CWE

Improper Neutralization of Invalid Characters in Identifiers in Web Pages

ID: 86		Date: (C)2012-05-14   (M)2022-10-10
Type: weakness		Status: DRAFT
Abstraction Type: Variant

Description

The software does not neutralize or incorrectly neutralizes invalid characters or byte sequences in the middle of tag names, URI schemes, and other identifiers.

Extended Description

Some web browsers may remove these sequences, resulting in output that may have unintended control implications. For example, the software may attempt to remove a "javascript:" URI scheme, but a "java%00script:" URI may bypass this check and still be rendered as active javascript by some browsers, allowing XSS or other attacks.

Applicable Platforms
Language Class: All

Time Of Introduction

• Implementation

Related Attack Patterns

• CAPEC-18
• CAPEC-199
• CAPEC-244
• CAPEC-247
• CAPEC-32
• CAPEC-63
• CAPEC-73
• CAPEC-85
• CAPEC-86

Common Consequences

Scope	Technical Impact	Notes
Confidentiality Integrity Availability	Read application data Execute unauthorized code or commands

Detection Methods
None

Potential Mitigations

Phase	Strategy	Description	Effectiveness	Notes
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	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.	Defense in Depth

Relationships

Related CWE	Type	View	Chain
CWE-86 ChildOf CWE-896	Category	CWE-888

Demonstrative Examples
None

Observed Examples

1. CVE-2004-0595 : XSS filter doesn't filter null characters before looking for dangerous tags, which are ignored by web browsers. Multiple Interpretation Error (MIE) and validate-before-cleanse.

For more examples, refer to CVE relations in the bottom box.

White Box Definitions
None

Black Box Definitions
None

Taxynomy Mappings

Taxynomy	Id	Name	Fit
PLOVER		Invalid Characters in Identifiers

References:
None