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CWE-176 Variante Rascunho
Improper Handling of Unicode Encoding
This vulnerability occurs when software fails to correctly process or interpret Unicode-encoded input, leading to security bypasses, data corruption, or unexpected behavior.
Definição
What is CWE-176?
This vulnerability occurs when software fails to correctly process or interpret Unicode-encoded input, leading to security bypasses, data corruption, or unexpected behavior.
Unicode allows characters from many languages to be represented, but this complexity can confuse security checks. For example, an application might filter dangerous characters like '../' in ASCII but miss equivalent Unicode representations, allowing attackers to bypass path traversal defenses. Similarly, visual lookalike characters (homoglyphs) can be used in phishing or to impersonate trusted data.
To prevent this, developers must normalize and validate all Unicode input consistently. Use established libraries for canonicalization to ensure characters are compared in their standard form, and apply security checks after normalization—not before. Always treat input handling and comparison logic as encoding-aware, never assuming text is only in basic ASCII.
Impacto no mundo real
Real-world CVEs caused by CWE-176
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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.
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Server allows a remote attacker to obtain source code of ASP files via a URL encoded with Unicode.
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Overlaps interaction error.
Como os atacantes a exploram
Trajeto do atacante passo a passo
- 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.
- 2
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.
- 3
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.
- 4
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.
- 5
If the username string contains more than UNLEN characters, the call to MultiByteToWideChar() will overflow the buffer unicodeUser.
Exemplo de código vulnerável
Vulnerable C
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.
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);
} Exemplo de código seguro
Secure pseudo
// Validate, sanitize, or use a safe API before reaching the sink.
function handleRequest(input) {
const safe = validateAndEscape(input);
return executeWithGuards(safe);
} What changed: the unsafe sink is replaced (or the input is validated/escaped) so the same payload no longer triggers the weakness.
Lista de verificação de prevenção
How to prevent CWE-176
- 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 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. This 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, denylists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright.
- 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.
Sinais de deteção
How to detect CWE-176
Executar testes dinâmicos de segurança de aplicações (DAST) contra o endpoint em execução.
Monitorizar os registos em tempo de execução para traços de exceção invulgares, input malformado ou tentativas de contornar a autorização.
Revisão de código: sinalizar qualquer novo código que trate input desta superfície sem usar os ajudantes validados do framework.
O Plexicus deteta automaticamente o CWE-176 e abre um PR de correção em menos de 60 segundos.
O Codex Remedium analisa cada commit, identifica esta fraqueza exata e entrega um pull request pronto para revisão com o patch. Sem tickets. Sem transferências.
Perguntas frequentes O que é o CWE-176?
Qual a gravidade do CWE-176?
Que linguagens ou plataformas são afetadas pelo CWE-176?
Como posso prevenir o CWE-176?
Como é que o Plexicus deteta e corrige o CWE-176?
Onde posso saber mais sobre o CWE-176?
Frequently asked questions
O que é o CWE-176?
This vulnerability occurs when software fails to correctly process or interpret Unicode-encoded input, leading to security bypasses, data corruption, or unexpected behavior.
Qual a gravidade do CWE-176?
A MITRE não publicou uma classificação de probabilidade de exploração para esta fraqueza. Trate-a como impacto médio até o seu modelo de ameaças provar o contrário.
Que linguagens ou plataformas são afetadas pelo CWE-176?
A MITRE não especificou as plataformas afetadas por este CWE — pode aplicar-se à maioria das stacks de aplicações.
Como posso prevenir o CWE-176?
Avoid making decisions based on names of resources (e.g. files) if those resources can have alternate names. 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…
Como é que o Plexicus deteta e corrige o CWE-176?
O motor SAST do Plexicus correlaciona a assinatura de fluxo de dados do CWE-176 em cada commit. Quando é encontrada uma correspondência, o nosso agente Codex Remedium abre um PR de correção com o código corrigido, testes e um resumo de uma linha para o revisor.
Onde posso saber mais sobre o CWE-176?
A MITRE publica a definição canónica em https://cwe.mitre.org/data/definitions/176.html. Pode também consultar a documentação da OWASP e do NIST para orientações adjacentes.
Fraquezas relacionadas
Weaknesses related to CWE-176
CWE-172 Pai
Encoding Error
This vulnerability occurs when software incorrectly transforms data between different formats, leading to corrupted or misinterpreted…
CWE-173 Irmão
Improper Handling of Alternate Encoding
This vulnerability occurs when software fails to correctly process input that arrives in a different character encoding than expected,…
CWE-174 Irmão
Double Decoding of the Same Data
This vulnerability occurs when an application decodes the same piece of data twice in sequence. This double processing can bypass or…
CWE-175 Irmão
Improper Handling of Mixed Encoding
This vulnerability occurs when software fails to correctly process input that contains multiple character encodings within the same data…
CWE-177 Irmão
Improper Handling of URL Encoding (Hex Encoding)
This vulnerability occurs when an application fails to correctly process URL-encoded (also known as percent-encoded or hex-encoded) input,…
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