diff --git a/lib/ruby_smb/gss/provider/ntlm.rb b/lib/ruby_smb/gss/provider/ntlm.rb
index 6def51ca..542596c2 100644
--- a/lib/ruby_smb/gss/provider/ntlm.rb
+++ b/lib/ruby_smb/gss/provider/ntlm.rb
@@ -194,10 +194,23 @@ def process_gss_type1(gss_api)
           # take the GSS blob, extract the NTLM type 3 message and pass it to the process method to build the response
           # which is then put back into a new GSS reply-blob
           def process_gss_type3(gss_api)
-            neg_token_init = Hash[RubySMB::Gss.asn1dig(gss_api, 0).value.map { |obj| [obj.tag, obj.value[0].value] }]
+            # a NegTokenResp carries every field as OPTIONAL (RFC 4178 section 4.2.2), and a tagged
+            # element may be empty, so a response_token is only present when a tag-[2] field exists
+            # and wraps a value
+            neg_token_init = {}
+            RubySMB::Gss.asn1dig(gss_api, 0).value.each do |obj|
+              inner = obj.value.is_a?(Array) ? obj.value[0] : nil
+              neg_token_init[obj.tag] = inner.value if inner.respond_to?(:value)
+            end
             raw_type3_msg = neg_token_init[2]
+            return if raw_type3_msg.nil?
 
-            type3_msg = Net::NTLM::Message.parse(raw_type3_msg)
+            begin
+              type3_msg = Net::NTLM::Message.parse(raw_type3_msg)
+            rescue StandardError => e
+              logger.error("Failed to parse the NTLM type 3 message (#{e.class}: #{e.message})")
+              return
+            end
             if type3_msg.flag & NTLM::NEGOTIATE_FLAGS[:UNICODE] == NTLM::NEGOTIATE_FLAGS[:UNICODE]
               type3_msg.domain.force_encoding('UTF-16LE')
               type3_msg.user.force_encoding('UTF-16LE')
diff --git a/spec/lib/ruby_smb/gss/provider/ntlm/authenticator_spec.rb b/spec/lib/ruby_smb/gss/provider/ntlm/authenticator_spec.rb
index b8c539bd..193de2f8 100644
--- a/spec/lib/ruby_smb/gss/provider/ntlm/authenticator_spec.rb
+++ b/spec/lib/ruby_smb/gss/provider/ntlm/authenticator_spec.rb
@@ -59,6 +59,61 @@
       expect(result.identity).to be_nil
       expect(authenticator.session_key).to be_nil
     end
+
+    context 'when the NegTokenResp is malformed' do
+      def wrap_neg_token_resp(fields)
+        OpenSSL::ASN1::ASN1Data.new(
+          [OpenSSL::ASN1::Sequence.new(fields)],
+          1,
+          :CONTEXT_SPECIFIC
+        ).to_der
+      end
+
+      it 'returns nil for an empty inner sequence' do
+        expect { authenticator.process(wrap_neg_token_resp([])) }.not_to raise_error
+        expect(authenticator.process(wrap_neg_token_resp([]))).to be_nil
+      end
+
+      it 'returns nil when the response_token is absent' do
+        buf = wrap_neg_token_resp([
+          OpenSSL::ASN1::ASN1Data.new(
+            [OpenSSL::ASN1::Enumerated.new(OpenSSL::BN.new(1))],
+            0,
+            :CONTEXT_SPECIFIC
+          )
+        ])
+        expect(authenticator.process(buf)).to be_nil
+      end
+
+      it 'returns nil when only mech_list_mic is present' do
+        buf = wrap_neg_token_resp([
+          OpenSSL::ASN1::ASN1Data.new(
+            [OpenSSL::ASN1::OctetString.new('AAAA')],
+            3,
+            :CONTEXT_SPECIFIC
+          )
+        ])
+        expect(authenticator.process(buf)).to be_nil
+      end
+
+      it 'returns nil when the response_token is empty' do
+        buf = wrap_neg_token_resp([
+          OpenSSL::ASN1::ASN1Data.new([], 2, :CONTEXT_SPECIFIC)
+        ])
+        expect(authenticator.process(buf)).to be_nil
+      end
+
+      it 'returns nil when the response_token is not an NTLM message' do
+        buf = wrap_neg_token_resp([
+          OpenSSL::ASN1::ASN1Data.new(
+            [OpenSSL::ASN1::OctetString.new('not-ntlm-bytes')],
+            2,
+            :CONTEXT_SPECIFIC
+          )
+        ])
+        expect(authenticator.process(buf)).to be_nil
+      end
+    end
   end
 
   describe '#process_ntlm_type1' do