Symlink Attack in Fiber with Jwt Tokens

A symlink attack in the Fiber web framework becomes high risk when JWT tokens are handled via files or temporary artifacts on disk. In this scenario, an attacker manipulates a filesystem path so that a trusted token file or cache location points to a file they control. Because Fiber applications often use JWT tokens for authentication and authorization, any token material written insecurely or resolved through user-supplied paths can be read, replaced, or poisoned.

The vulnerability arises when code writes JWT tokens or related secrets to a predictable location (for example, a cache directory or a file-based session store) and later reads that data using a path derived from user input without resolving symlinks. An attacker can create a symlink at that location before the application writes the token, causing the application to write sensitive JWT material to a location the attacker can later read. Conversely, if the application reads a token or key material from a path that may be symlinked, an attacker can redirect that read to a file they control, exfiltrating or altering the token context.

In a black-box scan, middleBrick tests such unauthenticated attack surfaces by attempting path traversals and symlink-based redirections where file operations intersect with authentication material. When JWT tokens or signing keys are stored or referenced via filesystem paths that do not canonicalize their targets, the scan can surface findings related to unsafe handling of sensitive authentication artifacts. This does not imply the JWT implementation itself is cryptographically broken, but that the surrounding file handling expands the attack surface.

Consider a Fiber route that stores a JWT signing key in a file under a user-provided directory path. If the application does not resolve symlinks before opening the file, an attacker can place a symlink at that path pointing elsewhere, leading to key leakage or substitution. Because JWT tokens are often used as bearer credentials, exposing or altering these files can undermine the entire authentication boundary.

middleBrick’s LLM/AI Security checks do not apply here, because this is a filesystem and token handling issue rather than a prompt injection or model output concern. However, the presence of JWT tokens in the API surface is noted in the inventory and data exposure checks, emphasizing the need to ensure that authentication material is never derived from or stored via unvalidated filesystem paths.

Remediation focuses on ensuring JWT tokens and related secrets are never read or written via paths influenced by user input, and that filesystem operations canonicalize and validate targets before use.

First, avoid writing JWT signing keys or tokens to disk unless absolutely necessary. If you must store keys on disk, load them at startup from a fixed, application-controlled path that is not derived from user input. For runtime token generation and validation, keep keys and tokens in memory only.

// Bad: key path derived from user input
string keyPath := c.Params("keyPath")
data, err := os.ReadFile(keyPath)
// Risk: symlink or traversal can lead to reading arbitrary files

// Better: key path is fixed and validated at startup
const jwtKeyPath = "./config/jwt_key.pem"
func loadKey() ([]byte, error) {
    return os.ReadFile(jwtKeyPath)
}

Second, if your application must resolve paths, use filepath.EvalSymlinks to canonicalize any user-supplied base paths before using them to locate authentication artifacts. Combine this with strict prefix checks to ensure the resolved path remains within an allowed directory.

import (
    "path/filepath"
    "strings"
)

func safeBasePath(userBase string) (string, error) {
    resolved, err := filepath.EvalSymlinks(userBase)
    if err != nil {
        return "", err
    }
    // Ensure resolved path is within an allowed directory
    allowedPrefix := "/app/securedata"
    if !strings.HasPrefix(resolved, allowedPrefix) {
        return "", errors.New("path outside allowed directory")
    }
    return resolved, nil
}

Third, when storing JWT tokens in files (for example, for revocation tracking), ensure that file creation uses flags that prevent symlink substitution. On Unix-like systems, use O_NOFOLLOW (via syscall or os-specific helpers) when opening files for write, and avoid creating files in world-writable directories. For Fiber handlers, prefer server-side stores that are not filesystem-based (e.g., in-memory caches with controlled access) over file-based token tracking.

// Example: secure token write to a fixed filename without symlink risk
func writeTokenFixed(token string) error {
    const tokenFile = "/app/tokens/app_jwt_store.dat"
    // Use O_WRONLY|O_CREATE|O_TRUNC without following symlinks where supported
    f, err := os.OpenFile(tokenFile, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, 0600)
    if err != nil {
        return err
    }
    defer f.Close()
    _, err = f.WriteString(token)
    return err
}

Finally, rotate JWT signing keys regularly and keep them in a secure secret store (such as environment variables injected at runtime or a secrets manager), rather than on disk. This reduces the impact of any path manipulation because the token material is never stored in a symlink-vulnerable location.