Path Traversal in Buffalo with Basic Auth

Path Traversal (commonly referenced as `CWE-22` and included in the OWASP API Top 10) occurs when user-controlled input is used to construct file system paths without proper validation or sanitization. In the Buffalo web framework for Go, this typically manifests when a route parameter or query string is passed directly to functions that open or serve files, such as `os.Open` or `http.ServeFile`.

When Basic Auth is used, it often creates a false sense of security. Developers may assume that because an endpoint is protected by HTTP Basic Authentication, the risk of unrestricted file access is reduced. However, authentication and path traversal are orthogonal concerns. middleBrick’s checks for Authentication and Path Traversal run independently; a valid Basic Auth credential does not mitigate an unchecked path traversal. An authenticated attacker can still leverage traversal sequences like ../../../etc/passwd if the application does not sanitize the input. The scanner tests the unauthenticated attack surface by design, but when credentials are supplied (e.g., via Authorization header), it can also evaluate whether those credentials inadvertently enable access to sensitive paths that would otherwise be restricted.

In Buffalo, a vulnerable pattern might look like reading a file relative to a user-supplied parameter:

// vulnerable example
func FilesHandler(c buffalo.Context) error {
    filename := c.Param("file") // user-controlled
    fullPath := filepath.Join("/var/data/", filename)
    http.ServeFile(c.Response(), c.Request(), fullPath)
    return nil
}

If the route is protected with HTTP Basic Auth, an authenticated user might still traverse outside /var/data/ using sequences like ../../../../etc/shadow. The framework does not inherently prevent this; it is the developer’s responsibility to sanitize and restrict file paths. middleBrick’s Property Authorization and Input Validation checks can surface such issues by probing endpoints with both authenticated and unauthenticated requests, highlighting whether traversal is possible regardless of the auth method.

Additionally, Basic Auth credentials are transmitted with each request in an easily decoded header. If logging or error handling inadvertently exposes path resolution logic or filesystem details, it can aid an attacker in refining traversal patterns. The scanner’s Data Exposure and Input Validation checks help identify whether path traversal is feasible and whether sensitive information is disclosed in the process.

To prevent Path Traversal in Buffalo when using Basic Auth, you must validate and sanitize all user-controlled path segments before using them in filesystem operations. Do not rely on authentication mechanisms to enforce path restrictions. Instead, use strict allowlisting and canonicalization techniques.

Recommended remediation steps:

• Avoid user input in file paths whenever possible. If you must use it, map input to a predefined set of safe resources.
• Use filepath.Clean and verify that the cleaned path remains within the intended directory.
• Use filepath.Rel to ensure the resolved path does not escape the base directory.
• Serve files via a controlled mapping (e.g., a database or a lookup table) rather than direct filename passthrough.

Secure code examples:

// secure example with base directory enforcement
import (
    "net/http"
    "os"
    "path/filepath"

    "github.com/gobuffalo/buffalo"
)

func SafeFilesHandler(c buffalo.Context) error {
    baseDir := "/var/data/"
    requested := c.Param("file")
    
    // Clean the path to remove redundant separators and dot segments
    cleanName := filepath.Clean(requested)
    
    // Build full path and ensure it remains inside baseDir
    fullPath := filepath.Join(baseDir, cleanName)
    rel, err := filepath.Rel(baseDir, fullPath)
    if err != nil || rel == ".." || filepath.IsAbs(rel) {
        http.Error(c.Response(), "invalid path", http.StatusBadRequest)
        return nil
    }
    
    // Ensure the file exists and is not a directory
    fileInfo, err := os.Stat(fullPath)
    if os.IsNotExist(err) || fileInfo.IsDir() {
        http.Error(c.Response(), "not found", http.StatusNotFound)
        return nil
    }
    
    http.ServeFile(c.Response(), c.Request(), fullPath)
    return nil
}

If you use Basic Auth, continue to enforce it as an access control layer, but do not treat it as a path validation substitute. Combine it with the above checks to ensure that authenticated users cannot read arbitrary files. For API endpoints that return sensitive data, also consider whether file-serving is necessary; often, returning data as structured JSON is safer than serving files directly.

When integrating with CI/CD, the middleBrick GitHub Action can be configured with a score threshold to fail builds if new endpoints introduce traversal risks, while the CLI allows you to scan specific routes during development. The MCP Server enables you to validate paths directly from your IDE as you code.