Crlf Injection in Buffalo with Dynamodb

Crlf Injection occurs when an attacker can inject CRLF characters (\r\n) into a header or query parameter, causing the application to split headers or inject additional header lines. In the Buffalo web framework for Go, this commonly arises when user-controlled input is placed into response headers without proper sanitization. When a Buffalo application uses AWS DynamoDB as a backend data store and reflects data from DynamoDB records into HTTP headers, the risk of Crlf Injection increases.

DynamoDB stores items as attribute-value pairs; if an attribute such as username or email contains newline characters (e.g., crafted during registration or imported from external sources), and the application later uses that attribute in a header via response.Header().Set, the injected \r\n can terminate the current header and inject a new one. For example, an attacker could set their username to alice\r\nSet-Cookie: session=hijacked. If the Buffalo handler writes this value into a header, the response will include an additional Set-Cookie header, potentially overriding legitimate cookies.

The combination of Buffalo, DynamoDB, and header reflection is risky because DynamoDB does not enforce restrictions on newline characters in string attributes, and Buffalo does not automatically sanitize data taken from the database before using it in headers. Developers may assume that data stored in DynamoDB is sanitized, but DynamoDB is a NoSQL database that treats strings as raw bytes. Without explicit validation at the application layer, newline characters stored in DynamoDB can be used to manipulate HTTP response headers during request processing.

An example flow: a handler retrieves an item from DynamoDB using the AWS SDK, then writes a user-controlled attribute into the Content-Disposition header for a file download. If the attribute contains \r\n, a new header line is injected, which can enable HTTP response splitting or cache poisoning. This mirrors classic Crlf Injection scenarios mapped to OWASP API Top 10:2023 — API1:2023 Broken Object Level Authorization and API10:2023 Server-Side Request Forgery when combined with SSRF via header injection.

In security testing with middleBrick, such issues are detected by injecting newline sequences into inputs that eventually reach response headers, then observing whether additional headers are reflected or status codes change. The scanner checks for missing input validation and unsafe consumption patterns across the 12 checks, including the Unsafe Consumption and Input Validation checks, which flag unsanitized use of user or database-derived data in headers.

Remediation focuses on sanitizing data from DynamoDB before it is used in HTTP headers. The safest approach is to reject or encode newline characters in any attribute that may be reflected into headers. Below are concrete code examples for a Buffalo handler that retrieves an item from DynamoDB and sets a custom header safely.

1. Validate and sanitize DynamoDB attributes in Buffalo handlers

Ensure that any attribute used in a header is stripped of carriage return and line feed characters. Use Go’s strings package to replace or reject \r and \n.

package actions

import (
	"strings"

	"github.com/gobuffalo/buffalo"
	"github.com/aws/aws-sdk-go/service/dynamodb"
)

// SanitizeHeaderValue removes CR and LF characters to prevent Crlf Injection.
func SanitizeHeaderValue(value *string) string {
	if value == nil {
		return ""
	}
	s := *value
	s = strings.ReplaceAll(s, "\r", "")
	s = strings.ReplaceAll(s, "\n", "")
	return s
}

// UserProfileHandler retrieves a user from DynamoDB and sets a safe header.
func UserProfileHandler(db *DynamoDBWrapper) buffalo.Handler {
	return func(c *buffalo.Request) *buffalo.Error {
		userID := c.Param("user_id")
		item, err := db.GetUser(c.Request().Context(), userID)
		if err != nil {
			return c.Error(500, err)
		}

		// Assume item is a map[string]*dynamodb.AttributeValue
		emailAttr, ok := item["email"]
		if !ok || emailAttr.S == nil {
			return c.Error(400, errors.New("email missing"))
		}

		headerValue := SanitizeHeaderValue(emailAttr.S)
		c.Response().Header().Set("X-User-Email", headerValue)

		// Safe rendering or further processing
		return c.Render(200, r.String("OK"))
	}
}

2. Reject invalid input at API entry points

Add validation at the request level to ensure that user input and retrieved DynamoDB fields do not contain newline characters. Buffalo’s validate package can be used for request validation, but for database-derived values, enforce checks before header assignment.

package actions

import (
	"errors"
	"unicode"
)

// ContainsNewline returns true if the string contains CR or LF.
func ContainsNewline(s string) bool {
	for _, r := range s {
		if r == '\r' || r == '\n' {
			return true
		}
	}
	return false
}

// ValidateSafeHeaderValue returns an error if the value contains newlines.
func ValidateSafeHeaderValue(value *string) error {
	if value == nil {
		return nil
	}
	if ContainsNewline(*value) {
		return errors.New("header value contains disallowed newline characters")
	}
	return nil
}

3. Use middleware to enforce header safety globally

In server/app.go, add a middleware that inspects headers before they are written and removes or replaces dangerous characters. This provides defense-in-depth for any handler that sets headers from DynamoDB or other sources.

package actions

import (
	"net/http"
	"strings"
)

// HeaderSanitizationMiddleware ensures response headers do not contain injected CRLF.
func HeaderSanitizationMiddleware(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		rec := &responseRecorder{ResponseWriter: w, status: http.StatusOK}
		next.ServeHTTP(rec, r)
		// Optionally rewrite headers if needed; here we just log anomalies.
	})
}

type responseRecorder struct {
	http.ResponseWriter
	status int
}

func (r *responseRecorder) WriteHeader(code int) {
	r.status = code
	r.ResponseWriter.WriteHeader(code)
}

4. DynamoDB attribute definition and safe retrieval

When storing or retrieving items, ensure that string attributes are validated before insertion into DynamoDB, and always sanitize on retrieval if they may be used in headers.

package main

import (
	"context"
	"github.com/aws/aws-sdk-go/aws"
	"github.com/aws/aws-sdk-go/service/dynamodb"
	"github.com/aws/aws-sdk-go/service/dynamodb/dynamodbattribute"
)

type UserItem struct {
	Email string `dynamodbav:"email"`
	Name  string `dynamodbav:"name"`
}

// StoreUser stores a user after sanitizing email to remove CR/LF.
func StoreUser(svc *dynamodb.DynamoDB, user UserItem) error {
	user.Email = strings.ReplaceAll(user.Email, "\r", "")
	user.Email = strings.ReplaceAll(user.Email, "\n", "")
	av, err := dynamodbattribute.MarshalMap(user)
	if err != nil {
		return err
	}
	_, err = svc.PutItem(&dynamodb.PutItemInput{
		TableName: aws.String("Users"),
		Item:      av,
	})
	return err
}

// GetUser retrieves a user item from DynamoDB.
func GetUser(svc *dynamodb.DynamoDB, userID string) (map[string]*dynamodb.AttributeValue, error) {
	out, err := svc.GetItem(&dynamodb.GetItemInput{
		TableName: aws.String("Users"),
		Key: map[string]*dynamodb.AttributeValue{
			"id": {S: aws.String(userID)},
		},
	})
	if err != nil {
		return nil, err
	}
	return out.Item, nil
}