CWE-80 Variant Incomplete High likelihood

Improper Neutralization of Script-Related HTML Tags in a Web Page (Basic XSS)

This vulnerability, commonly known as Basic Cross-Site Scripting (XSS), occurs when a web application fails to properly sanitize user input containing HTML and JavaScript tags. When untrusted data…

Definition

What is CWE-80?

This vulnerability, commonly known as Basic Cross-Site Scripting (XSS), occurs when a web application fails to properly sanitize user input containing HTML and JavaScript tags. When untrusted data containing characters like <, >, and & is rendered directly into a webpage, a browser may execute it as active code, not just display it as text.
At its core, this flaw allows attackers to inject malicious scripts into web pages viewed by other users. These scripts execute within the victim's browser context, enabling actions like stealing session cookies, logging keystrokes, defacing websites, or redirecting users to malicious sites. The risk is highest in any part of an application that displays user-provided data without proper validation, such as comment sections, user profiles, or search result pages. Preventing Basic XSS requires a consistent strategy of output encoding or contextual escaping. Instead of trying to filter out 'bad' input, you should proactively encode all user-controlled data before inserting it into HTML. This means converting special characters into their safe HTML entity equivalents (e.g., < for <). Rely on well-tested libraries for your framework to perform this encoding, and never use insecure methods like `innerHTML` or `document.write()` with raw user input.
Vulnerability Diagram CWE-80
Reflected XSS (Basic) Crafted URL ?q=<script>…</script> Server echoes q into HTML <p>You searched: q</p> no encoding Victim browser runs reflected script Session cookies leak The malicious URL is shared (email/chat); reply page reflects the script.
Real-world impact

Real-world CVEs caused by CWE-80

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 JSP

In the following example, a guestbook comment isn't properly encoded, filtered, or otherwise neutralized for script-related tags before being displayed in a client browser.

Vulnerable JSP 
<% for (Iterator i = guestbook.iterator(); i.hasNext(); ) {
  	Entry e = (Entry) i.next(); %>
  	<p>Entry #<%= e.getId() %></p>
  	<p><%= e.getText() %></p>
  	<%
  	} %>
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-80

  • Implementation Carefully check each input parameter against a rigorous positive specification (allowlist) defining the specific characters and format allowed. All input should be neutralized, not just parameters that the user is supposed to specify, but all data in the request, including hidden fields, cookies, headers, the URL itself, and so forth. A common mistake that leads to continuing XSS vulnerabilities is to validate only fields that are expected to be redisplayed by the site. We often encounter data from the request that is reflected by the application server or the application that the development team did not anticipate. Also, a field that is not currently reflected may be used by a future developer. Therefore, validating ALL parts of the HTTP request is recommended.
  • Implementation Use and specify an output encoding that can be handled by the downstream component that is reading the output. Common encodings include ISO-8859-1, UTF-7, and UTF-8. When an encoding is not specified, a downstream component may choose a different encoding, either by assuming a default encoding or automatically inferring which encoding is being used, which can be erroneous. When the encodings are inconsistent, the downstream component might treat some character or byte sequences as special, even if they are not special in the original encoding. Attackers might then be able to exploit this discrepancy and conduct injection attacks; they even might be able to bypass protection mechanisms that assume the original encoding is also being used by the downstream component. The problem of inconsistent output encodings often arises in web pages. If an encoding is not specified in an HTTP header, web browsers often guess about which encoding is being used. This can open up the browser to subtle XSS attacks.
  • Implementation With Struts, write all data from form beans with the bean's filter attribute set to true.
  • Implementation To help mitigate XSS attacks against the user's session cookie, set the session cookie to be HttpOnly. In browsers that support the HttpOnly feature (such as more recent versions of Internet Explorer and Firefox), this attribute can prevent the user's session cookie from being accessible to malicious client-side scripts that use document.cookie. This is not a complete solution, since HttpOnly is not supported by all browsers. More importantly, XMLHTTPRequest and other powerful browser technologies provide read access to HTTP headers, including the Set-Cookie header in which the HttpOnly flag is set.
Detection signals

How to detect CWE-80

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.)

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Frequently asked questions

Frequently asked questions

What is CWE-80?

This vulnerability, commonly known as Basic Cross-Site Scripting (XSS), occurs when a web application fails to properly sanitize user input containing HTML and JavaScript tags. When untrusted data containing characters like , and & is rendered directly into a webpage, a browser may execute it as active code, not just display it as text.

How serious is CWE-80?

MITRE rates the likelihood of exploit as High — this weakness is actively exploited in the wild and should be prioritized for remediation.

What languages or platforms are affected by CWE-80?

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

How can I prevent CWE-80?

Carefully check each input parameter against a rigorous positive specification (allowlist) defining the specific characters and format allowed. All input should be neutralized, not just parameters that the user is supposed to specify, but all data in the request, including hidden fields, cookies, headers, the URL itself, and so forth. A common mistake that leads to continuing XSS vulnerabilities is to validate only fields that are expected to be redisplayed by the site. We often encounter data…

How does Plexicus detect and fix CWE-80?

Plexicus's SAST engine matches the data-flow signature for CWE-80 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-80?

MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/80.html. You can also reference OWASP and NIST documentation for adjacent guidance.

Related weaknesses

Weaknesses related to CWE-80

CWE-79 Parent

Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')

This vulnerability occurs when a web application fails to properly sanitize or encode user-supplied input before displaying it on a…

CWE-81 Sibling

Improper Neutralization of Script in an Error Message Web Page

This vulnerability occurs when an application fails to properly sanitize user-supplied input before displaying it within an error message…

CWE-83 Sibling

Improper Neutralization of Script in Attributes in a Web Page

This vulnerability occurs when a web application fails to properly sanitize or block JavaScript URIs (like 'javascript:') within HTML tag…

CWE-84 Sibling

Improper Neutralization of Encoded URI Schemes in a Web Page

This vulnerability occurs when a web application fails to properly sanitize user-supplied input that contains malicious scripts disguised…

CWE-85 Sibling

Doubled Character XSS Manipulations

This vulnerability occurs when a web application fails to properly sanitize user input that contains doubled characters, allowing…

CWE-86 Sibling

Improper Neutralization of Invalid Characters in Identifiers in Web Pages

This vulnerability occurs when an application fails to properly filter or escape invalid characters within web identifiers like HTML tag…

CWE-87 Sibling

Improper Neutralization of Alternate XSS Syntax

This vulnerability occurs when an application fails to properly sanitize user-supplied input that uses alternative methods to execute…

Resources

Further reading

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