CWE-113

HTTP ヘッダにおける CRLF シーケンスの不適切な中和 ('HTTP リクエスト/レスポンスの分割')

Improper Neutralization of CRLF Sequences in HTTP Headers ('HTTP Request/Response Splitting')

脆弱性作成中

日本語説明

この製品は、HTTPエージェント/コンポーネント（ウェブサーバー、プロキシ、ブラウザーなど）からデータを受信するが、データが送信HTTPヘッダーに含まれる前にCRとLF文字を中和しないか、間違って中和する。

HTTPエージェントやコンポーネントは、ウェブサーバ、ロードバランサ、リバースプロキシ、ウェブキャッシングプロキシ、アプリケーションファイアウォール、ウェブブラウザなどを含むかもしれない。どのような役割であっても、すべてのコンポーネントで一貫した一貫性のある HTTP 通信状態を維持することが期待されています。しかしながら、HTTP ヘッダーに予期せぬデータを含めることで、攻撃者はクライアント HTTP エージェント (例: ウェブブラウザ) やバックエンド HTTP エージェント (例: ウェブサーバ) によってレンダリングされる HTTP メッセージの全体を、メッセージがリクエストの一部であるかレスポンスの一部であるかに関わらず指定することができます。

HTTP リクエストが予期しない CR や LF 文字を含む場合、サーバはストリームを 1 つではなく 2 つの異なる HTTP メッセージに "分割" して解釈する出力ストリームで応答するかもしれません。CR はキャリッジリターンで、%0d または ˶ˆ꒳ˆ˵ で与えられ、LF はラインフィードで、%0a または ˶ˆ꒳ˆ˵ で与えられます。

CRとLFに加えて、HT(水平タブ、%09または%tで指定)やSP(スペース、+記号または%20で指定)など、有効な/RFC準拠の特殊文字や固有の文字エンコーディングも使用できます。

このような HTTP メッセージヘッダ内の有効でない予期しないデータにより、攻撃者はサーバーサイドリクエストフォージェリ、クロスサイトスクリプティング、キャッシュポイズニング攻撃などの攻撃を行うために、2 番目の "split" メッセージを制御することができます。

HTTP レスポンス分割の弱点は、以下のような場合に存在する可能性があります：

原文 (English)

The product receives data from an HTTP agent/component (e.g., web server, proxy, browser, etc.), but it does not neutralize or incorrectly neutralizes CR and LF characters before the data is included in outgoing HTTP headers.

HTTP agents or components may include a web server, load balancer, reverse proxy, web caching proxy, application firewall, web browser, etc. Regardless of the role, they are expected to maintain coherent, consistent HTTP communication state across all components. However, including unexpected data in an HTTP header allows an attacker to specify the entirety of the HTTP message that is rendered by the client HTTP agent (e.g., web browser) or back-end HTTP agent (e.g., web server), whether the message is part of a request or a response.

When an HTTP request contains unexpected CR and LF characters, the server may respond with an output stream that is interpreted as "splitting" the stream into two different HTTP messages instead of one. CR is carriage return, also given by %0d or \r, and LF is line feed, also given by %0a or \n.

In addition to CR and LF characters, other valid/RFC compliant special characters and unique character encodings can be utilized, such as HT (horizontal tab, also given by %09 or \t) and SP (space, also given as + sign or %20).

These types of unvalidated and unexpected data in HTTP message headers allow an attacker to control the second "split" message to mount attacks such as server-side request forgery, cross-site scripting, and cache poisoning attacks.

HTTP response splitting weaknesses may be present when:

関連する脆弱性

想定される影響

Scope: Integrity, Access Control / Impact: Modify Application Data; Gain Privileges or Assume Identity

対策・緩和策

Construct HTTP headers very carefully, avoiding the use of non-validated input data.

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. If an input does not strictly conform to specifications, reject it or transform it into something that conforms.

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.

Use and specify an output encoding that can be handled by the downstream component that is reading the output. Common encodings include ISO-8859-1, UTF-7, and UTF-8. When an encoding is not specified, a downstream component may choose a different encoding, either by assuming a default encoding or automatically inferring which encoding is being used, which can be erroneous. When the encodings are inconsistent, the downstream component might treat some character or byte sequences as special, even if they are not special in the original encoding. Attackers might then be able to exploit this discrepancy and conduct injection attacks; they even might be able to bypass protection mechanisms that assume the original encoding is also being used by the downstream component.

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.

外部リンク

MITRE公式ページ — CWE-113