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

  • CVE-2005-4155

    NUL byte bypasses PHP regular expression check

  • CVE-2005-3153

    inserting SQL after a NUL byte bypasses allowlist regexp, enabling SQL injection

How attackers exploit it

Step-by-step attacker path

  1. 1

    Identify a code path that handles untrusted input without validation.

  2. 2

    Craft a payload that exercises the unsafe behavior — injection, traversal, overflow, or logic abuse.

  3. 3

    Deliver the payload through a normal request and observe the application's reaction.

  4. 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.

Vulnerable pseudo 
// 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

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

SAST High

Run static analysis (SAST) on the codebase looking for the unsafe pattern in the data flow.

DAST Moderate

Run dynamic application security testing against the live endpoint.

Runtime Moderate

Watch runtime logs for unusual exception traces, malformed input, or authorization bypass attempts.

Code review Moderate

Code review: flag any new code that handles input from this surface without using the validated framework helpers.

Plexicus auto-fix

Plexicus auto-detects CWE-626 and opens a fix PR in under 60 seconds.

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Frequently asked questions

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

CWE-147 Parent

Improper Neutralization of Input Terminators

This vulnerability occurs when an application accepts external input but fails to properly handle special characters that downstream…

Resources

Further reading

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