diff --git a/app/models/concerns/has_custom_fields.rb b/app/models/concerns/has_custom_fields.rb
index 5330abdd694..1b592733528 100644
--- a/app/models/concerns/has_custom_fields.rb
+++ b/app/models/concerns/has_custom_fields.rb
@@ -60,12 +60,19 @@ module HasCustomFields
     end
   end
 
-  included do
-    has_many :_custom_fields, dependent: :destroy, class_name: "#{name}CustomField"
-    after_save :save_custom_fields
+  CUSTOM_FIELDS_MAX_ITEMS = 100
+  CUSTOM_FIELDS_MAX_VALUE_LENGTH = 10_000_000
 
+  included do
     attr_reader :preloaded_custom_fields
 
+    has_many :_custom_fields, dependent: :destroy, class_name: "#{name}CustomField"
+
+    validate :custom_fields_max_items, unless: :custom_fields_clean?
+    validate :custom_fields_value_length, unless: :custom_fields_clean?
+
+    after_save :save_custom_fields
+
     def custom_fields_fk
       @custom_fields_fk ||= "#{_custom_fields.reflect_on_all_associations(:belongs_to)[0].name}_id"
     end
@@ -133,6 +140,28 @@ module HasCustomFields
           end
       end
     end
+
+    private
+
+    def custom_fields_max_items
+      if custom_fields.size > CUSTOM_FIELDS_MAX_ITEMS
+        errors.add(
+          :base,
+          I18n.t("custom_fields.validations.max_items", max_items_number: CUSTOM_FIELDS_MAX_ITEMS),
+        )
+      end
+    end
+
+    def custom_fields_value_length
+      return if custom_fields.values.all? { _1.to_s.size <= CUSTOM_FIELDS_MAX_VALUE_LENGTH }
+      errors.add(
+        :base,
+        I18n.t(
+          "custom_fields.validations.max_value_length",
+          max_value_length: CUSTOM_FIELDS_MAX_VALUE_LENGTH,
+        ),
+      )
+    end
   end
 
   def reload(options = nil)
diff --git a/config/locales/server.en.yml b/config/locales/server.en.yml
index 366981d9536..edc1f98d33e 100644
--- a/config/locales/server.en.yml
+++ b/config/locales/server.en.yml
@@ -245,6 +245,10 @@ en:
     errors:
       <<: *errors
 
+  custom_fields:
+    validations:
+      max_items: "Maximum number of custom fields for this entity has been reached (%{max_items_number})"
+      max_value_length: "Maximum length for a custom field value has been reached (%{max_value_length})"
   invite:
     expired: "Your invite token has expired. Please <a href='%{base_url}/about'>contact staff</a>."
     not_found: "Your invite token is invalid. Please <a href='%{base_url}/about'>contact staff</a>."
diff --git a/spec/models/category_spec.rb b/spec/models/category_spec.rb
index d434bb886a7..d59d7abb984 100644
--- a/spec/models/category_spec.rb
+++ b/spec/models/category_spec.rb
@@ -4,6 +4,8 @@
 RSpec.describe Category do
   fab!(:user) { Fabricate(:user) }
 
+  it_behaves_like "it has custom fields"
+
   it { is_expected.to validate_presence_of :user_id }
   it { is_expected.to validate_presence_of :name }
 
diff --git a/spec/models/group_spec.rb b/spec/models/group_spec.rb
index cc0a09b3922..0db630cd921 100644
--- a/spec/models/group_spec.rb
+++ b/spec/models/group_spec.rb
@@ -5,6 +5,8 @@ RSpec.describe Group do
   let(:user) { Fabricate(:user) }
   let(:group) { Fabricate(:group) }
 
+  it_behaves_like "it has custom fields"
+
   describe "Validations" do
     it { is_expected.to allow_value("#{"a" * 996}.com").for(:automatic_membership_email_domains) }
     it do
diff --git a/spec/models/post_spec.rb b/spec/models/post_spec.rb
index 6cbf7e4f19b..66ece7915df 100644
--- a/spec/models/post_spec.rb
+++ b/spec/models/post_spec.rb
@@ -7,6 +7,8 @@ RSpec.describe Post do
 
   before { Oneboxer.stubs :onebox }
 
+  it_behaves_like "it has custom fields"
+
   it { is_expected.to have_many(:reviewables).dependent(:destroy) }
 
   describe "#hidden_reasons" do
diff --git a/spec/models/topic_spec.rb b/spec/models/topic_spec.rb
index 1bc71fa6334..d5af3d60ec6 100644
--- a/spec/models/topic_spec.rb
+++ b/spec/models/topic_spec.rb
@@ -16,6 +16,8 @@ RSpec.describe Topic do
     Fabricate(:user, trust_level: SiteSetting.min_trust_level_to_allow_invite)
   end
 
+  it_behaves_like "it has custom fields"
+
   describe "Validations" do
     let(:topic) { Fabricate.build(:topic) }
 
diff --git a/spec/models/user_spec.rb b/spec/models/user_spec.rb
index bbb216ad541..fd993516cf8 100644
--- a/spec/models/user_spec.rb
+++ b/spec/models/user_spec.rb
@@ -1,9 +1,11 @@
 # frozen_string_literal: true
 
 RSpec.describe User do
+  subject(:user) { Fabricate(:user, last_seen_at: 1.day.ago) }
+
   fab!(:group) { Fabricate(:group) }
 
-  subject(:user) { Fabricate(:user, last_seen_at: 1.day.ago) }
+  it_behaves_like "it has custom fields"
 
   def user_error_message(*keys)
     I18n.t(:"activerecord.errors.models.user.attributes.#{keys.join(".")}")
diff --git a/spec/support/shared_examples_for_custom_fields.rb b/spec/support/shared_examples_for_custom_fields.rb
new file mode 100644
index 00000000000..c3f98f72d74
--- /dev/null
+++ b/spec/support/shared_examples_for_custom_fields.rb
@@ -0,0 +1,27 @@
+# frozen_string_literal: true
+
+RSpec.shared_examples_for "it has custom fields" do
+  let(:record) { described_class.new }
+
+  describe "Max number of custom fields" do
+    let(:custom_fields) { (1..101).to_a.product(["value"]).to_h }
+
+    before { record.custom_fields = custom_fields }
+
+    it "can't have more than 100 custom fields" do
+      expect(record).to be_invalid
+      expect(record.errors[:base]).to include(/Maximum number.*\(100\)/)
+    end
+  end
+
+  describe "Max length of a custom field" do
+    let(:bad_value) { "a" * 10_000_001 }
+
+    before { record.custom_fields[:my_custom_field] = bad_value }
+
+    it "can't have more than 10,000,000 characters" do
+      expect(record).to be_invalid
+      expect(record.errors[:base]).to include(/Maximum length.*\(10000000\)/)
+    end
+  end
+end