Symlink Attack in Echo Go with Dynamodb

A symlink attack in an Echo Go service that uses DynamoDB typically occurs when the application builds file paths for DynamoDB local development or integration layers using user-controlled input, then creates or follows symbolic links in the filesystem. An attacker can manipulate the path so that a file write resolves to a location the application did not intend, potentially reaching DynamoDB configuration files, credentials, or data stores used by the service. Because DynamoDB is often accessed via local file system artifacts during development (e.g., local endpoint configurations, SDK credential files), a symlink can redirect writes to these sensitive artifacts, exposing credentials or altering endpoint behavior.

In Echo Go, routes that accept file uploads or path parameters and then write content to disk before interacting with DynamoDB can be vulnerable if path cleaning is insufficient. For example, if a handler uses filepath.Join with an untrusted segment to construct a path to a local DynamoDB dump or cache file, an attacker-supplied ../ sequence can escape the intended directory. If the application then opens the file for writing without resolving symlinks, it may overwrite a configuration file that the DynamoDB client reads on subsequent calls. This can lead to credential leakage or redirection of API calls to unintended endpoints, changing how the application communicates with DynamoDB.

The risk is not in DynamoDB itself but in how the Go service manages temporary files and symbolic links before issuing SDK calls. If the service runs with higher privileges or if the DynamoDB endpoint configuration is stored on the filesystem, a symlink can redirect writes to those files. This becomes a chain that exposes the unauthenticated attack surface when the service exposes file-system-assisted operations without validating canonical paths. middleBrick detects such risks by correlating path-handling patterns and unauthenticated endpoint behavior, highlighting findings related to path traversal and insecure file operations that could affect DynamoDB interactions.

Remediation focuses on strict path validation, avoiding symlink-following writes, and isolating DynamoDB configuration from user-controlled paths. Use absolute path resolution with filepath.EvalSymlinks before opening files, and ensure directory boundaries are enforced. Prefer in-memory or isolated temporary directories for DynamoDB-related artifacts, and never derive filesystem paths from user input that could traverse outside the intended scope.

// Secure handler example in Echo Go
package main

import (
    "fmt"
    "io/ioutil"
    "net/http"
    "os"
    "path/filepath"

    "github.com/labstack/echo/v4"
)

func uploadHandler(c echo.Context) error {
    // Receive a filename and body from request (example only)
    filename := c.FormValue("filename")
    content, err := ioutil.ReadAll(c.Request().Body)
    if err != nil {
        return c.String(http.StatusBadRequest, "failed to read body")
    }

    // Validate and sanitize filename: allow only alphanumeric and limited safe chars
    cleanName := filepath.Base(filename) // strips path separators
    if cleanName != filename {
        return c.String(http.StatusBadRequest, "invalid filename")
    }

    // Define a secure base directory for temporary artifacts
    baseDir := "/srv/app/dynamodb-tmp"
    if err := os.MkdirAll(baseDir, 0700); err != nil {
        return c.String(http.StatusInternalServerError, "cannot create temp dir")
    }

    // Build target path safely using filepath.Join with the base directory
    target := filepath.Join(baseDir, cleanName)

    // Resolve symlinks and ensure the target remains inside baseDir
    absBase, err := filepath.Abs(baseDir)
    if err != nil {
        return c.String(http.StatusInternalServerError, "cannot resolve base")
    }
    resolved, err := filepath.EvalSymlinks(absBase)
    if err != nil {
        return c.String(http.StatusInternalServerError, "cannot resolve base symlinks")
    }
    absTarget, err := filepath.Abs(target)
    if err != nil {
        return c.String(http.StatusInternalServerError, "cannot resolve target")
    }
    finalPath, err := filepath.Rel(resolved, absTarget)
    if err != nil || finalPath == ".." || filepath.IsAbs(finalPath) && finalPath[:3] == ".." {
        return c.String(http.StatusBadRequest, "path escapes base directory")
    }

    // Write content without following symlinks in the target directory
    if err := ioutil.WriteFile(target, content, 0600); err != nil {
        return c.String(http.StatusInternalServerError, "failed to write file")
    }

    // Example: load DynamoDB configuration from a fixed, non-user path
    cfgPath := "/etc/app/dynamodb_config.json"
    cfgData, err := ioutil.ReadFile(cfgPath)
    if err != nil {
        return c.String(http.StatusInternalServerError, "cannot read DynamoDB config")
    }
    fmt.Printf("Loaded DynamoDB config from fixed path: %s\n", cfgPath)

    return c.JSON(http.StatusOK, map[string]string{"status": "ok", "file": target})
}

func main() {
    e := echo.New()
    e.POST("/upload", uploadHandler)
    e.Logger.Fatal(e.Start(":8080"))
}

Key practices:

• Use filepath.Base or strict allowlists for filenames and never concatenate user input into paths.
• Resolve the base directory with filepath.Abs and filepath.EvalSymlinks to detect symlinks before creating files.
• Ensure the resolved target path remains within the intended directory using filepath.Rel checks.
• Keep DynamoDB configuration and credentials in fixed, non-user-writable locations and reference them by absolute paths that are not derived from user input.