CWE-473 Variant Draft

PHP External Variable Modification

This vulnerability occurs when a PHP application fails to properly validate or sanitize variables that originate from outside the application, such as HTTP query strings, cookies, or POST data.…

Definition

What is CWE-473?

This vulnerability occurs when a PHP application fails to properly validate or sanitize variables that originate from outside the application, such as HTTP query strings, cookies, or POST data. Attackers can exploit this to inject unexpected values, altering the program's logic and security controls.
PHP's historical feature of automatically registering global variables from user input (like `$_GET`, `$_POST`) created a major security pitfall. Even with this feature deprecated, the core risk remains: if developers directly trust external data without validation, attackers can overwrite critical internal variables. This opens the door to authentication bypass, privilege escalation, or logic flaws that shouldn't exist in a properly isolated codebase. To prevent this, developers must adopt a clear separation between external input and internal application state. Always initialize variables explicitly, disable legacy features like `register_globals`, and treat all user-supplied data as untrusted. Implement strict validation and use allow-lists for expected values, ensuring external sources cannot arbitrarily modify the variables that control your application's behavior and security decisions.
Real-world impact

Real-world CVEs caused by CWE-473

  • CVE-2000-0860

    File upload allows arbitrary file read by setting hidden form variables to match internal variable names.

  • CVE-2001-0854

    Mistakenly trusts $PHP_SELF variable to determine if include script was called by its parent.

  • CVE-2002-0764

    PHP remote file inclusion by modified assumed-immutable variable.

  • CVE-2001-1025

    Modify key variable when calling scripts that don't load a library that initializes it.

  • CVE-2003-0754

    Authentication bypass by modifying array used for authentication.

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-473

  • Requirements / Implementation Carefully identify which variables can be controlled or influenced by an external user, and consider adopting a naming convention to emphasize when externally modifiable variables are being used. An application should be reluctant to trust variables that have been initialized outside of its trust boundary. Ensure adequate checking is performed when relying on input from outside a trust boundary. Do not allow your application to run with register_globals enabled. If you implement a register_globals emulator, be extremely careful of variable extraction, dynamic evaluation, and similar issues, since weaknesses in your emulation could allow external variable modification to take place even without register_globals.
Detection signals

How to detect CWE-473

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-473 and opens a fix PR in under 60 seconds.

Codex Remedium scans every commit, identifies this exact weakness, and ships a reviewer-ready pull request with the patch. No tickets. No hand-offs.

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Frequently asked questions

Frequently asked questions

What is CWE-473?

This vulnerability occurs when a PHP application fails to properly validate or sanitize variables that originate from outside the application, such as HTTP query strings, cookies, or POST data. Attackers can exploit this to inject unexpected values, altering the program's logic and security controls.

How serious is CWE-473?

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-473?

MITRE lists the following affected platforms: PHP.

How can I prevent CWE-473?

Carefully identify which variables can be controlled or influenced by an external user, and consider adopting a naming convention to emphasize when externally modifiable variables are being used. An application should be reluctant to trust variables that have been initialized outside of its trust boundary. Ensure adequate checking is performed when relying on input from outside a trust boundary. Do not allow your application to run with register_globals enabled. If you implement a…

How does Plexicus detect and fix CWE-473?

Plexicus's SAST engine matches the data-flow signature for CWE-473 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-473?

MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/473.html. You can also reference OWASP and NIST documentation for adjacent guidance.

Related weaknesses

Weaknesses related to CWE-473

CWE-471 Parent

Modification of Assumed-Immutable Data (MAID)

This vulnerability occurs when an application fails to protect data it assumes cannot be changed, allowing an attacker to alter it.

CWE-472 Sibling

External Control of Assumed-Immutable Web Parameter

This vulnerability occurs when a web application incorrectly trusts data that appears to be fixed or hidden from the user, such as values…

CWE-607 Sibling

Public Static Final Field References Mutable Object

This vulnerability occurs when a class exposes a public or protected static final field that points to a changeable object. Because the…

CWE-98 Can precede

Improper Control of Filename for Include/Require Statement in PHP Program ('PHP Remote File Inclusion')

This vulnerability occurs when a PHP application uses unvalidated or insufficiently restricted user input directly within file inclusion…

Resources

Further reading

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