Variant weakness

Incomplete

CWE-81: Improper Neutralization of Script in an Error Message Web Page

The product receives input from an upstream component, but it does not neutralize or incorrectly neutralizes special characters that could be interpreted as web-scripting elements when they are sent to an error page.

Recorded CVEs

With this primary weakness

KEVs

In the CISA KEV catalog

Not rated

Likelihood of exploit

Per MITRE

Variant

Abstraction

MITRE classification

Overview

Error pages may include customized 403 Forbidden or 404 Not Found pages. When an attacker can trigger an error that contains script syntax within the attacker's input, then cross-site scripting attacks may be possible.

How to prevent it

Practical mitigations for CWE-81, grouped by where in the lifecycle they apply.

Implementation

Do not write user-controlled input to error pages.

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

Output Encoding

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

Attack Surface Reduction

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.

Effectiveness: Defense in Depth

CWE-81: Improper Neutralization of Script in an Error Message Web Page

Overview

Real-world CVEs

Common consequences

How it happens

When it is introduced

Applies to

How to prevent it

How to detect it

Automated Static Analysis

Illustrative examples

Terminology & mappings

Mapped taxonomies

Attack patterns

Frequently asked questions

What is CWE-81?

What CVEs are caused by CWE-81?

How do you prevent CWE-81?

How is CWE-81 detected?

What are the consequences of CWE-81?

Is CWE-81 actively exploited?

References

Stay ahead of CWE-81

Overview

Real-world CVEs

Common consequences

How it happens

When it is introduced

Applies to

How to prevent it

How to detect it

Automated Static Analysis

Illustrative examples

Related weaknesses

Parent

Can also be

Terminology & mappings

Mapped taxonomies

Attack patterns

Frequently asked questions

What is CWE-81?

What CVEs are caused by CWE-81?

How do you prevent CWE-81?

How is CWE-81 detected?

What are the consequences of CWE-81?

Is CWE-81 actively exploited?

References

Stay ahead of CWE-81