CWE-176 Unicode编码处理不恰当

Improper Handling of Unicode Encoding

结构: Simple

Abstraction: Variant

状态: Draft

被利用可能性: unkown

基本描述

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

相关缺陷

• cwe_Nature: ChildOf cwe_CWE_ID: 172 cwe_View_ID: 1000 cwe_Ordinal: Primary

• cwe_Nature: ChildOf cwe_CWE_ID: 172 cwe_View_ID: 699 cwe_Ordinal: Primary

适用平台

Language: {'cwe_Class': 'Language-Independent', 'cwe_Prevalence': 'Undetermined'}

常见的影响

范围	影响	注释
Integrity	Unexpected State

可能的缓解方案

MIT-44 Architecture and Design

策略: Input Validation

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

MIT-5 Implementation

策略: 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.

MIT-20 Implementation

策略: 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.

示例代码

例

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 C

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.

分析过的案例

标识	说明	链接
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.	https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2000-0884
CVE-2001-0709	Server allows a remote attacker to obtain source code of ASP files via a URL encoded with Unicode.	https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2001-0709
CVE-2001-0669	Overlaps interaction error.	https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2001-0669

分类映射

映射的分类名	ImNode ID	Fit	Mapped Node Name
PLOVER			Unicode Encoding
CERT C Secure Coding	MSC10-C		Character Encoding - UTF8 Related Issues

相关攻击模式

• CAPEC-71

引用

• REF-62 The Art of Software Security Assessment