Symlink Attack in Echo Go with Basic Auth

A symlink attack in Echo Go with Basic Auth occurs when an authenticated or unauthenticated attacker can leverage predictable file paths and weak access controls to replace a legitimate file or directory with a symbolic link that points to a sensitive system location. In Echo Go, routes that handle file uploads, downloads, or temporary file creation can become vectors if file paths are derived from user input without canonicalization or directory traversal checks.

When Basic Auth is used, developers may assume that the presence of a password is sufficient to protect endpoints. However, if the authenticated handler writes files to a user-controlled destination (e.g., a filename provided via query parameter or header), an attacker who knows or guesses the Basic Auth credentials might craft requests that create symlinks on the filesystem before the handler runs. For example, an attacker could first issue a request that creates a symlink at a path the application later uses to serve or process files, and then use the same credentials to trigger file reads or writes through the compromised path.

The combination is risky because Basic Auth transmits credentials in base64 with each request. If credentials are intercepted or reused, an attacker can repeatedly issue crafted requests that exploit path manipulation. Even without intercepting traffic, if the application does not validate whether a path is a symlink before opening it, an authenticated session can unwittingly read or overwrite arbitrary files on the host, such as application configuration files or logs that may contain sensitive data. This maps to common OWASP API Top 10 categories like Broken Object Level Authorization (BOLA/IDOR) and Security Misconfiguration, and can be surfaced by middleBrick scans as findings related to Path Traversal and Insecure Direct Object References.

Real-world patterns include using os.Link or os.Symlink in Go to create links based on user input, and failing to check os.Stat results for the Mode() & os.ModeSymlink bit. middleBrick tests such unauthenticated attack surfaces and, when credentials are provided, can surface authenticated scenarios where symlink manipulation leads to unauthorized data exposure or modification.

Remediation focuses on eliminating path manipulation opportunities and hardening how Basic Auth protects endpoints. Do not rely on Basic Auth alone to enforce access control; combine it with strict input validation and filesystem safeguards.

• Validate and sanitize all user-supplied path components: use filepath.Clean and filepath.Rel to ensure paths remain within an allowed directory.
• Check for symlinks before opening files by calling os.Lstat and verifying that the file is not a symlink using os.ModeSymlink.
• Use absolute base directories and avoid concatenating user input directly into filesystem paths.

Example of a vulnerable Echo Go handler with Basic Auth that does not protect against symlink attacks:

// WARNING: vulnerable example — do not use in production
package main

import (
    "net/http"
    "os"
    "path/filepath"

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

func downloadHandler(c echo.Context) error {
    user := c.Get("user").(*basicUser) // set by Basic Auth middleware
    filename := c.QueryParam("file")
    base := "/var/app/uploads"
    path := filepath.Join(base, filename) // unsafe if filename contains ".."

    file, err := os.Open(path)
    if err != nil {
        return echo.NewHTTPError(http.StatusInternalServerError, "unable to open file")
    }
    defer file.Close()

    return c.Stream(http.StatusOK, "application/octet-stream", file)
}

type basicUser struct {
    username string
}

Fixed version that sanitizes input and checks for symlinks:

package main

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

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

func safeDownloadHandler(c echo.Context) error {
    // Basic Auth middleware should have already set identity in c.Get("user")
    user := c.Get("user").(*basicUser)
    filename := c.QueryParam("file")

    base := "/var/app/uploads"
    // Clean and ensure the resolved path stays within base
    rel, err := filepath.Rel(".", filename)
    if err != nil || rel == ".." || len(rel) > 0 && rel[0] == '.' {
        return echo.NewHTTPError(http.StatusBadRequest, "invalid file parameter")
    }
    path := filepath.Join(base, filepath.Clean(rel))

    // Ensure the path does not escape base
    if !isWithinBase(path, base) {
        return echo.NewHTTPError(http.StatusForbidden, "access denied")
    }

    // Defensive symlink check
    info, err := os.Lstat(path)
    if err != nil {
        return echo.NewHTTPError(http.StatusInternalServerError, "unable to stat file")
    }
    if info.Mode()&os.ModeSymlink != 0 {
        return echo.NewHTTPError(http.StatusForbidden, "symlink not allowed")
    }

    file, err := os.Open(path)
    if err != nil {
        return echo.NewHTTPError(http.StatusInternalServerError, "unable to open file")
    }
    defer file.Close()

    return c.Stream(http.StatusOK, "application/octet-stream", file)
}

func isWithinBase(path, base string) bool {
    rel, err := filepath.Rel(base, path)
    return err == nil && len(rel) > 0 && rel[0] != '.'
}

type basicUser struct {
    username string
}

In addition to code fixes, integrate middleBrick’s CLI to scan endpoints from the terminal with middlebrick scan <url> and include the GitHub Action to fail builds if risk scores drop below your chosen threshold. For continuous protection, the Pro plan enables scheduled scans and alerts, helping you detect regressions that could reintroduce symlink or authentication bypass risks.