Email Injection in Chi with Dynamodb

Email Injection in the context of a Chi web application that uses Amazon DynamoDB as a persistence layer typically arises when user-controlled input is incorporated into email headers or message bodies without validation, and the resulting data is stored in or retrieved from DynamoDB. Chi is a lightweight Clojure web framework where route parameters, query strings, and JSON payloads can all become sources of untrusted data. When this data is used to construct email headers—such as To, Cc, Subject, or Reply-To—without canonicalization or allowlisting, attackers can inject additional headers or control the message flow.

DynamoDB stores the email templates or user-provided fields that feed into these headers. Because DynamoDB is a NoSQL database, it does not enforce a rigid schema; therefore, an application might store a value like user-supplied-email-header and later interpolate it into an email without re-validation. If the stored value contains newline characters (e.g., %0aCc: [email protected] or \r\nBcc: [email protected]), the email generation logic in Chi may treat these as header separators, leading to header injection. Common attack patterns include injecting extra recipients, manipulating reply-to addresses to hijack responses, or smuggling malicious content into the message body via crafted newlines.

The risk is amplified when the application retrieves data from DynamoDB and passes it directly to an email-sending library, such as clj-email or an SMTP client, without sanitizing line breaks or enforcing header encoding. For example, a stored DynamoDB attribute containing test\r\nSubject: Urgent Password Reset could cause the email engine to treat part of the user input as a new subject line. Because DynamoDB does not inherently filter control characters, the application must explicitly validate and encode any data used in email construction. The combination of Chi’s flexible routing and DynamoDB’s schema-less nature creates a clear path for email injection if input handling is not strict at both storage and rendering stages.

An attacker might first probe the API endpoint managed by Chi to identify how user data is persisted in DynamoDB. Using a tool like middleBrick, which scans unauthenticated attack surfaces and checks input validation and data exposure, the attacker can detect whether email fields are reflected in responses or stored without sanitization. middleBrick’s checks for Input Validation and Data Exposure help identify whether newlines or suspicious patterns are accepted and stored. If the application later uses that data in email generation, the stored malicious header sequences can be triggered during legitimate message composition, leading to unauthorized redirection or information leakage.

Remediation focuses on two areas: preventing malicious sequences at the point of DynamoDB storage and hardening email construction in Chi. At storage, normalize user input by removing or encoding carriage returns and line feeds, and apply strict allowlists for email header fields. At the rendering stage, use dedicated libraries that separate headers from content and enforce RFC-compliant formatting. MiddleBrick’s findings can guide developers to the exact vectors where injection is possible, and its integration with CI/CD via the GitHub Action can ensure that future changes do not reintroduce the issue.

To remediate email injection in Chi with DynamoDB, implement input validation and output encoding at both storage and usage boundaries. Below are concrete code examples demonstrating safe handling of user-supplied email data.

1. Sanitization before storing in DynamoDB

When persisting user input that may be used in email contexts, strip or replace control characters that could enable header injection. Use Clojure’s clojure.string/replace to remove carriage returns and line feeds, and validate format with a regular expression.

(ns app.email-store
  (:require [aws.sdk.dynamodb :as ddb]
            [clojure.string :as str]))

(defn sanitize-email-header [input]
  ;; Remove CR and LF characters to prevent header injection
  (-> input
      (str/replace #"[\r\n]" "")
      str/trim))

(defn store-email-preference [user-id email-header]
  (let [safe-header (sanitize-email-header email-header)]
    (ddb/put-item {:table-name "UserPreferences"
                   :item {:user-id {:s user-id}
                          :email-header {:s safe-header}}})))

2. Safe retrieval and usage in Chi routes

When reading from DynamoDB and constructing emails, use a dedicated email library that separates headers from body. Avoid string concatenation for headers. The following example uses buddy.core.codecs for safe encoding and a hypothetical email function that expects pre-sanitized data.

(ns app.email-handler
  (:require [aws.sdk.dynamodb :as ddb]
            [buddy.core.codecs :as codecs]
            [app.email :as email]))

(defn get-email-header-from-db [user-id]
  (let [resp (ddb/get-item {:table-name "UserPreferences"
                            :key {:user-id {:s user-id}}})]
    (-> resp :item :email-header :s)))

(defn send-welcome-email [user-id recipient-email]
  (let [header-from-db (get-email-header-from-db user-id)
        safe-subject (if (str/blank? header-from-db)
                       "Welcome"
                       (codecs/url-encode header-from-db))]
    (email/send {:to recipient-email
                 :subject safe-subject
                 :body "Welcome to our service."})))

3. Enforce allowlists for known-safe values

For critical headers like Subject, prefer an allowlist approach rather than trying to sanitize arbitrary input. Define a set of permitted subjects and map user choices to them.

(def allowed-subjects
  {"welcome" "Welcome"
   "reminder" "Account Reminder"
   "promo" "Special Offer"})

(defn get-safe-subject [user-key]
  (get allowed-subjects user-key "Welcome"))

4. Use middleware to validate route parameters

In Chi, apply validation early in the request lifecycle to reject malformed input before it reaches DynamoDB. This prevents storage of invalid data that could later cause email injection.

(ns app.middleware
  (:require [ring.util.response :as resp]))

(defn validate-email-header [handler]
  (fn [request]
    (let [header-value (get-in request [:params :email-header])]
      (if (and header-value (re-matches #"[^\r\n]+" header-value))
        (handler request)
        (resp/bad-request "Invalid email header")))))

By combining sanitization at write time, safe encoding at read time, and strict validation in Chi routes, the application mitigates email injection risks while continuing to leverage DynamoDB for persistence. Regular scans with middleBrick can verify that input validation controls are effective and that no new vectors are introduced through changes to the data model or email logic.