Zip Slip in Echo Go with Basic Auth

Zip Slip is a path traversal vulnerability that occurs when an application constructs file paths using unsanitized user input, allowing an attacker to navigate outside the intended directory. When Basic Authentication is used in an Echo Go application, the username and password are typically extracted from the Authorization header and may influence path construction or access control decisions. If the application uses the authenticated username to build file paths—such as storing user-specific uploads under a directory derived from the username—unsanitized input from the request (e.g., a filename parameter) combined with the username can lead to directory traversal sequences like ../../../. An attacker who provides a malicious filename such as ../../../etc/passwd can escape the user’s intended directory and access or overwrite sensitive files. Because Basic Auth is stateless and relies on each request carrying credentials, the Echo handler may not enforce strict path validation between authentication and file operations, increasing the risk. The combination creates a scenario where authentication context is leveraged in path resolution without adequate sanitization, enabling unauthorized file access or manipulation.

For example, consider an Echo Go handler that writes an uploaded file to a user-specific folder using the username from Basic Auth:

username := userFromBasicAuth(c) // extracted from header
target := filepath.Join("uploads/", username, c.FormValue("filename"))

If filename contains traversal sequences and the handler does not sanitize or validate it, the resolved path can escape uploads/[username]/. MiddleBrick’s 12 security checks, including Input Validation and Property Authorization, are designed to detect such risky input handling patterns and provide remediation guidance. Even when authentication is in place, missing path normalization and validation remain a critical exposure.

To mitigate Zip Slip in Echo Go when using Basic Authentication, always validate and sanitize user-controlled input before using it in file paths. Do not rely on the presence of Basic Auth to enforce path boundaries. Use strict allowlists for filenames, normalize paths, and ensure joined paths remain within the intended base directory.

Secure handler example with Basic Auth and path validation

The following example demonstrates a safer approach in Echo Go. It extracts credentials via Basic Auth, validates the filename against a strict pattern, cleans the path, and confirms the final location is within the base directory before writing the file.

package main

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

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

// extractUsernameFromBasicAuth returns the username from the Authorization header.
func extractUsernameFromBasicAuth(c echo.Context) (string, error) {
    auth := c.Request().Header.Get("Authorization")
    if auth == "" {
        return "", echo.ErrUnauthorized
    }
    const prefix = "Basic "
    if !strings.HasPrefix(auth, prefix) {
        return "", echo.ErrUnauthorized
    }
    // In production, decode the credential and parse username:password.
    // This simplified example assumes a helper function decodes and returns username.
    return decodeBasicAuthUsername(auth), nil
}

// decodeBasicAuthUsername is a placeholder for safe decoding logic.
func decodeBasicAuthUsername(auth string) string {
    // Implement proper base64 decoding and extraction.
    return "validated_user"
}

// isValidFilename allows only alphanumeric, underscore, dash, and common extensions.
var validFilename = regexp.MustCompile(`^[A-Za-z0-9._-]+\.[A-Za-z0-9]+$`)

func uploadHandler(c echo.Context) error {
    username, err := extractUsernameFromBasicAuth(c)
    if err != nil {
        return err
    }

    filename := c.FormValue("filename")
    if filename == "" || !validFilename.MatchString(filename) {
        return echo.NewHTTPError(http.StatusBadRequest, "invalid filename")
    }

    baseDir := "uploads"
    usernameDir := filepath.Join(baseDir, username)
    // Ensure the username directory exists and is within baseDir.
    if err := os.MkdirAll(usernameDir, 0750); err != nil {
        return echo.NewHTTPError(http.StatusInternalServerError, "cannot create directory")
    }

    // Clean each component and rebuild to ensure no traversal.
    cleanFilename := filepath.Clean(filename)
    target := filepath.Join(usernameDir, cleanFilename)

    // Ensure the final path is within the intended base.
    rel, err := filepath.Rel(baseDir, target)
    if err != nil || strings.HasPrefix(rel, "..") {
        return echo.NewHTTPError(http.StatusBadRequest, "path traversal detected")
    }

    // Proceed to save the uploaded file to target.
    // file, err := c.FormFile("file")
    // ...
    return c.JSON(http.StatusOK, map[string]string{"path": target})
}

Key remediation steps include: validating the filename with a strict regex, using filepath.Clean to remove . and .. elements, and confirming with filepath.Rel that the resolved path does not escape the base directory. These controls reduce the risk even when Basic Auth identifies the user. Security findings from MiddleBrick’s checks, such as Input Validation and Property Authorization, will highlight whether such safeguards are absent or insufficient in your implementation.