Symlink Attack in Buffalo with Jwt Tokens

A symlink attack in Buffalo when JWT tokens are handled via file-based storage or session linkage occurs when an attacker manipulates symbolic links so that a JWT token written by the application is placed where an attacker can read or replace it. In Buffalo, this can happen if tokens are stored as files in user-writable directories or if temporary files used during token generation or refresh are created insecurely. An attacker who can control path components (for example, through a predictable file path or a directory traversal vector) may create or replace a symlink so that the application writes a JWT to a location the attacker can later read, or reads an attacker-controlled file expecting a JWT.

Buffalo’s typical JWT usage involves signing tokens with a secret or private key and validating them on each request. The framework does not inherently create symlinks, but if the application writes JWT-related artifacts (e.g., refresh token files, debug dumps, or cached public keys) to a path derived from user input without resolving symlinks securely, the token material can be leaked or substituted. For example, if the application uses a path like ./tmp/token_{userId}.key and an attacker can make tmp a symlink to a sensitive file, a token intended for one user may overwrite or be read from a privileged location. Similarly, if public keys or certificates used to verify JWTs are loaded from a directory where symlinks are not resolved safely, an attacker may substitute a malicious key and forge tokens.

Because Buffalo is a Go framework, developers must ensure file operations related to JWT material use functions that avoid symlink-based races (e.g., using os.OpenFile with O_NOFOLLOW on file creation and avoiding ioutil.ReadFile on paths that may be symlinks). The attack surface is not in JWT cryptography itself but in how token artifacts are materialized on the filesystem. An unauthenticated scanner (such as middleBrick) can surface indicators like world-writable directories or predictable temp file paths that facilitate symlink attacks, while authenticated scans can detect whether JWT validation and storage routines are resilient to path manipulation.

When combined with other findings such as IDOR or BFLA, a symlink weakness can amplify impact: an attacker able to control file paths might redirect token writes to exfiltrate JWTs or replace them with attacker-signed tokens. middleBrick’s checks for BOLA/IDOR and Property Authorization help highlight endpoints where token handling intersects with path-controlled resources, and its LLM/AI Security probes test for inadvertent exposure of token generation logic via model endpoints.

Remediation centers on ensuring JWT material is never written to or read from paths an attacker can influence, and that filesystem operations avoid symlink-based races. Below are concrete examples for Buffalo applications.

1. Secure temporary file creation for JWT refresh material:

import (
    "os"
    "path/filepath"
)

func safeTokenFile(baseDir string) (string, error) {
    // Use a directory that is not user-writable; ensure it exists with 0700.
    dir := filepath.Join(baseDir, "tokens")
    if err := os.MkdirAll(dir, 0700); err != nil {
        return "", err
    }
    f, err := os.OpenFile(filepath.Join(dir, "refresh.tmp"), os.O_CREATE|os.O_EXCL|os.O_WRONLY, 0600)
    if err != nil {
        return "", err
    }
    // f.Name() is a path guaranteed not to be followed symlinks at open time.
    return f.Name(), f.Close()
}

Using O_EXCL prevents symlink attacks that rely on creating a file between path resolution and opening; storing files in a dedicated, non-shared directory reduces leakage risk.

2. Loading JWT public keys without symlink following:

import (
    "io/ioutil"
    "os"
    "path/filepath"
)

func loadPublicKey(dir, filename string) ([]byte, error) {
    // Clean and restrict to dir to avoid directory traversal.
    cleanName := filepath.Clean(filename)
    if cleanName != filename || filepath.IsAbs(cleanName) {
        return nil, os.ErrInvalid
    }
    // Ensure the final path stays within dir.
    keyPath := filepath.Join(filepath.Clean(dir), cleanName)
    // Follow symlinks only if the resolved path remains within dir.
    // Use O_NOFOLLOW when opening; here we resolve then validate.
    info, err := os.Lstat(keyPath)
    if err != nil {
        return nil, err
    }
    if info.Mode()&os.ModeSymlink != 0 {
        return nil, os.ErrPermission
    }
    return ioutil.ReadFile(keyPath)
}

Rejecting symlinks for key material prevents substitution attacks where a malicious key replaces the legitimate public key.

3. JWT signing configuration in Buffalo handlers:

import (
    "github.com/gobuffalo/buffalo"
    "github.com/golang-jwt/jwt/v4"
)

func ValidateToken(next buffalo.Handler) buffalo.Handler {
    return func(c *buffalo.Context) error {
        raw := c.Request().Header.Get("Authorization")
        // Expect Bearer prefix; avoid file-based token storage.
        if len(raw) < 7 || raw[:7] != "Bearer " {
            return c.Render(401, r.JSON(map[string]string{"error": "invalid_auth"}))
        }
        tokenStr := raw[7:]
        claims := jwt.MapClaims{}
        token, err := jwt.ParseWithClaims(tokenStr, claims, func(token *jwt.Token) (interface{}, error) {
            // Load from a secure, non-symlink path; do not derive key path from token header.
            key, err := loadPublicKey("/etc/app/jwks", "rsa.pub")
            if err != nil {
                return nil, err
            }
            return jwt.ParseRSAPublicKeyFromPEM(key)
        })
        if err != nil || !token.Valid {
            return c.Render(401, r.JSON(map[string]string{"error": "invalid_token"}))
        }
        // Ensure claims and bindings are validated before use.
        if sub, ok := claims["sub"].(string); !ok || sub == "" {
            return c.Render(401, r.JSON(map[string]string{"error": "invalid_claims"}))
        }
        return next(c)
    }
}

Keep secrets and keys in memory where possible; avoid writing JWTs or keys to disk. If disk storage is necessary, use the secure temp file pattern above and enforce strict permissions (0600) and ownership.