CWE-86: Improper Neutralization of Invalid Characters in Identifiers in Web Pages
Weakness ID: 86
The software does not neutralize or incorrectly neutralizes invalid characters or byte sequences in the middle of tag names, URI schemes, and other identifiers.
Time of Introduction
Technical Impact: Read application
data; Execute unauthorized code or
XSS filter doesn't filter null characters before
looking for dangerous tags, which are ignored by web browsers. Multiple
Interpretation Error (MIE) and
Strategy: 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
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.
Strategy: Identify and Reduce Attack Surface
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.