Path Traversal in Echo Go with Dynamodb

Path Traversal occurs when user-controlled input is used to construct file or object paths without proper validation, allowing an attacker to access files or resources outside the intended directory. In a Go service built with the Echo framework that stores or retrieves objects from Amazon DynamoDB, the risk arises when request parameters such as userID or documentID are directly used to build DynamoDB keys or to derive object paths in application logic.

Consider an endpoint designed to fetch user documents from DynamoDB. If the documentID from the request is concatenated with a base path to form a key without canonicalization or validation, an attacker can supply inputs like ../../../etc/passwd. Even though DynamoDB itself does not have a classic filesystem path, the application layer may use the supplied identifier to build S3 paths, local cache keys, or internal resource identifiers. Echo routes typically bind parameters directly into handler functions, so without explicit validation, these values flow straight into DynamoDB key construction or subsequent internal path assembly.

For example, a handler might create a DynamoDB attribute value by joining a static prefix with the user-supplied ID. If the ID traverses directory boundaries, the application may unintentionally reference other users' data or configuration files. While DynamoDB enforces its own access controls via IAM policies and condition expressions, the vulnerability lies in how the application maps HTTP parameters to DynamoDB keys or related resources. A misconfigured route like GET /documents/:documentID can thus become an indirect vector for unauthorized data access across logical boundaries, even when the underlying database permissions appear correct.

In the context of middleBrick's checks, this pattern is flagged under BOLA/IDOR and Input Validation categories. The scanner tests whether path-like inputs are safely handled before being used in key generation or resource resolution. Because Echo Go routes often bind parameters implicitly, developers must explicitly sanitize and validate every user-supplied segment before using it in any key construction logic, whether that key targets DynamoDB or any downstream resource.

To remediate Path Traversal in Echo Go with DynamoDB, validate and sanitize all user input before constructing keys, and avoid direct concatenation of user data into paths or key components. Use allowlists for acceptable characters, enforce strict length limits, and canonicalize inputs where applicable. Below are concrete code examples demonstrating secure patterns.

1. Validate and sanitize input before using it as a DynamoDB key

Ensure that identifiers do not contain path traversal characters or sequences. Use a strict allowlist (e.g., alphanumeric and safe delimiters) and reject anything that does not match.

package handlers

import (
    "net/http"
    "regexp"
    "strings"

    "github.com/labstack/echo/v4"
    "github.com/aws/aws-sdk-go-v2/service/dynamodb"
)

var safeID = regexp.MustCompile(`^[a-zA-Z0-9_-]{1,64}$`)

func GetDocument(c echo.Context) error {
    documentID := strings.TrimSpace(c.Param("documentID"))
    if !safeID.MatchString(documentID) {
        return echo.NewHTTPError(http.StatusBadRequest, "invalid document identifier")
    }

    // Safe to use as part of a DynamoDB key
    key := map[string]interface{}{
        "PK": map[string]interface{}{
            "S": "DOC#" + documentID,
        },
    }

    // Example DynamoDB GetItem call
    // out, err := svc.GetItem(context.TODO(), &dynamodb.GetItemInput{
    //     TableName: aws.String("Documents"),
    //     Key:       key,
    // })
    _ = key
    return c.JSON(http.StatusOK, map[string]string{"id": documentID})
}

2. Use explicit key construction with a prefix and UUID or hash

Instead of using raw user input as a key, derive a safe key using a hash or UUID. This prevents traversal attempts and reduces exposure of internal structures.

import (
    "crypto/sha256"
    "encoding/hex"
)

func buildKey(userID, documentID string) map[string]interface{} {
    hasher := sha256.New()
    hasher.Write([]byte(userID + ":" + documentID))
    safeKey := hex.EncodeToString(hasher.Sum(nil))

    return map[string]interface{}{
        "PK": map[string]interface{}{
            "S": "DOC#" + safeKey,
        },
        "SK": map[string]interface{}{
            "S": "METADATA",
        },
    }
}

3. Enforce separation of concerns and avoid path joins

Do not join user input to form filesystem-like paths for DynamoDB operations. If you must store objects in S3, validate and restrict bucket and key construction separately, using IAM conditions to scope access per user.

// Example of safe S3 key construction (if needed)
// bucket := "my-secure-bucket"
// userPrefix := "user-" + userID // userID already validated
// key := userPrefix + "/documents/" + safeDocumentUUID

4. Combine with DynamoDB condition expressions for ownership

Even with safe key construction, use condition expressions to ensure users can only access their own items. This adds a layer of defense-in-depth beyond path traversal prevention.

// conditionExpression := "userID = :uid"
// expressionAttributeValues := map[string]interface{}{
//     ":uid": map[string]interface{}{"S": userID},
// }

By applying these patterns, you eliminate path traversal risks while maintaining compatibility with DynamoDB's key model. Echo Go routes remain clean, and the attack surface for IDOR and BOLA is reduced through strict input validation and safe key derivation.