Http Request Smuggling in Buffalo with Dynamodb

HTTP Request Smuggling arises when a backend service processes HTTP messages differently depending on whether they are interpreted as front-end (e.g., a load balancer or reverse proxy) or back-end requests. In a Buffalo application that uses Amazon DynamoDB as its persistence layer, the risk is shaped by how requests flow through infrastructure, how routes are defined, and how DynamoDB operations are constructed from user-supplied input.

Buffalo does not provide a built-in web server for production; it relies on an underlying HTTP handler. If you place a proxy that buffers or normalizes requests differently than the application parses them, header ordering and chunked encoding handling can diverge. A common pattern is to forward requests to a Buffalo app while also storing or referencing items in DynamoDB based on parsed body parameters or headers. If the proxy merges headers differently (for example, handling Content-Length and Transfer-Encoding inconsistently), an attacker can smuggle a request that the proxy interprets as part of the current request while the Buffalo handler parses it as a separate, second request.

DynamoDB-specific exposure occurs when smuggled requests manipulate database operations. For example, if a route uses user-controlled values to construct a GetItem or Query input without strict validation, a smuggled parameter could change the table name, key schema, or filter expressions. Consider a route that retrieves a document by ID from a table named by a tenant identifier. If a header is smuggled to change the table name or the key attribute, the application might read or write data belonging to another tenant, effectively turning the issue into an IDOR via DynamoDB. Moreover, if request bodies are bound to DynamoDB condition expressions or update paths, smuggling can modify those expressions, leading to unauthorized updates or deletes.

The combination is risky when the application constructs DynamoDB input structs directly from request headers or body fields without canonicalization. For instance, if a smuggled X-Target-Table header influences the table name in a Scan or Query, and the application does not validate the header against an allowlist, the attacker can probe for accessible tables or exfiltrate data. Even with unauthenticated scanning, middleBrick can surface such parameter pollution and header-handling inconsistencies as findings, especially when spec-defined request shapes do not align with runtime behavior.

Remediation focuses on strict input validation, canonical request handling, and avoiding user-controlled influence over low-level HTTP parsing or DynamoDB primitives. Below are concrete patterns for a Buffalo application using the AWS SDK for Go (v2).

1. Validate and constrain table names

Never allow a header or parameter to directly set the DynamoDB table name. Use a mapping or allowlist.

package actions

import (
    "context"
    "strings"

    "github.com/aws/aws-sdk-go-v2/aws"
    "github.com/aws/aws-sdk-go-v2/feature/dynamodb/attributevalue"
    "github.com/aws/aws-sdk-go-v2/service/dynamodb"
    "github.com/aws/aws-sdk-go-v2/service/dynamodb/types"
    "github.com/gobuffalo/buffalo"
)

var allowedTables = map[string]bool{
    "prod_users": true,
    "staging_users": true,
}

func getTable(name string) (string, bool) {
    // Normalize and check against allowlist
    name = strings.TrimPrefix(name, "_")
    ok := allowedTables[name]
    return name, ok
}

2. Use strongly-typed input structs and reject unknown fields

Define request payloads that explicitly declare expected fields and use decoder options to reject extra fields, reducing the surface for parameter smuggling.

package actions

import (
    "context"

    "github.com/aws/aws-sdk-go-v2/service/dynamodb"
    "github.com/gobuffalo/buffalo"
    "github.com/gobuffalo/validate/v3"
    "github.com/gorilla/schema"
)

type UserQuery struct {
    UserID string `schema:"user_id" validate:"required"`
    // No extra fields; decoder will error if unknown keys are present
}

func DecodeUserQuery(req *buffalo.Request) (*UserQuery, *validate.Errors, error) {
    dec := schema.NewDecoder()
    dec.IgnoreUnknownKeys(true) // or false to reject unknown keys
    var uq UserQuery
    err := dec.Decode(&uq, req.Params())
    if err != nil {
        return nil, nil, err
    }
    verrs, _ := validate.ValidateStruct(&uq)
    return &uq, verrs, nil
}

3. Build DynamoDB input safely inside handlers

Construct GetItemInput or QueryInput using constants for table and key expressions, and derive key values only from validated parameters.

package actions

import (
    "context"
    "github.com/aws/aws-sdk-go-v2/service/dynamodb"
    "github.com/gobuffalo/buffalo"
)

func GetUserHandler(c buffalo.Context) error {
    userID := c.Param("user_id")
    tableName, ok := getTable("prod_users")
    if !ok {
        return c.Render(400, r.String("invalid table"))
    }
    svc := dynamodb.NewFromConfig(awsConfig)
    out, err := svc.GetItem(c.Request().Context(), &dynamodb.GetItemInput{
        TableName: aws.String(tableName),
        Key: map[string]types.AttributeValue{
            "id": &types.AttributeValueMemberS{Value: userID},
        },
    })
    if err != nil {
        return c.Error(500, err)
    }
    // process out.Item
    return c.Render(200, r.JSON(out.Item))
}

4. Enforce canonical header processing

Configure your proxy to normalize headers (e.g., lowercase, single-valued) before routing to Buffalo. In the app, avoid merging headers; treat each header as a single source of truth. Do not allow header values to influence protocol-level decisions like Content-Length parsing.

5. Use middleware to reject malformed encodings

Add a Buffalo middleware that inspects Transfer-Encoding and Content-Length and rejects requests where both are present or where chunked encoding is used in an unexpected way. This reduces the chance that a smuggled chunked request reaches the router.

package middleware

import (
    "github.com/gobuffalo/buffalo"
)

func SmugglingProtection(next buffalo.Handler) buffalo.Handler {
    return func(c buffalo.Context) error {
        req := c.Request()
        te := req.Header.Get("Transfer-Encoding")
        cl := req.Header.Get("Content-Length")
        if te != "" && cl != "" {
            return c.Error(400, &InvalidRequestError{"conflicting headers"})
        }
        // Reject chunked if not expected
        if te == "chunked" && !expectedChunked(req) {
            return c.Error(400, &InvalidRequestError{"unexpected chunked encoding"})
        }
        return next(c)
    }
}