {
const safe = validateAndEscape(input);
return executeWithGuards(safe);
} What changed: the unsafe sink is replaced (or the input is validated/escaped) so the same payload no longer triggers the weakness.
Prevention checklist
How to prevent CWE-626
- Implementation Remove null bytes from all incoming strings.
Detection signals
How to detect CWE-626
Run dynamic application security testing against the live endpoint.
Watch runtime logs for unusual exception traces, malformed input, or authorization bypass attempts.
Code review: flag any new code that handles input from this surface without using the validated framework helpers.
CWE-626
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Frequently asked questions What is CWE-626?
How serious is CWE-626?
What languages or platforms are affected by CWE-626?
How can I prevent CWE-626?
How does Plexicus detect and fix CWE-626?
Where can I learn more about CWE-626?
Frequently asked questions
What is CWE-626?
This vulnerability occurs when software incorrectly processes null bytes (NUL characters) as data moves between different systems or programming languages, leading to unexpected and potentially dangerous behavior.
How serious is CWE-626?
MITRE has not published a likelihood-of-exploit rating for this weakness. Treat it as medium-impact until your threat model proves otherwise.
What languages or platforms are affected by CWE-626?
MITRE lists the following affected platforms: PHP, Perl, ASP.NET.
How can I prevent CWE-626?
Remove null bytes from all incoming strings.
How does Plexicus detect and fix CWE-626?
Plexicus's SAST engine matches the data-flow signature for CWE-626 on every commit. When a match is found, our Codex Remedium agent opens a fix PR with the corrected code, tests, and a one-line summary for the reviewer.
Where can I learn more about CWE-626?
MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/626.html. You can also reference OWASP and NIST documentation for adjacent guidance.
Related weaknesses
Weaknesses related to CWE-626
Resources
Further reading
- MITRE — official CWE-626 https://cwe.mitre.org/data/definitions/626.html
- Perl CGI problems https://phrack.org/issues/55/7
- 0x00 vs ASP file upload scripts http://www.security-assessment.com/Whitepapers/0x00_vs_ASP_File_Uploads.pdf
- ShAnKaR: multiple PHP application poison NULL byte vulnerability https://seclists.org/fulldisclosure/2006/Sep/185
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Ready to validate what matters?
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