CWE-87: Improper Neutralization of Alternate XSS Syntax

Code Examples & CVEs

Demonstrative Examples

In the following example, an XSS neutralization method intends to replace script tags in user-supplied input with a safe equivalent:

The code only works when the "script" tag is in all lower-case, forming an incomplete denylist (CWE-184). Equivalent tags such as "SCRIPT" or "ScRiPt" will not be neutralized by this method, allowing an XSS attack.

BadJava

public String preventXSS(String input, String mask) {return input.replaceAll("script", mask);}

Observed CVE Examples (1)

CVE-2002-0738

XSS using "&={script}".

View Details

Source-backed guidance

Additional facts reviewed against primary or authoritative security sources.

Verify controls for CWE-87 with SSDF evidence

Use NIST SSDF verification and vulnerability-response practices to detect CWE-87, Improper Neutralization of Alternate XSS Syntax, throughout the product lifecycle. Derive review questions, static or dynamic checks, and negative tests from the CWE's causal behavior; define the components and lifecycle stages each check covers; and retain findings with enough evidence to distinguish the root cause from symptoms and impacts. Track escapes and false negatives, then improve the verification plan after every confirmed occurrence.

NIST SP 800-218 Secure Software Development Framework — National Institute of Standards and Technology

Apply Output Encoding controls for Improper Neutralization of Alternate XSS Syntax

MITRE associates mitigation with Implementation; the listed strategies include Output Encoding, and Attack Surface Reduction; recorded impacts include Read Application Data, and Execute Unauthorized Code or Commands. Use these source-defined anchors to turn CWE-87 into implementation, review, and verification checks for the affected component.

CWE-87: Improper Neutralization of Alternate XSS Syntax — MITRE CWE

Triage CWE-87 against known exploitation evidence

Use CISA's Known Exploited Vulnerabilities catalog to test whether a vulnerability mapped to CWE-87, Improper Neutralization of Alternate XSS Syntax, has evidence of exploitation in the wild. Confirm the CVE-to-CWE root-cause mapping independently before attaching the example, then capture the affected product, required action, and remediation deadline. A missing KEV match is not evidence that the weakness is unexploited, and a KEV entry must not be generalized to every occurrence of this CWE.

Known Exploited Vulnerabilities Catalog — Cybersecurity and Infrastructure Security Agency

Apply precise root-cause mapping to CWE-87

Apply MITRE's full root-cause mapping guidance when using CWE-87, Improper Neutralization of Alternate XSS Syntax. Separate weakness language from attacker prerequisites and technical impact, check the entry's abstraction and vulnerability-mapping notes, and prefer the most specific Base or Variant supported by the evidence. Record the rejected alternatives and require an independent review before the mapping is used for remediation trends or program metrics.

CVE to CWE Root Cause Mapping Guidance — MITRE CWE

Validate CWE-87 with root-cause mapping checks

Apply MITRE's root-cause mapping quick tips to CWE-87, Improper Neutralization of Alternate XSS Syntax. Confirm the finding describes the causal weakness rather than an impact or attack pattern, compare the abstraction and mapping notes with plausible alternatives, and have a second reviewer challenge the selection. Preserve the evidence and reasoning so recurring defects can be measured against one consistent identifier.

CVE to CWE Root Cause Mapping Quick Tips — MITRE CWE

Frequently Asked Questions

What is CWE-87: Improper Neutralization of Alternate XSS Syntax?+

CWE-87: Improper Neutralization of Alternate XSS Syntax is a Common Weakness Enumeration (CWE) entry maintained by MITRE. The product does not neutralize or incorrectly neutralizes user-controlled input for alternate script syntax.

What are the security consequences of Improper Neutralization of Alternate XSS Syntax?+

If exploited, CWE-87 (Improper Neutralization of Alternate XSS Syntax) it can compromise Confidentiality, Integrity and Availability, leading to outcomes such as Read Application Data and Execute Unauthorized Code or Commands.

How do you prevent or mitigate Improper Neutralization of Alternate XSS Syntax?+

Recommended mitigations for CWE-87 include: Resolve all input to absolute or canonical representations before processing. 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 tag attributes, 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.

Which programming languages are affected by Improper Neutralization of Alternate XSS Syntax?+

CWE-87 commonly affects Not Language-Specific. Note that weaknesses are often language-agnostic patterns, so secure coding practices apply broadly.

What are real-world examples of Improper Neutralization of Alternate XSS Syntax?+

MITRE documents real CVEs mapped to CWE-87, including CVE-2002-0738. You can look up the full details of each CVE, including CVSS scores and remediation guidance, on our CVE Lookup tool.

What is the difference between a CWE and a CVE?+

A CWE (Common Weakness Enumeration) like CWE-87 describes a category of software weakness — the underlying flaw type. A CVE (Common Vulnerabilities and Exposures) identifies a specific, real-world vulnerability in a particular product. In short, a CWE is the kind of mistake, and a CVE is an instance of that mistake being found in software.

CWE-87: Improper Neutralization of Alternate XSS Syntax

Technical Details

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