The product does not neutralize or incorrectly neutralizes user-controlled input for alternate script syntax.
View on MITREResolve 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.
No detection method information available for this CWE.
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.
No relationship information available for this CWE.
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.
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.
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.
CWE-87 commonly affects Not Language-Specific. Note that weaknesses are often language-agnostic patterns, so secure coding practices apply broadly.
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.
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.