Out Of Bounds Write in Chi with Jwt Tokens

An Out Of Bounds Write in the context of Chi and JWT tokens occurs when an application processes JWT data in a way that writes memory past the intended allocation boundaries. This typically arises when a developer manipulates JWT payloads—such as header, claims, or signature fields—using low-level or unsafe operations that do not enforce strict length or type constraints. In Chi, a functional web framework for Clojure, routes and middleware can inadvertently expose this risk when deserializing or transforming JWTs without validating payload sizes or structures before further processing.

JWT tokens are structured as three dot-separated base64url-encoded parts: header, claims (payload), and signature. If an application decodes these parts and uses the resulting byte slices in operations like copying into fixed-size buffers or constructing new binary structures without bounds checking, an Out Of Bounds Write can be triggered. For example, if a route handler in Chi directly reads the decoded bytes of the JWT payload and writes them into a preallocated array assuming a fixed size, an attacker could supply an oversized payload, causing writes beyond the allocated memory region.

This becomes particularly dangerous when combined with Chi’s composability, where middleware may transform or forward JWT data across layers. An attacker might exploit an unbounded JWT claim—such as embedding a large string in the sub or custom field—to induce memory corruption. Because Chi encourages immutable data transformations, the risk often surfaces when integrating with libraries or native functions that perform unchecked writes. The vulnerability is not in JWT itself but in how the framework and application code handle decoded token bytes without validating lengths or enforcing safe memory practices.

Consider a scenario where a Chi handler decodes a JWT and passes the payload to a library expecting a fixed-size record. If the library uses unchecked array assignment, an oversized payload can corrupt adjacent memory, potentially leading to arbitrary code execution or information disclosure. Real-world patterns like CVE-2021-28082 illustrate how improper bounds handling in token processing can be leveraged, even if the framework itself does not directly manage memory.

middleBrick detects such risky patterns during black-box scanning by analyzing endpoint behavior with oversized or malformed JWT tokens. It identifies inconsistencies in token handling, such as missing length validation or unsafe deserialization, and maps findings to the BFLA/Privilege Escalation and Input Validation checks. This helps developers understand how an Out Of Bounds Write might be triggered through JWT manipulation in a Chi-based service.

To mitigate, always validate the size and structure of JWT components before using them in memory-sensitive operations. Never assume fixed sizes for payload fields, and avoid direct byte-level manipulations unless necessary and properly guarded. Using high-level abstractions and validated libraries reduces the chance of introducing such vulnerabilities in Chi routes.

Remediation focuses on strict validation and safe handling of JWT data within Chi handlers. Avoid raw byte assumptions and enforce explicit size and format checks. Below are concrete code examples demonstrating secure practices.

1. Validate JWT Payload Size Before Processing

Ensure decoded payloads do not exceed expected limits. Use Clojure’s when and count to enforce constraints.

(ns myapp.handler
  (:require [cheshire.core :as json]
            [buddy.hashers :as hashers]))

(defn safe-jwt-handler [request]
  (let [token (get-in request [:headers "authorization"])
        parts (when token (clojure.string/split token #"\\."))
        payload (when (and parts (= 3 (count parts)))
                  (try
                    (json/parse-string (String. (js/atob (second parts))) true)
                    (catch Exception _ nil)))]
    (if (and payload (<= (count (pr-str payload)) 1024)) ; enforce max size
      (do-something-safe payload)
      {:status 400 :body "Invalid token"})))

2. Use Typed Libraries Instead of Manual Byte Handling

Prefer libraries like clj-jwt that abstract unsafe operations and provide structured access.

(ns myapp.auth
  (:require [clj-jwt.core :as jwt]))

(defn verify-token [token secret]
  (try
    (jwt/verify token secret)
    (catch Exception e
      {:error :invalid-token})))

3. Avoid Direct Memory Manipulation

Never convert JWT claims into raw byte arrays for storage or copying. Use maps and strings instead.

(defn extract-subject [token secret]
  (let [claims (jwt/verify token secret)]
    (:sub claims))) ; safe access without byte buffers

By combining input validation, size checks, and high-level libraries, you eliminate the conditions that enable Out Of Bounds Writes. middleBrick’s scans can verify that these controls are present by testing with oversized or malformed JWTs and reviewing the application’s response handling.