Symlink Attack in Chi with Basic Auth

A symlink attack in the Chi framework becomes notably more consequential when routes are protected only by HTTP Basic Auth. Chi is a lightweight router for Go that relies on explicit route registration. When a developer uses Basic Auth via a middleware that validates credentials per request, they may assume that file-system operations performed as part of a handler are safe. This assumption is risky if the handler builds file paths from user-controlled input without canonicalization.

Consider a handler that receives a filename via a URL parameter and serves content from a directory protected by Basic Auth. If the handler does not clean the path, an attacker can supply a crafted path such as ../../../etc/passwd. Because the route is protected by Basic Auth, the attacker must first supply valid credentials. Once authenticated, the attacker’s crafted path may traverse outside the intended directory and resolve via a symbolic link to a sensitive file on the host. The handler’s file operations then read or reveal data that should remain restricted.

The critical interaction here is not between Chi and Basic Auth, but between Chi’s routing and path handling when Basic Auth gates access. Basic Auth ensures the request includes valid credentials, but it does not change how Chi resolves paths or how the underlying operating system follows symlinks. If the application logic trusts user input for filesystem locations, the presence of Basic Auth may give a false sense of security, because the auth check passes while the file operation escapes the intended directory.

In the context of middleBrick’s security checks, this pattern would surface findings in multiple categories. The Input Validation check would flag path traversal risks, Property Authorization would note that access controls do not restrict filesystem scope, and BFLA/Privilege Escalation would highlight that authenticated access to dangerous endpoints can lead to unauthorized data exposure. Even though the scan is unauthenticated, middleBrick’s OpenAPI/Swagger analysis can detect routes with authentication markers and highlight endpoints that accept user input for filesystem operations, drawing attention to the combined risk.

Real-world analogies include scenarios where an authenticated API endpoint uses user-supplied paths to read logs or configuration files. If the endpoint is implemented in Chi and uses http.Dir or similar without cleaning, a symlink placed in the accessible filesystem can redirect reads to sensitive locations. The Basic Auth layer does not mitigate this; it only ensures the requester is known, not that the operation is confined to the intended scope.

Remediation centers on strict input validation, path cleaning, and scope confinement rather than relying on authentication alone. When using HTTP Basic Auth in Chi, you should treat authenticated requests as potentially hostile and ensure filesystem operations cannot escape intended directories.

Example 1: Safe file serving with Basic Auth in Chi

Instead of directly concatenating user input into file paths, use path.Clean and ensure the resolved path remains within a base directory. The following example demonstrates a handler protected by Basic Auth that safely serves files:

package main

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

    "github.com/go-chi/chi/v5"
    "github.com/go-chi/basicauth"
)

func main() {
    r := chi.NewRouter()

    // Basic Auth middleware with a simple checker
    r.Use(basicauth.BasicAuth(&basicauth.Credentials{
        "alice": "correcthorsebatterystaple",
        "bob":   "drumsticksandtrumpets",
    }, func(w http.ResponseWriter, r *http.Request) bool {
        // You can integrate with your user store here
        return true
    }))

    // Safe file handler
    r.Get("/files/*", func(w http.ResponseWriter, r *http.Request) {
        // Extract the filename safely
        filename := chi.URLParam(r, "wildcard")
        if filename == "" {
            http.Error(w, "missing file parameter", http.StatusBadRequest)
            return
        }

        // Clean and restrict to a base directory
        base := "/var/app/public"
        cleaned := path.Clean("/" + filename)
        if cleaned == "/" || strings.HasPrefix(cleaned, "..") {
            http.Error(w, "invalid path", http.StatusBadRequest)
            return
        }

        filePath := path.Join(base, cleaned)
        // Ensure the resolved path is still within base
        if !strings.HasPrefix(filePath, base) {
            http.Error(w, "forbidden", http.StatusForbidden)
            return
        }

        http.ServeFile(w, r, filePath)
    })

    http.ListenAndServe(":8080", r)
}

This approach ensures that even when authenticated, a user cannot escape the /var/app/public directory via symlinks or path traversal. The check strings.HasPrefix(filePath, base) provides an additional safety layer after joining paths.

Example 2: Rejecting suspicious input patterns

Complement path cleaning with explicit validation to reject inputs containing dangerous patterns. The following snippet shows how to block paths that include .. or attempt null bytes:

func isSafePath(p string) bool {
    if strings.Contains(p, "..") || strings.Contains(p, "\x00") {
        return false
    }
    // Additional checks can be added here
    return true
}

Combine this validator with the Basic Auth middleware so that authenticated requests still undergo the same scrutiny. middleBrick’s Input Validation checks can help identify endpoints that lack such guards, and its BFLA/Privilege Escalation analysis can surface routes where authenticated access may lead to privilege issues if path handling is weak.

Finally, prefer using http.Dir with Open and ReadDir when serving directories, and avoid passing user input directly to functions like os.Stat without canonicalization. These practices reduce the risk of symlink-based information disclosure even when routes are protected by Basic Auth.