Auth Bypass in Chi with Jwt Tokens

Chi is a lightweight HTTP router for the Nim programming language. When JWT tokens are used for authentication in Chi-based services, several implementation details can lead to an authentication bypass. A common pattern is to rely on a middleware that extracts the token from the Authorization header, validates its signature, and then places a claims payload into the request context for downstream handlers to use.

Vulnerabilities arise when the validation step is incomplete. For example, if the service only verifies the token signature but does not enforce required claims such as iss (issuer), aud (audience), or exp (expiration), an attacker can reuse a token issued for one service or tenant against another endpoint. In Chi, this often occurs when routes are grouped with shared middleware and the authorization check is applied at a higher level but omitted on a subset of routes, creating an unauthenticated path through an otherwise protected API.

Another specific risk with JWT usage in Chi is algorithm confusion. If a server accepts tokens signed with HS256 but also processes tokens that lack a signature or use an unexpected algorithm without rejecting them, an attacker can supply an unsigned token or a token signed with a different key. Because Chi handlers typically decode the token into a claims record to access roles or permissions, failing to enforce strict algorithm validation can lead to privilege escalation or full auth bypass when the secret is weak or not enforced.

Runtime testing by middleBrick illustrates this category of issue. One of its twelve security checks targets authentication and BOLA/IDOR patterns, validating that endpoints which should require a token indeed reject requests without valid credentials. It also inspects token handling for missing or weak validation of standard claims and algorithms. Findings from such scans highlight missing guards in Chi routes where tokens are accepted but not rigorously verified, and they provide remediation guidance tied to common attack patterns like CWE-287 and OWASP API Top 10 Broken Object Level Authorization.

Additionally, JWTs with overly broad scopes or roles, when coupled with weak route-level authorization in Chi, can enable horizontal or vertical privilege escalation. For instance, a token containing a role claim of user might be accepted by an admin route if the handler does not explicitly check the role. Because middleBrick’s Authentication and Property Authorization checks correlate spec definitions from OpenAPI/Swagger with runtime behavior, it can surface cases where the declared security schemes in the Chi service’s spec do not match the actual enforcement in handlers.

In summary, an auth bypass in Chi using JWT tokens typically results from missing claim validation, weak or missing algorithm enforcement, inconsistent middleware application across routes, and insufficient role or scope checks at the handler level. These issues manifest when developers assume token presence equals authentication, rather than verifying token integrity, audience, issuer, expiration, and authorization context for each request.

To remediate JWT-related authentication bypass issues in Chi, adopt strict validation for every token accepted by your service. Use a well-audited JWT library, enforce expected algorithms, validate standard claims, and apply authorization checks at the route or handler level. The following examples assume you are using the jwt and chi packages in Nim.

First, define a procedure to verify the token and extract claims with required fields:

import jwt, strutils, chronicles

type
  Claims = object
    sub*: string
    role*: string
    exp*: int
    iss*: string
    aud*: string

proc verifyToken(token: string, secret: string, expectedIssuer: string, expectedAudience: string): Claims =
  let key = newKey(secret, HmacSHA256)
  let validated = decode[Claims](token, key, verifySignature = true)

  # Enforce required claims and validation
  if validated.iss != expectedIssuer:
    raise newException(ValueError, "Invalid issuer")
  if validated.aud != expectedAudience:
    raise newException(ValueError, "Invalid audience")
  if validated.exp < getTime().toUnix:
    raise newException(ValueError, "Token expired")

  return validated

Next, integrate this verifier into a Chi middleware that protects routes. Apply the middleware to route groups and ensure it runs before handlers that rely on identity or permissions:

import chi, httpclient, strutils

let router = newRouter()
let secretKey = "your-strong-secret"
let expectedIssuer = "my-auth-server"
let expectedAudience = "my-chi-service"

proc jwtMiddleware(req: Request, res: Response) =
  let authHeader = req.headers.get("Authorization")
  if authHeader.isNil or not authHeader.startsWith("Bearer "):
    res.status = Http401
    res.write("Missing or invalid Authorization header")
    return

  let token = authHeader[7..^1]  # strip "Bearer "
  try:
    let claims = verifyToken(token, secretKey, expectedIssuer, expectedAudience)
    req.context["claims"] = claims
    proceed(req, res)
  except ValueError as e:
    res.status = Http401
    res.write("Invalid token: " & e.msg)

router.use(jwtMiddleware)

For route-specific authorization, explicitly check roles or scopes in handlers rather than relying on middleware alone:

router.get("/admin/dashboard") do (req: Request, res: Response) =
  let claims = req.context.get("claims", Claims)
  if claims.role != "admin":
    res.status = Http403
    res.write("Insufficient permissions")
    return
  res.write("Welcome admin")

When using the middleBrick CLI to scan a Chi service, the tool can correlate the declared security scheme in an OpenAPI spec with runtime behavior to highlight missing validation. For teams on the Pro plan, continuous monitoring and CI/CD integration via the GitHub Action can fail builds if a new route lacks proper authentication checks, helping maintain consistent JWT enforcement across services.