Crlf Injection in Chi with Dynamodb

Crlf Injection occurs when an attacker can inject carriage return (CR, \r) and line feed (\n) sequences into an HTTP response header. In a Chi application that interacts with Amazon DynamoDB, this typically arises when user-controlled data retrieved from a DynamoDB item is placed into a header, a redirect location, or into a rendered response without proper sanitization. Because DynamoDB stores values as strings or binary data, if an application writes a DynamoDB attribute directly into a header—such as a custom X-Request-ID or a Location value—the injected CRLF sequences can split the header chain and inject new headers or the response body. In Chi, routes are composed as middleware; if a handler builds a response header using values from a DynamoDB record without validation, an attacker can control subsequent lines in the response.

Consider a scenario where the application retrieves a user profile from a DynamoDB table and sets a custom header X-User-Data based on an attribute. A malicious user could have previously stored a value containing \r\nSet-Cookie: malicious=1 in that attribute. When Chi reads the item and sets the header, the injected CRLF can introduce a new header, potentially leading to session fixation or response splitting. The DynamoDB scan performed by middleBrick checks for indicators that responses may reflect unsafe data usage, including header-like patterns found in DynamoDB attributes. This is part of the Data Exposure and Input Validation checks, which run in parallel during a scan. The scanner does not rely on internal architecture, but it does highlight risks when responses include values that resemble HTTP headers and originate from DynamoDB fields.

Another vector involves redirects. If a DynamoDB item contains a URL intended for a redirect, and the application uses that URL in a Location header via chi.urls.Redirect, an attacker who can influence the stored URL can inject \r\n to append new headers or a new response body. The CRLF sequences will be interpreted by the HTTP server and client as header termination and new header injection. Because DynamoDB does not enforce a strict schema, string values can contain characters that break header formatting. The combination of Chi’s flexible route composition and DynamoDB’s schemaless storage increases the surface for response splitting when developers concatenate user-influenced data into headers or redirect targets. middleBrick’s LLM/AI Security checks specifically probe for output exposure of sensitive patterns and will flag cases where DynamoDB-sourced data appears in response headers without encoding or validation.

To remediate Crlf Injection when using DynamoDB and Chi, ensure that any data sourced from DynamoDB is sanitized before being placed into headers, redirects, or any line-based protocol context. The safest approach is to treat all DynamoDB string attributes as opaque and to apply strict allow-lists or percent-encode characters that are not valid in the target header. Below are concrete code examples for a Chi application that demonstrate secure handling of DynamoDB values.

First, define a function that sanitizes strings for header use by removing or replacing CRLF characters. Then, use this function whenever setting headers from DynamoDB attributes. Here is an example in Common Lisp using the cl-ppcre library to remove CR and LF characters:

(ql:quickload '(:cl-ppcre :aws-sdk2/dynamodb))

(defun sanitize-header-value (value)
  "Remove CR and LF characters to prevent header injection."
  (cl-ppcre:regex-replace-all "[\r\n]" value ""))

(defun get-item-safe (table-name key)
  (let ((client (aws-sdk2/dynamodb:make-client))
        (request (aws-sdk2/dynamodb:make-get-item-request
                  :table-name table-name
                  :key (list (cons "id" (cons :s key))))))
    (aws-sdk2/dynamodb:get-item client request)))

;; In a Chi handler
(let* ((item (get-item-safe "users" "user123"))
       (raw-value (gethash "custom-header" (aws-sdk2/dynamodb:item-attributes item)))
       (safe-value (sanitize-header-value (or raw-value ""))))
  (setf (hunchentoot:header-out "X-Custom-Data") safe-value)
  (format nil "Safe header set: ~a" safe-value))

For redirects, always validate or reconstruct the URL instead of directly using a DynamoDB attribute. Use Chi’s built-in URL utilities to ensure the Location header contains a safe absolute or relative path. The following example shows how to redirect safely:

(defun safe-redirect (url)
  "Ensure the URL does not contain CRLF before redirecting."
  (let ((clean-url (sanitize-header-value url)))
    (chi:redirect clean-url)))

;; Handler using a DynamoDB-stored target
(let* ((item (get-item-safe "redirects" "home"))
       (target (gethash "url" (aws-sdk2/dynamodb:item-attributes item))))
  (safe-redirect target))

Additionally, consider schema validation for DynamoDB items at the application layer. If a field is expected to be a URL, enforce a strict pattern and reject items that do not conform. This reduces the risk of storing malicious values that could later be used in headers. middleBrick’s OpenAPI/Swagger spec analysis can highlight endpoints that accept user input destined for headers, and its Data Exposure checks can help identify DynamoDB attributes that appear in response contexts. By combining these practices with the remediation steps above, you can effectively mitigate Crlf Injection in Chi applications that use DynamoDB.