Path Traversal in Buffalo with Jwt Tokens

Path Traversal occurs when user-controlled input is used to construct file system paths without proper validation, allowing an attacker to navigate outside the intended directory. In a Buffalo application, this typically arises when a filename or path parameter is directly concatenated into file operations such as os.Open or http.ServeFile. JWT tokens are often used for authentication and authorization; if the token payload (e.g., a username, subject, or role) is reflected into file paths without sanitization, an attacker can manipulate the token’s claims to traverse directories.

Consider a scenario where a Buffalo handler uses a JWT claim to locate user-specific assets. If the token contains a claim like sub or a custom field such as username, and that value is appended to a base directory without validation, an attacker who crafts a token with sequences like ../../../etc/passwd can read arbitrary files. Even if the token is signed, signature verification only ensures the token hasn’t been altered in transit; it does not sanitize the content. Therefore, a valid JWT with maliciously crafted claims can still lead to Path Traversal when used to build file paths in Buffalo handlers.

Moreover, if the JWT is parsed and its claims are used to authorize access to resources (for example, mapping a subject to a directory path), improper handling of encoded slashes or null bytes can bypass naive checks. Buffalo’s middleware stack may verify the token and attach claims to the context, but if the downstream handler uses those claims naively, the attack surface remains. The risk is compounded when file operations occur in a shared or parent directory, as relative path resolution can climb outside the intended sandbox. This illustrates why JWT tokens must be treated as untrusted input when they influence file system paths, and why explicit path sanitization is required in Buffalo handlers.

To mitigate Path Traversal when using JWT tokens in Buffalo, always treat claims as untrusted and sanitize any value used in file system operations. Use Go’s path/filepath utilities to clean and restrict paths, and avoid direct string concatenation. Below is a secure example of a Buffalo handler that validates a JWT and safely serves a file based on a claim.

import (
    "github.com/gobuffalo/buffalo"
    "github.com/gobuffalo/buffalo/middleware"
    "github.com/golang-jwt/jwt/v4"
    "path/filepath"
    "strings"
)

func ServeUserAsset(c buffalo.Context) error {
    token := c.Param("token")
    parsed, err := jwt.Parse(token, func(token *jwt.Token) (interface{}, error) {
        return []byte("your-secret"), nil
    })
    if err != nil || !parsed.Valid {
        return c.Error(401, fmt.Errorf("invalid token"))
    }

    claims, ok := parsed.Claims.(jwt.MapClaims)
    if !ok {
        return c.Error(400, fmt.Errorf("invalid claims"))
    }

    username, ok := claims["username"].(string)
    if !ok || username == "" {
        return c.Error(400, fmt.Errorf("missing username claim"))
    }

    // Sanitize the username to prevent path traversal
    cleanUsername := filepath.Clean("/" + strings.Trim(username, "/"))
    if cleanUsername == "/" || strings.Contains(cleanUsername, "..") {
        return c.Error(400, fmt.Errorf("invalid username"))
    }

    filePath := filepath.Join("/safe/base/dir", cleanUsername, "asset.txt")
    // Further ensure the resolved path is within the allowed directory
    absBase, _ := filepath.Abs("/safe/base/dir")
    absPath, _ := filepath.Abs(filePath)
    if !strings.HasPrefix(absPath, absBase) {
        return c.Error(403, fmt.Errorf("access denied"))
    }

    return c.Render(200, r.String("attachment; filename=asset.txt"), http.Dir("/safe/base/dir").Open(filePath))
}

This approach ensures that the JWT claim used in the path is cleaned, checked for directory traversal patterns, and verified to remain within an allowed base directory. Additionally, always use absolute path comparisons to prevent symlink-based escapes. For API token usage, consider embedding a non-sensitive identifier rather than user-controlled data in file paths, and leverage Buffalo’s middleware to enforce strict validation before handlers execute.