CWE-363 Base Draft

Race Condition Enabling Link Following

This vulnerability occurs when a program checks a file's status before using it, creating a brief window where an attacker can replace that file with a malicious link. This causes the program to…

Definition

What is CWE-363?

This vulnerability occurs when a program checks a file's status before using it, creating a brief window where an attacker can replace that file with a malicious link. This causes the program to follow the link and access an unintended, potentially dangerous location.
Developers often assume the gap between checking a file and using it is too small to exploit, but this race condition is very real. Attackers can widen this window by slowing the system down—for example, by consuming memory—or simply by launching repeated attacks until one succeeds. This flaw is particularly dangerous because it bypasses intended security checks. The program verifies a legitimate file, but in that instant, an attacker swaps it for a link pointing to a sensitive system file or a malicious payload, tricking the application into performing unauthorized actions.
Real-world impact

Real-world CVEs caused by CWE-363

No public CVE references are linked to this CWE in MITRE's catalog yet.

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 PHP

This code prints the contents of a file if a user has permission.

Vulnerable PHP 
function readFile($filename){
  		$user = getCurrentUser();
```
//resolve file if its a symbolic link* 
  		if(is_link($filename)){
  		```
  			$filename = readlink($filename);
  		}
  		if(fileowner($filename) == $user){
  			echo file_get_contents($realFile);
  			return;
  		}
  		else{
  			echo 'Access denied';
  			return false;
  		}
  }
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-363

  • Architecture Use safe-by-default frameworks and APIs that prevent the unsafe pattern from being expressible.
  • Implementation Validate input at trust boundaries; use allowlists, not denylists.
  • Implementation Apply the principle of least privilege to credentials, file paths, and runtime permissions.
  • Testing Cover this weakness in CI: SAST rules + targeted unit tests for the data flow.
  • Operation Monitor logs for the runtime signals listed in the next section.
Detection signals

How to detect CWE-363

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-363 and opens a fix PR in under 60 seconds.

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Frequently asked questions

Frequently asked questions

What is CWE-363?

This vulnerability occurs when a program checks a file's status before using it, creating a brief window where an attacker can replace that file with a malicious link. This causes the program to follow the link and access an unintended, potentially dangerous location.

How serious is CWE-363?

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-363?

MITRE has not specified affected platforms for this CWE — it can apply across most application stacks.

How can I prevent CWE-363?

Use safe-by-default frameworks, validate untrusted input at trust boundaries, and apply the principle of least privilege. Cover the data-flow signature in CI with SAST.

How does Plexicus detect and fix CWE-363?

Plexicus's SAST engine matches the data-flow signature for CWE-363 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-363?

MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/363.html. You can also reference OWASP and NIST documentation for adjacent guidance.

Related weaknesses

Weaknesses related to CWE-363

CWE-367 Parent

Time-of-check Time-of-use (TOCTOU) Race Condition

This vulnerability occurs when a program verifies a resource's state (like a file's permissions or existence) but then uses it after that…

CWE-59 Can precede

Improper Link Resolution Before File Access ('Link Following')

This vulnerability occurs when an application uses a filename to access a file but fails to properly check if that name points to a…

Resources

Further reading

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