Path Traversal in Buffalo with Dynamodb

Path Traversal occurs when user-controlled input is used to construct file system paths without proper validation, allowing an attacker to access files outside the intended directory. In a Buffalo application using Amazon DynamoDB as a persistence layer, the risk typically arises at the application-design boundary rather than within DynamoDB itself, because DynamoDB does not provide traditional file system access. However, a vulnerable endpoint that uses user input to derive logical keys, object references, or even indirect file paths (for example, when exporting or importing data to local storage) can expose sensitive files.

Consider a scenario where a Buffalo handler accepts a document_id parameter to retrieve a user’s profile image stored locally and referenced by a key stored in a DynamoDB table. If the handler directly concatenates the user input into a filesystem path, an attacker can supply sequences like ../../etc/passwd to traverse directories. Even though DynamoDB stores only structured data, the application layer that maps DynamoDB items to local resources becomes the attack surface. The DynamoDB table might contain a field such as avatar_s3_key or local_file_path; if that field is derived from or influenced by user input without canonicalization, the traversal can lead to unauthorized file reads or writes.

Additionally, if the application uses AWS SDK operations that reference object keys derived from user input (for example, constructing an S3 key prefix from a path-like parameter), improper validation may allow access to unintended objects that logically reside outside the permitted prefix. While this is not a classic OS-level path traversal, the effect is similar: data exposure across trust boundaries. The scanning capabilities of middleBrick include checks for Path Traversal within the context of API input validation and Data Exposure; when scanning a Buffalo endpoint that interacts with DynamoDB, it tests whether user-supplied identifiers can escape intended logical boundaries and maps findings to relevant OWASP API Top 10 categories such as Broken Object Level Authorization (BOLA) and Injection.

Remediation focuses on strict input validation, canonical paths, and safe mapping between user requests and DynamoDB keys. Avoid using user input directly in filesystem paths or in constructing object keys that infer directory traversal. Instead, use a controlled mapping layer and enforce allowlists.

Example: Safe DynamoDB key construction in Buffalo

Assume a User model stored in DynamoDB with a partition key user_id. The application should derive the key from authenticated session or token claims, never from raw user-supplied path segments.

// handlers/user_avatar.go
package handlers

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

func GetAvatar(c buffalo.Context) error {
    // Authenticated user ID from session; never from URL path directly
    userID := c.Value("user_id").(string)
    // Validate and sanitize any additional component, e.g., file name
    filename := c.Param("filename")
    if filename == "" {
        return c.Error(400, errors.New("filename is required"))
    }
    // Allowlist safe characters and reject path separators
    if strings.ContainsAny(filename, "/\\") {
        return c.Error(400, errors.New("invalid filename"))
    }
    // Construct a safe key for DynamoDB or S3; no filesystem traversal possible
    key := "avatar/" + userID + "/" + filename
    // Use the AWS SDK to get the object using 'key' (e.g., S3 GetObject)
    // ...
    return c.Render(200, r.JSON(map[string]string{"key": key}))
}

Example: Parameterized query with DynamoDB GetItem

When retrieving items from DynamoDB, use the SDK’s built-in expression builders and never interpolate user input into key strings without validation.

// services/user_service.go
package services

import (
    "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 UserService struct {
    dynamoClient *dynamodb.DynamoDB
    tableName  string
}

func (s *UserService) GetUser(userID string) (*User, error) {
    // Enforce a simple pattern for userID if necessary (e.g., UUID)
    if userID == "" || strings.ContainsAny(userID, "/\\") {
        return nil, errors.New("invalid user identifier")
    }
    input := &dynamodb.GetItemInput{
        TableName: aws.String(s.tableName),
        Key: map[string]*dynamodb.AttributeValue{
            "user_id": {
                S: aws.String(userID),
            }},
    }
    result, err := s.dynamoClient.GetItem(input)
    if err != nil {
        return nil, err
    }
    var user User
    err = dynamodbattribute.UnmarshalMap(result.Item, &user)
    if err != nil {
        return nil, err
    }
    return &user, nil
}

General practices

• Treat user input as identifiers, not paths. Map identifiers to internal keys via a database or index.
• Use allowlists for filenames and reject characters that enable directory traversal (/, \, .., . in certain positions).
• Apply Principle of Least Privilege to AWS credentials used by the Buffalo app, restricting access to specific DynamoDB tables and S3 prefixes.

middleBrick’s checks for Property Authorization and Input Validation help detect whether endpoints expose indirect path traversal risks when integrating with DynamoDB; its findings include remediation guidance tailored to API design patterns.