CWE-87: Improper Neutralization of Alternate XSS Syntax
Description
The product does not neutralize or incorrectly neutralizes user-controlled input for alternate script syntax.
- Abstraction
- VariantA weakness tied to a specific language, technology or resource.
- Likelihood of exploit
- Not rated by MITRE
- Status
- Draft
Common consequences
- ConfidentialityIntegrityAvailability
Read Application Data; Execute Unauthorized Code or Commands
Potential mitigations
- Implementation
Resolve all input to absolute or canonical representations before processing.
- 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 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...
- ImplementationOutput 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...
- Implementation
With Struts, write all data from form beans with the bean's filter attribute set to true.
- ImplementationAttack Surface ReductionEffectiveness: Defense in Depth
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...