CWE-176: Improper Handling of Unicode Encoding

Description

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

Potential Impact

Integrity

Unexpected State

Demonstrative Examples

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.

In a multibyte character string, each character occupies a varying number of bytes, and therefore the size of such strings is most easily specified as a total number of bytes. In Unicode, however, characters are always a fixed size, and string lengths are typically given by the number of characters they contain. Mistakenly specifying the wrong units in a size argument can lead to a buffer overflow.

The following function takes a username specified as a multibyte string and a pointer to a structure for user information and populates the structure with information about the specified user. Since Windows authentication uses Unicode for usernames, the username argument is first converted from a multibyte string to a Unicode string.

Bad

void getUserInfo(char *username, struct _USER_INFO_2 info){WCHAR unicodeUser[UNLEN+1];MultiByteToWideChar(CP_ACP, 0, username, -1, unicodeUser, sizeof(unicodeUser));NetUserGetInfo(NULL, unicodeUser, 2, (LPBYTE *)&info);}

This function incorrectly passes the size of unicodeUser in bytes instead of characters. The call to MultiByteToWideChar() can therefore write up to (UNLEN+1)*sizeof(WCHAR) wide characters, or (UNLEN+1)*sizeof(WCHAR)*sizeof(WCHAR) bytes, to the unicodeUser array, which has only (UNLEN+1)*sizeof(WCHAR) bytes allocated.

If the username string contains more than UNLEN characters, the call to MultiByteToWideChar() will overflow the buffer unicodeUser.

Mitigations & Prevention

Architecture and Design

Avoid making decisions based on names of resources (e.g. files) if those resources can have alternate names.

Implementation

Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a list 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 relat

Implementation

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 allowlist validation schemes by introducing dangerous inputs after they have been checked.

Real-World CVE Examples

CVE ID	Description
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.
CVE-2001-0709	Server allows a remote attacker to obtain source code of ASP files via a URL encoded with Unicode.
CVE-2001-0669	Overlaps interaction error.

Taxonomy Mappings

PLOVER: — Unicode Encoding
CERT C Secure Coding: MSC10-C — Character Encoding - UTF8 Related Issues

Frequently Asked Questions

What is CWE-176?

CWE-176 (Improper Handling of Unicode Encoding) is a software weakness identified by MITRE's Common Weakness Enumeration. It is classified as a Variant-level weakness. The product does not properly handle when an input contains Unicode encoding.

How can CWE-176 be exploited?

Attackers can exploit CWE-176 (Improper Handling of Unicode Encoding) to unexpected state. This weakness is typically introduced during the Implementation phase of software development.

How do I prevent CWE-176?

Key mitigations include: Avoid making decisions based on names of resources (e.g. files) if those resources can have alternate names.

What is the severity of CWE-176?

CWE-176 is classified as a Variant-level weakness (Low-Medium abstraction). It has been observed in 3 real-world CVEs.