diff --git a/plugins/chat/app/models/chat/message.rb b/plugins/chat/app/models/chat/message.rb
index 5ba54f666b6..965c2fa09a3 100644
--- a/plugins/chat/app/models/chat/message.rb
+++ b/plugins/chat/app/models/chat/message.rb
@@ -73,6 +73,7 @@ module Chat
 
     before_save { ensure_last_editor_id }
 
+    validates :cooked, length: { maximum: 20_000 }
     validate :validate_message
 
     def self.polymorphic_class_mapping = { "ChatMessage" => Chat::Message }
diff --git a/plugins/chat/app/models/chat/message_revision.rb b/plugins/chat/app/models/chat/message_revision.rb
index 3b01ee03339..8a273443e1a 100644
--- a/plugins/chat/app/models/chat/message_revision.rb
+++ b/plugins/chat/app/models/chat/message_revision.rb
@@ -4,6 +4,9 @@ module Chat
   class MessageRevision < ActiveRecord::Base
     self.table_name = "chat_message_revisions"
 
+    validates :old_message, length: { maximum: 50_000 }
+    validates :new_message, length: { maximum: 50_000 }
+
     belongs_to :chat_message, class_name: "Chat::Message"
     belongs_to :user
   end
diff --git a/plugins/chat/spec/models/chat/message_revision_spec.rb b/plugins/chat/spec/models/chat/message_revision_spec.rb
new file mode 100644
index 00000000000..b5e4bba527d
--- /dev/null
+++ b/plugins/chat/spec/models/chat/message_revision_spec.rb
@@ -0,0 +1,6 @@
+# frozen_string_literal: true
+
+RSpec.describe Chat::MessageRevision do
+  it { is_expected.to validate_length_of(:old_message).is_at_most(50_000) }
+  it { is_expected.to validate_length_of(:new_message).is_at_most(50_000) }
+end
diff --git a/plugins/chat/spec/models/chat/message_spec.rb b/plugins/chat/spec/models/chat/message_spec.rb
index 7b44fa94271..5c4b0fb974d 100644
--- a/plugins/chat/spec/models/chat/message_spec.rb
+++ b/plugins/chat/spec/models/chat/message_spec.rb
@@ -7,6 +7,12 @@ describe Chat::Message do
 
   it { is_expected.to have_many(:chat_mentions).dependent(:destroy) }
 
+  describe "validations" do
+    subject(:message) { described_class.new(message: "") }
+
+    it { is_expected.to validate_length_of(:cooked).is_at_most(20_000) }
+  end
+
   describe ".cook" do
     it "does not support HTML tags" do
       cooked = described_class.cook("<h1>test</h1>")