Allow a leeway to be given for the iat claim verification

CHANGELOG.md

@@ -6,6 +6,7 @@
 
 **Features:**
 
+- Allow a leeway to be given for the `iat` claim verification [#747](https://github.com/jwt/ruby-jwt/pull/747) - ([@denis1011101](https://github.com/denis1011101))
 - Your contribution here
 
 **Fixes and enhancements:**

README.md

@@ -548,6 +548,12 @@ rescue JWT::InvalidIatError
 end
 ```
 
+The comparison is exact by default. When the clock of the issuer can drift ahead of the clock of the verifier, the tolerated drift can be given explicitly:
+
+```ruby
+decoded_token = JWT.decode(token, hmac_secret, true, { verify_iat: { leeway: 30 }, algorithm: 'HS256' })
+```
+
 ### Subject Claim
 
 From [Oauth JSON Web Token 4.1.2. "sub" (Subject) Claim](https://tools.ietf.org/html/rfc7519#section-4.1.2):

lib/jwt/claims/decode_verifier.rb

@@ -15,7 +15,7 @@ module DecodeVerifier
         verify_expiration: ->(options) { Claims::Expiration.new(leeway: options[:exp_leeway] || options[:leeway]) },
         verify_not_before: ->(options) { Claims::NotBefore.new(leeway: options[:nbf_leeway] || options[:leeway]) },
         verify_iss: ->(options) { options[:iss] && Claims::Issuer.new(issuers: options[:iss]) },
-        verify_iat: ->(*) { Claims::IssuedAt.new },
+        verify_iat: ->(options) { Claims::IssuedAt.new(leeway: options[:verify_iat].is_a?(Hash) ? options[:verify_iat][:leeway] : nil) },
         verify_jti: ->(options) { Claims::JwtId.new(validator: options[:verify_jti]) },
         verify_aud: ->(options) { options[:aud] && Claims::Audience.new(expected_audience: options[:aud]) },
         verify_sub: ->(options) { options[:sub] && Claims::Subject.new(expected_subject: options[:sub]) },

lib/jwt/claims/issued_at.rb

@@ -4,6 +4,13 @@ module JWT
   module Claims
     # The IssuedAt class is responsible for validating the issued at claim ('iat') in a JWT token.
     class IssuedAt
+      # Initializes a new IssuedAt instance.
+      #
+      # @param leeway [Integer] the drift (in seconds) to allow between the clock of the issuer and the clock of the verifier. Default: 0.
+      def initialize(leeway: 0)
+        @leeway = leeway || 0
+      end
+
       # Verifies the issued at claim ('iat') in the JWT token.
       #
       # @param context [Object] the context containing the JWT payload.
@@ -15,8 +22,12 @@ def verify!(context:, **_args)
         return unless context.payload.key?('iat')
 
         iat = context.payload['iat']
-        raise(JWT::InvalidIatError, 'Invalid iat') if !iat.is_a?(::Numeric) || iat.to_f > Time.now.to_f
+        raise(JWT::InvalidIatError, 'Invalid iat') if !iat.is_a?(::Numeric) || iat.to_f > (Time.now.to_f + leeway)
       end
+
+      private
+
+      attr_reader :leeway
     end
   end
 end

lib/jwt/claims/verifier.rb

@@ -8,7 +8,7 @@ module Verifier
         exp: ->(options) { Claims::Expiration.new(leeway: options.dig(:exp, :leeway)) },
         nbf: ->(options) { Claims::NotBefore.new(leeway: options.dig(:nbf, :leeway)) },
         iss: ->(options) { Claims::Issuer.new(issuers: options[:iss]) },
-        iat: ->(*)       { Claims::IssuedAt.new },
+        iat: ->(options) { Claims::IssuedAt.new(leeway: options.dig(:iat, :leeway)) },
         jti: ->(options) { Claims::JwtId.new(validator: options[:jti]) },
         aud: ->(options) { Claims::Audience.new(expected_audience: options[:aud]) },
         sub: ->(options) { Claims::Subject.new(expected_subject: options[:sub]) },

lib/jwt/configuration/decode_configuration.rb

@@ -11,7 +11,7 @@ class DecodeConfiguration
       # @!attribute [rw] verify_iss
       #   @return [Boolean] whether to verify the issuer claim.
       # @!attribute [rw] verify_iat
-      #   @return [Boolean] whether to verify the issued at claim.
+      #   @return [Boolean, Hash] whether to verify the issued at claim. A hash can be given to configure the claim, currently only `leeway` is supported.
       # @!attribute [rw] verify_jti
       #   @return [Boolean] whether to verify the JWT ID claim.
       # @!attribute [rw] verify_aud

spec/jwt/claims/issued_at_spec.rb

@@ -3,7 +3,9 @@
 RSpec.describe JWT::Claims::IssuedAt do
   let(:payload) { { 'iat' => Time.now.to_f } }
 
-  subject(:verify!) { described_class.new.verify!(context: SpecSupport::Token.new(payload: payload)) }
+  let(:options) { {} }
+
+  subject(:verify!) { described_class.new(**options).verify!(context: SpecSupport::Token.new(payload: payload)) }
 
   context 'when iat is now' do
     it 'passes validation' do
@@ -18,6 +20,42 @@
       verify!
     end
   end
+  context 'when the issuer clock is ahead of the verifier clock' do
+    let(:now) { Time.at(1_609_459_200.5) }
+    let(:payload) { { 'iat' => 1_609_459_201 } }
+
+    before { allow(Time).to receive(:now) { now } }
+
+    it 'fails validation' do
+      expect { verify! }.to raise_error(JWT::InvalidIatError)
+    end
+
+    context 'when a leeway covering the drift is given' do
+      let(:options) { { leeway: 1 } }
+
+      it 'passes validation' do
+        verify!
+      end
+    end
+
+    context 'when a leeway smaller than the drift is given' do
+      let(:payload) { { 'iat' => 1_609_459_260 } }
+      let(:options) { { leeway: 1 } }
+
+      it 'fails validation' do
+        expect { verify! }.to raise_error(JWT::InvalidIatError)
+      end
+    end
+  end
+
+  context 'when iat is positive infinity' do
+    let(:payload) { { 'iat' => Float::INFINITY } }
+
+    it 'fails validation' do
+      expect { verify! }.to raise_error(JWT::InvalidIatError)
+    end
+  end
+
   context 'when iat is not a number' do
     let(:payload) { { 'iat' => 'not_a_number' } }
 

spec/jwt/claims_spec.rb

@@ -9,6 +9,15 @@
       end
     end
 
+    context 'iat claim' do
+      let(:payload) { { 'iat' => Time.now.to_i + 10, 'pay' => 'load' } }
+
+      it 'verifies the iat' do
+        expect { described_class.verify_payload!(payload, iat: {}) }.to raise_error(JWT::InvalidIatError, 'Invalid iat')
+        described_class.verify_payload!(payload, iat: { leeway: 1000 })
+      end
+    end
+
     context 'exp claim' do
       let(:payload) { { 'exp' => Time.now.to_i - 10, 'pay' => 'load' } }
 

spec/jwt/jwt_spec.rb

@@ -645,11 +645,19 @@
       end
     end
 
-    context 'when iat is 1 second before Time.now' do
+    context 'when iat is 1 second after Time.now' do
       let(:iat) { time_now.to_i + 1 }
       it 'raises an error' do
         expect { decoded_token }.to raise_error(JWT::InvalidIatError, 'Invalid iat')
       end
+
+      context 'when a leeway covering the drift is given' do
+        subject(:decoded_token) { JWT.decode(token, 'secret', true, verify_iat: { leeway: 1 }) }
+
+        it 'considers iat valid' do
+          expect(decoded_token).to be_an(Array)
+        end
+      end
     end
   end