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Fix JWT Vulnerabilities (Weak Signing, None Algo) in Roda

JWT implementations in Roda often fall victim to 'alg: none' bypasses and HMAC brute-forcing due to weak secrets. In the Ruby ecosystem, the 'ruby-jwt' gem is the standard, but its default configuration can be dangerously permissive. This guide demonstrates how to harden Roda routes against signature stripping and weak cryptographic keys.

The Vulnerable Pattern

require 'roda'
require 'jwt'

class App < Roda
plugin :json


WEAK: Hardcoded secret makes HMAC brute-forcing trivial


SECRET = ‘secret123’


route do |r|
r.on ‘api/data’ do
token = r.headers[‘Authorization’]&.split(’ ’)&.last
# VULNERABLE: Second argument ‘false’ disables signature verification,
# or calling decode without specifying algorithms allows ‘alg: none’ bypass.
payload, header = JWT.decode(token, SECRET, false)
{ data: ‘sensitive_info’, user: payload[‘user_id’] }
end
end
end

The Secure Implementation

The secure implementation mitigates two primary attack vectors. First, by passing { algorithm: 'HS256' } as the fourth argument to JWT.decode, we force the library to validate the 'alg' header against our whitelist, effectively killing 'alg: none' bypasses. Second, we move the signing key to an environment variable. A production secret should be a high-entropy string (at least 32-64 bytes) to prevent offline HMAC cracking. Finally, we wrap the decoding in a rescue block to handle malformed tokens gracefully, preventing stack trace leakage.

require 'roda'
require 'jwt'

class App < Roda
plugin :json


SECURE: Load high-entropy secret from environment


SECRET = ENV.fetch(‘JWT_SECRET’) { raise ‘FATAL: JWT_SECRET not set’ }


route do |r|
r.on ‘api/data’ do
token = r.headers[‘Authorization’]&.split(’ ’)&.last


  begin
    # SECURE: Explicitly set verify to true and whitelist allowed algorithms.
    # This prevents 'alg: none' attacks and RS256/HS256 confusion.
    options = { algorithm: 'HS256', verify_iat: true }
    decoded_token = JWT.decode(token, SECRET, true, options)
    payload = decoded_token[0]
    
    { data: 'protected_content', user: payload['user_id'] }
  rescue JWT::DecodeError, JWT::VerificationError => e
    response.status = 401
    { error: 'Unauthorized: Invalid or missing token' }
  end
end


end
end

System Alert • ID: 3296
Target: Roda API
Potential Vulnerability

Your Roda API might be exposed to JWT Vulnerabilities (Weak Signing, None Algo)

74% of Roda apps fail this check. Hackers use automated scanners to find this specific flaw. Check your codebase before they do.

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