CWE

Improper Handling of Unicode Encoding

ID: 176		Date: (C)2012-05-14   (M)2022-10-10
Type: weakness		Status: DRAFT
Abstraction Type: Variant

Description

The software does not properly handle when an input contains Unicode encoding.

Applicable Platforms
Language Class: All

Time Of Introduction

• Implementation

Related Attack Patterns

• CAPEC-71

Common Consequences

Scope	Technical Impact	Notes
Integrity	Unexpected state

Detection Methods
None

Potential Mitigations

Phase	Strategy	Description	Effectiveness	Notes
Architecture and Design	Input Validation	Avoid making decisions based on names of resources (e.g. files) if those resources can have alternate names.
Implementation	Input Validation	Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a whitelist of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does. When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, "boat" may be syntactically valid because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as "red" or "blue." Do not rely exclusively on looking for malicious or malformed inputs (i.e., do not rely on a blacklist). A blacklist is likely to miss at least one undesirable input, especially if the code's environment changes. This can give attackers enough room to bypass the intended validation. However, blacklists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright.
Implementation	Input Validation	Inputs should be decoded and canonicalized to the application's current internal representation before being validated (CWE-180). Make sure that the application does not decode the same input twice (CWE-174). Such errors could be used to bypass whitelist validation schemes by introducing dangerous inputs after they have been checked.

Relationships

Related CWE	Type	View	Chain
CWE-176 ChildOf CWE-896	Category	CWE-888

Demonstrative Examples   (Details)

1. Windows provides the MultiByteToWideChar(), WideCharToMultiByte(), UnicodeToBytes(), and BytesToUnicode() functions to convert between arbitrary multibyte (usually ANSI) character strings and Unicode (wide character) strings. The size arguments to these functions are specified in different units, (one in bytes, the other in characters) making their use prone to error.

Observed Examples

1. CVE-2000-0884 : Server allows remote attackers to read documents outside of the web root, and possibly execute arbitrary commands, via malformed URLs that contain Unicode encoded characters.
2. CVE-2001-0709 : Server allows a remote attacker to obtain source code of ASP files via a URL encoded with Unicode.
3. CVE-2001-0669 : Overlaps interaction error.

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		Unicode Encoding
CERT C Secure Coding	MSC10-C	Character Encoding - UTF8 Related Issues
CERT C++ Secure Coding	MSC10-CPP	Character Encoding - UTF8 Related Issues

References:

1. Mark Dowd John McDonald Justin Schuh .The Art of Software Security Assessment 1st Edition. Addison Wesley. Section:'Chapter 8, "Character Sets and Unicode", Page 446.'. Published on 2006.