Symlink Attack in Chi with Jwt Tokens

A symlink attack in the Chi routing context becomes critical when JWT tokens are involved, for example when sensitive claims are written to temporary files or when token introspection relies on file system paths. Chi is a minimal, composable router for Clojure web applications, and routes often invoke handlers that may write data to disk (e.g., logging tokens, caching decoded payloads, or storing refresh material). If an attacker can influence file or directory paths through user-supplied input and the application resolves those paths insecurely, they can create a symbolic link (symlink) that redirects writes to a location of the attacker’s choosing. This can expose JWT tokens or overwrite configuration expected by the application.

Consider a handler that decodes a JWT and writes the claims to a per-request temporary file for auditing. If the temporary file path is derived from an unvalidated parameter and the application resolves symlinks naively, an attacker can place a symlink at that path before the write occurs. When the handler writes the JWT claims, the symlink redirects the content to a sensitive file, potentially exposing tokens or altering runtime behavior. In Chi, this risk arises when route parameters, query strings, or headers influence filesystem operations without canonicalization, and JWT tokens are handled as file-backed data. Because Chi encourages explicit route definitions and composable middleware, developers may inadvertently introduce filesystem interactions—such as logging or caching—that combine poorly with untrusted input.

During a middleBrick scan, such issues can appear in findings related to Input Validation, Property Authorization, and BFLA/Privilege Escalation. The scanner does not assume an authenticated context and tests the unauthenticated attack surface, which can surface path manipulation risks that lead to token exposure. Findings include severity, remediation guidance, and mapping to frameworks like OWASP API Top 10 and SOC2. middleBrick supports OpenAPI/Swagger spec analysis (2.0, 3.0, 3.1) with full $ref resolution, cross-referencing spec definitions with runtime behavior to highlight mismatches that could enable symlink-related exposures when JWT tokens are processed via filesystem operations.

Remediation centers on avoiding filesystem operations for JWT material, validating and sanitizing all paths, and using language-provided facilities that avoid symlink races. Prefer in-memory representations for JWTs and ensure any file interactions use absolute paths with no user-controlled components. Below are concrete code examples for Chi that demonstrate secure handling of JWT tokens.

Example 1: Decode and use JWT in memory without filesystem writes

Avoid writing JWT claims to disk entirely. Process tokens in memory and pass claims as data structures.

(ns secure-handler
  (:require [cheshire.core :as json]
            [buddy.hashers :as hashers]
            [ring.util.response :as resp]
            [reitit.core :as reitit]
            [reitit.ring :as ring]))

(defn decode-jwt-safe [token secret]
  ;; Use a library that validates signature and returns claims map in memory.
  ;; Example using buddy (replace with your JWT library of choice).
  (try
    (hashers/unsign token secret) ;; simplified; use proper JWT lib in practice
    (catch Exception e
      (throw (ex-info "Invalid token" {:status 401})))))

(def app
  (ring/ring-handler
    (ring/router
      ["/api"
       ["/profile"
        {:get (fn [request]
                (let [auth-header (get-in request [:headers "authorization"])
                      token (when auth-header (subs auth-header 7))
                      claims (decode-jwt-safe token "")]
                  (-> (resp/response (json/generate-string claims))
                      (resp/content-type "application/json")))}
         ["/admin"
          {:get (fn [request]
                  (let [auth-header (get-in request [:headers "authorization"])
                        token (when auth-header (subs auth-header 7))
                        claims (decode-jwt-safe token "")]
                    (if (get-in claims [:admin])
                      (resp/response "admin data")
                      (resp/response "forbidden")))})]]])

Example 2: If filesystem logging is unavoidable, canonicalize paths and avoid symlinks

If you must log JWT-related events to files, resolve paths to their canonical, real forms and ensure no symlink can intervene between resolution and use. Do not rely on user input for filenames or directories.

(ns secure-logging
  (:require [clojure.java.io :as io]
            [clojure.string :as str]))

(defn safe-log-token-event [event]
  (let [log-dir (io/file "/var/log/myapp")
        ;; Ensure the directory is absolute and real
        base-path (.getCanonicalPath log-dir)
        filename (str "events-" (System/currentTimeMillis) ".log")
        log-file (io/file base-path filename)]
    ;; Double-check canonical path to prevent traversal or symlink surprises
    (when (str/starts-with? (.getCanonicalPath log-file) base-path)
      (spit log-file (str event "\n") :append true))))

;; Usage in a Chi handler
(defn audit-handler [request]
  (let [token (get-in request [:headers "authorization"])
        event (str "Token accessed at " (System/currentTimeMillis))]
    (safe-log-token-event event)
    (-> (resp/response "ok")
        (resp/content-type "text/plain"))))

General defenses

• Never construct file paths from user-controlled data (headers, params, body) without strict validation and canonicalization.
• Use in-memory data stores or secure temporary file APIs that create unlinked, non-symlink files when persistence is required.
• Apply the principle of least privilege to processes that handle JWT material, reducing the impact of accidental exposure.