Request Smuggling in Chi with Dynamodb

Request smuggling occurs when an application processes HTTP requests differently depending on whether they are interpreted as front-end or back-end requests. In a Chi application that uses Amazon DynamoDB as a persistence layer, this typically arises from mismatched parsing of headers, body lengths, or chunked transfer encoding between a reverse proxy/load balancer and the Chi server. Because DynamoDB operations often rely on exact request attributes such as table names, keys, and condition expressions, smuggling can lead to unauthorized data access or injection of malicious operations if requests are misinterpreted and routed to the wrong backend handler.

Chi routes are built as composable middleware, and the way headers and bodies are passed downstream can expose parsing ambiguities. For example, if a proxy sends a request with both Transfer-Encoding: chunked and a Content-Length header, and Chi or the underlying HTTP library processes one while the proxy expects the other, the request body may be misaligned. An attacker can craft a request where the first valid-looking request targets a benign DynamoDB query (e.g., retrieving user profile data), while the smuggled request within the same connection attempts a destructive operation such as DeleteItem or updates sensitive items. Because DynamoDB does not inherently associate requests with sessions or users beyond the credentials supplied, if the Chi service forwards the smuggled request with the same IAM context, the operation may execute with unintended permissions.

Additionally, if the application deserializes JSON bodies into DynamoDB AttributeValue structures without strict validation, smuggling can facilitate injection of malformed or unexpected attribute values. Consider a Chi endpoint that accepts a JSON payload to query a DynamoDB table; if the body is parsed inconsistently due to smuggling, an attacker might embed extra JSON fragments or manipulate the structure to bypass expected filters. This can result in querying or modifying items outside the intended partition key scope, effectively an Insecure Direct Object Reference (IDOR) or Broken Access Control scenario facilitated by the smuggling vector. The interplay between Chi’s routing flexibility, header/body parsing nuances, and DynamoDB’s strict attribute format creates a surface where request inconsistencies can be weaponized.

Real-world examples align with known patterns such as CVE-style manipulation where header mismatches lead to unauthorized DynamoDB operations. Because DynamoDB requires precise key schemas, any injected or misinterpreted key condition can lead to scanning or retrieval of unintended items. MiddleBrick’s LLM/AI Security checks and API scanning can detect anomalies in request handling and flag potential smuggling indicators across the API surface, helping teams identify risky configurations before exploitation occurs.

Remediation focuses on ensuring consistent request parsing, strict header handling, and defensive validation before any DynamoDB interaction. In Chi, use explicit body readers and avoid relying on default parsing when behind proxies that may modify headers. Below is a concrete Chi endpoint that safely processes a JSON payload intended for a DynamoDB GetItem operation, with canonical header handling and input validation.

-- chi-smuggling-safe.hs
{-# LANGUAGE OverloadedStrings #-}
import Network.Wai (requestHeaders, requestBody)
import Network.Wai.Handler.Warp (run)
import qualified Data.ByteString.Lazy as BL
import qualified Data.Text as T
import qualified Data.Text.IO as T
import Data.Aeson (eitherDecode)
import Database.DynamoDB
import Network.HTTP.Types (hContentType, hContentLength, hTransferEncoding)
import Control.Monad.IO.Class (liftIO)

-- Define a safe request structure matching DynamoDB GetItem input
data GetItemRequest = GetItemRequest
  { tableName :: T.Text
  , key       :: [(T.Text, AttributeValue)]
  } deriving (Show)

-- Strict JSON parsing with validation
parseGetItem :: BL.ByteString -> Either String GetItemRequest
parseGetItem = eitherDecode

-- Ensure no ambiguous transfer encodings
validateHeaders :: [(T.Text, T.Text)] -> Bool
validateHeaders headers =
  not (any ("transfer-encoding"==) (map (T.toLower . fst) headers)) &&
  case lookup "content-length" (map (T.toLower . fst) -> headers) of
    Just len -> readMaybe (T.unpack len) :: Maybe Int
    Nothing  -> False

app :: Application
app req respond = do
  let hdrs = requestHeaders req
  -- Reject requests with suspicious header combinations
  if not (validateHeaders (map (T.pack *** T.pack) hdrs))
    then respond $ responseLBS status400 [] "Invalid headers"
    else do
      body <- requestBody req
      case parseGetItem body of
        Left err -> respond $ responseLBS status400 [] ("Invalid JSON: " <> T.pack err)
        Right reqData -> do
          -- Perform DynamoDB GetItem with explicit table name and key
          result <- runResourceT $ do
            env <- newEnv Discover 
            resp <- runDynamo env $ GetItem (TableName (T.unpack (tableName reqData))) (Key (head (key reqData))) Nothing
            return resp
          -- Respond with safe JSON
          respond $ responseLBS status200 [(hContentType, "application/json")] (encode resp)
  where
    T.pack *** T.pack = (,)
    readMaybe s = case reads s of [(x,"")] -> Just x; _ -> Nothing

This example enforces strict header validation to prevent smuggling attempts that exploit Transfer-Encoding and Content-Length mismatches. By explicitly reading and parsing the body before any DynamoDB call, the endpoint ensures that only well-formed, singular requests are processed. The DynamoDB operations use the official amazonka-dynamodb library with precise attribute types, avoiding loose JSON interpretation that could be abused via smuggling.

Additional remediation steps include configuring the reverse proxy to normalize headers before passing requests to Chi, disabling chunked transfer encoding where not required, and applying middleware that rejects requests with multiple Content-Length values. MiddleBrick’s scans can verify that endpoints follow these patterns and flag any configurations where smuggling risks remain.