CWE-93 Base Rascunho

Improper Neutralization of CRLF Sequences ('CRLF Injection')

This vulnerability occurs when an application uses carriage return and line feed characters (CRLF) to structure data, like separating lines or records, but fails to properly sanitize these sequences…

Definição

What is CWE-93?

This vulnerability occurs when an application uses carriage return and line feed characters (CRLF) to structure data, like separating lines or records, but fails to properly sanitize these sequences from user-supplied input before processing.
CRLF injection, often called 'HTTP Response Splitting' in web contexts, happens because attackers can inject the characters \r\n (CRLF) into input fields. When this unsanitized input is later used to construct structured outputs like HTTP headers, log files, or email headers, the injected characters can create new lines or entries. This allows attackers to manipulate the application's behavior, forge log entries, or inject malicious content that other users might see. To prevent this, developers must rigorously validate and sanitize all user-controlled data that influences output formatting. This involves encoding or stripping CRLF sequences (\r, \n, and their encoded equivalents like %0d%0a) before the data is used in any context where these characters have special meaning, such as HTTP responses, generated files, or system commands.
Vulnerability Diagram CWE-93
CRLF Injection / HTTP Response Splitting Input contains value\r\nSet-Cookie: a=b HTTP Response Set-Header: value ↳ \r\n ← splits headers Set-Cookie: a=b \r\n\r\n <injected body> Browser sees 2 responses / fake page CR/LF chars forge new headers or split response bodies.
Impacto no mundo real

Real-world CVEs caused by CWE-93

  • CVE-2002-1771

    CRLF injection enables spam proxy (add mail headers) using email address or name.

  • CVE-2002-1783

    CRLF injection in API function arguments modify headers for outgoing requests.

  • CVE-2004-1513

    Spoofed entries in web server log file via carriage returns

  • CVE-2006-4624

    Chain: inject fake log entries with fake timestamps using CRLF injection

  • CVE-2005-1951

    Chain: Application accepts CRLF in an object ID, allowing HTTP response splitting.

  • CVE-2004-1687

    Chain: HTTP response splitting via CRLF in parameter related to URL.

Como os atacantes a exploram

Trajeto do atacante passo a passo

  1. 1

    The following code segment reads the name of the author of a weblog entry, author, from an HTTP request and sets it in a cookie header of an HTTP response.

  2. 2

    Assuming a string consisting of standard alpha-numeric characters, such as "Jane Smith", is submitted in the request the HTTP response including this cookie might take the following form:

  3. 3

    However, because the value of the cookie is composed of unvalidated user input, the response will only maintain this form if the value submitted for AUTHOR_PARAM does not contain any CR and LF characters. If an attacker submits a malicious string, such as

  4. 4

    then the HTTP response would be split into two responses of the following form:

  5. 5

    The second response is completely controlled by the attacker and can be constructed with any header and body content desired. The ability to construct arbitrary HTTP responses permits a variety of resulting attacks, including:

Exemplo de código vulnerável

Vulnerable Java

The following code segment reads the name of the author of a weblog entry, author, from an HTTP request and sets it in a cookie header of an HTTP response.

Vulnerável Java 
String author = request.getParameter(AUTHOR_PARAM);
  ...
  Cookie cookie = new Cookie("author", author);
  cookie.setMaxAge(cookieExpiration);
  response.addCookie(cookie);
Payload do atacante

However, because the value of the cookie is composed of unvalidated user input, the response will only maintain this form if the value submitted for AUTHOR_PARAM does not contain any CR and LF characters. If an attacker submits a malicious string, such as

Payload do atacante 
Wiley Hacker\r\nHTTP/1.1 200 OK\r\n
Exemplo de código seguro

Secure pseudo

Seguro 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-93

  • Implementation Avoid using CRLF as a special sequence.
  • Implementation Appropriately filter or quote CRLF sequences in user-controlled input.
Sinais de deteção

How to detect CWE-93

Automated Static Analysis High

Automated static analysis, commonly referred to as Static Application Security Testing (SAST), can find some instances of this weakness by analyzing source code (or binary/compiled code) without having to execute it. Typically, this is done by building a model of data flow and control flow, then searching for potentially-vulnerable patterns that connect "sources" (origins of input) with "sinks" (destinations where the data interacts with external components, a lower layer such as the OS, etc.)

Correção automática do Plexicus

O Plexicus deteta automaticamente o CWE-93 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.

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Perguntas frequentes

Frequently asked questions

O que é o CWE-93?

This vulnerability occurs when an application uses carriage return and line feed characters (CRLF) to structure data, like separating lines or records, but fails to properly sanitize these sequences from user-supplied input before processing.

Qual a gravidade do CWE-93?

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-93?

A MITRE não especificou as plataformas afetadas por este CWE — pode aplicar-se à maioria das stacks de aplicações.

Como posso prevenir o CWE-93?

Avoid using CRLF as a special sequence. Appropriately filter or quote CRLF sequences in user-controlled input.

Como é que o Plexicus deteta e corrige o CWE-93?

O motor SAST do Plexicus correlaciona a assinatura de fluxo de dados do CWE-93 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-93?

A MITRE publica a definição canónica em https://cwe.mitre.org/data/definitions/93.html. Pode também consultar a documentação da OWASP e do NIST para orientações adjacentes.

Fraquezas relacionadas

Weaknesses related to CWE-93

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CWE-79 Irmão

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CWE-88 Irmão

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CWE-91 Irmão

XML Injection (aka Blind XPath Injection)

XML Injection occurs when an application fails to properly validate or escape user-controlled input before including it in XML documents…

Recursos

Further reading

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