CWE-1021 Base Incomplete

Improper Restriction of Rendered UI Layers or Frames

This vulnerability occurs when a web application fails to properly control whether its pages can be embedded within frames or UI layers from other websites, potentially misleading users about which…

Definition

What is CWE-1021?

This vulnerability occurs when a web application fails to properly control whether its pages can be embedded within frames or UI layers from other websites, potentially misleading users about which site they are interacting with.
Modern web applications should actively define which external sites, if any, are permitted to display their content within frames, iframes, or similar embedding elements. Without these security headers (like X-Frame-Options or Content-Security-Policy frame-ancestors), your site becomes vulnerable to clickjacking or UI redressing attacks. This allows malicious actors to overlay their own interface on top of yours, tricking users into performing actions unintentionally. To prevent this, developers must implement server-side restrictions that explicitly deny or whitelist permissible domains for embedding. This is a critical client-side security control that protects user interactions and data integrity by ensuring your application's interface is only rendered in a trusted context, preventing confusion and unauthorized actions.
Real-world impact

Real-world CVEs caused by CWE-1021

  • CVE-2017-7440

    E-mail preview feature in a desktop application allows clickjacking attacks via a crafted e-mail message

  • CVE-2017-5697

    Hardware/firmware product has insufficient clickjacking protection in its web user interface

  • CVE-2017-4015

    Clickjacking in data-loss prevention product via HTTP response header.

  • CVE-2016-2496

    Tapjacking in permission dialog for mobile OS allows access of private storage using a partially-overlapping window.

  • CVE-2015-1241

    Tapjacking in web browser related to page navigation and touch/gesture events.

  • CVE-2017-0492

    System UI in mobile OS allows a malicious application to create a UI overlay of the entire screen to gain privileges.

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-1021

  • Implementation The use of X-Frame-Options allows developers of web content to restrict the usage of their application within the form of overlays, frames, or iFrames. The developer can indicate from which domains can frame the content. The concept of X-Frame-Options is well documented, but implementation of this protection mechanism is in development to cover gaps. There is a need for allowing frames from multiple domains.
  • Implementation A developer can use a "frame-breaker" script in each page that should not be framed. This is very helpful for legacy browsers that do not support X-Frame-Options security feature previously mentioned. It is also important to note that this tactic has been circumvented or bypassed. Improper usage of frames can persist in the web application through nested frames. The "frame-breaking" script does not intuitively account for multiple nested frames that can be presented to the user.
  • Implementation This defense-in-depth technique can be used to prevent the improper usage of frames in web applications. It prioritizes the valid sources of data to be loaded into the application through the usage of declarative policies. Based on which implementation of Content Security Policy is in use, the developer should use the "frame-ancestors" directive or the "frame-src" directive to mitigate this weakness. Both directives allow for the placement of restrictions when it comes to allowing embedded content.
Detection signals

How to detect CWE-1021

Automated Static Analysis High

Automated static analysis, commonly referred to as Static Application Security Testing (SAST), can find some instances of this weakness by analyzing source code (or binary/compiled code) without having to execute it. Typically, this is done by building a model of data flow and control flow, then searching for potentially-vulnerable patterns that connect "sources" (origins of input) with "sinks" (destinations where the data interacts with external components, a lower layer such as the OS, etc.)

Plexicus auto-fix

Plexicus auto-detects CWE-1021 and opens a fix PR in under 60 seconds.

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Frequently asked questions

Frequently asked questions

What is CWE-1021?

This vulnerability occurs when a web application fails to properly control whether its pages can be embedded within frames or UI layers from other websites, potentially misleading users about which site they are interacting with.

How serious is CWE-1021?

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-1021?

MITRE lists the following affected platforms: Web Based.

How can I prevent CWE-1021?

The use of X-Frame-Options allows developers of web content to restrict the usage of their application within the form of overlays, frames, or iFrames. The developer can indicate from which domains can frame the content. The concept of X-Frame-Options is well documented, but implementation of this protection mechanism is in development to cover gaps. There is a need for allowing frames from multiple domains. A developer can use a "frame-breaker" script in each page that should not be framed.…

How does Plexicus detect and fix CWE-1021?

Plexicus's SAST engine matches the data-flow signature for CWE-1021 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-1021?

MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/1021.html. You can also reference OWASP and NIST documentation for adjacent guidance.

Related weaknesses

Weaknesses related to CWE-1021

CWE-441 Parent

Unintended Proxy or Intermediary ('Confused Deputy')

A confused deputy vulnerability occurs when a system receives a request from a client and forwards it to an external destination without…

CWE-918 Sibling

Server-Side Request Forgery (SSRF)

Server-Side Request Forgery (SSRF) occurs when a web application fetches a remote resource based on user-controlled input, but fails to…

Resources

Further reading

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