diff --git a/Gemfile b/Gemfile
index bf748f0183..8b25414561 100644
--- a/Gemfile
+++ b/Gemfile
@@ -167,6 +167,9 @@ gem 'aws-sdk-s3', require: false
 gem 'azure-storage', require: false
 gem 'google-cloud-storage', '~> 1.45', require: false
 
+# Storage content analyzers
+gem 'excel_analyzer', path: 'gems/excel_analyzer'
+
 group :test do
   gem 'fivemat', '~> 1.3.7'
   gem 'webmock', '~> 3.19.1'
diff --git a/Gemfile.lock b/Gemfile.lock
index 710e002303..bbc3907d13 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -51,6 +51,14 @@ PATH
       mime-types (< 4.0.0)
       rails (~> 7.0.4)
 
+PATH
+  remote: gems/excel_analyzer
+  specs:
+    excel_analyzer (0.0.1)
+      activestorage
+      nokogiri
+      rubyzip
+
 GEM
   remote: https://rubygems.org/
   specs:
@@ -575,6 +583,7 @@ DEPENDENCIES
   capybara (~> 3.39.2)
   charlock_holmes (~> 0.7.7)
   dalli (~> 3.2.6)
+  excel_analyzer!
   exception_notification (~> 4.5.0)
   factory_bot_rails (~> 6.4.2)
   fancybox-rails (~> 0.3.0)
diff --git a/doc/CHANGES.md b/doc/CHANGES.md
index e67fc5ac5a..8198d14c80 100644
--- a/doc/CHANGES.md
+++ b/doc/CHANGES.md
@@ -2,6 +2,8 @@
 
 ## Highlighted Features
 
+* Add XSLX spreadsheet analyser to automatically detect hidden data (Helen
+  Cross, Graeme Porteous)
 * Update attachment processing to automatically rebuild if cached file goes
   missing (Graeme Porteous)
 * Allow `InfoRequest` to be categorised (Graeme Porteous)
diff --git a/gems/excel_analyzer/.gitignore b/gems/excel_analyzer/.gitignore
new file mode 100644
index 0000000000..a85f3fae79
--- /dev/null
+++ b/gems/excel_analyzer/.gitignore
@@ -0,0 +1 @@
+.rspec_status
diff --git a/gems/excel_analyzer/Gemfile b/gems/excel_analyzer/Gemfile
new file mode 100644
index 0000000000..68e9bd0ef0
--- /dev/null
+++ b/gems/excel_analyzer/Gemfile
@@ -0,0 +1,6 @@
+# frozen_string_literal: true
+
+source "https://rubygems.org"
+
+# Specify your gem's dependencies in excel_analyzer.gemspec
+gemspec
diff --git a/gems/excel_analyzer/README.md b/gems/excel_analyzer/README.md
new file mode 100644
index 0000000000..f321fba0a1
--- /dev/null
+++ b/gems/excel_analyzer/README.md
@@ -0,0 +1,16 @@
+# ExcelAnalyzer
+
+This gem packages an analyzer to inspects `XLSX` files (uploaded and analyzed as
+`ActiveStorage::Blob` objects) for hidden data, adding the results to the blob
+metadata.
+
+## Development
+
+After checking out the repo, run `bin/setup` to install dependencies. Then, run
+`rake spec` to run the tests. You can also run `bin/console` for an interactive
+prompt that will allow you to experiment.
+
+## Contributing
+
+Bug reports and pull requests are welcome on GitHub at
+https://github.com/mysociety/alaveteli
diff --git a/gems/excel_analyzer/Rakefile b/gems/excel_analyzer/Rakefile
new file mode 100644
index 0000000000..b6ae734104
--- /dev/null
+++ b/gems/excel_analyzer/Rakefile
@@ -0,0 +1,8 @@
+# frozen_string_literal: true
+
+require "bundler/gem_tasks"
+require "rspec/core/rake_task"
+
+RSpec::Core::RakeTask.new(:spec)
+
+task default: :spec
diff --git a/gems/excel_analyzer/bin/console b/gems/excel_analyzer/bin/console
new file mode 100755
index 0000000000..d8b31c8448
--- /dev/null
+++ b/gems/excel_analyzer/bin/console
@@ -0,0 +1,15 @@
+#!/usr/bin/env ruby
+# frozen_string_literal: true
+
+require "bundler/setup"
+require "excel_analyzer"
+
+# You can add fixtures and/or initialization code here to make experimenting
+# with your gem easier. You can also use a different console, if you like.
+
+# (If you use this, don't forget to add pry to your Gemfile!)
+# require "pry"
+# Pry.start
+
+require "irb"
+IRB.start(__FILE__)
diff --git a/gems/excel_analyzer/bin/setup b/gems/excel_analyzer/bin/setup
new file mode 100755
index 0000000000..dce67d860a
--- /dev/null
+++ b/gems/excel_analyzer/bin/setup
@@ -0,0 +1,8 @@
+#!/usr/bin/env bash
+set -euo pipefail
+IFS=$'\n\t'
+set -vx
+
+bundle install
+
+# Do any other automated setup that you need to do here
diff --git a/gems/excel_analyzer/excel_analyzer.gemspec b/gems/excel_analyzer/excel_analyzer.gemspec
new file mode 100644
index 0000000000..3ac499a707
--- /dev/null
+++ b/gems/excel_analyzer/excel_analyzer.gemspec
@@ -0,0 +1,34 @@
+# frozen_string_literal: true
+
+lib = File.expand_path('lib', __dir__)
+$LOAD_PATH.unshift(lib) unless $LOAD_PATH.include?(lib)
+
+Gem::Specification.new do |spec|
+  spec.name = "excel_analyzer"
+  spec.version = "0.0.1"
+  spec.authors = ["mySociety"]
+  spec.email = ["alaveteli@mysociety.org"]
+
+  spec.summary = "File analysers for ActiveStorage"
+  spec.description = "Extra ActiveStorage Analysers for Alaveteli"
+  spec.homepage = "https://alaveteli.org"
+  spec.required_ruby_version = ">= 3.0.0"
+
+  spec.metadata["homepage_uri"] = spec.homepage
+
+  spec.files = Dir.chdir(__dir__) do
+    `git ls-files -z`.split("\x0").reject do |f|
+      (File.expand_path(f) == __FILE__) || f.start_with?(*%w[bin/ spec/ .git])
+    end
+  end
+  spec.require_paths = ["lib"]
+
+  spec.add_dependency "activestorage"
+  spec.add_dependency "nokogiri"
+  spec.add_dependency "rubyzip"
+
+  spec.add_development_dependency "bundler"
+  spec.add_development_dependency "pry"
+  spec.add_development_dependency "rake"
+  spec.add_development_dependency "rspec"
+end
diff --git a/gems/excel_analyzer/lib/excel_analyzer.rb b/gems/excel_analyzer/lib/excel_analyzer.rb
new file mode 100644
index 0000000000..92cb43e5cb
--- /dev/null
+++ b/gems/excel_analyzer/lib/excel_analyzer.rb
@@ -0,0 +1,2 @@
+require "excel_analyzer/analyzer"
+require "excel_analyzer/railtie" if defined?(Rails)
diff --git a/gems/excel_analyzer/lib/excel_analyzer/analyzer.rb b/gems/excel_analyzer/lib/excel_analyzer/analyzer.rb
new file mode 100644
index 0000000000..3ef3d11669
--- /dev/null
+++ b/gems/excel_analyzer/lib/excel_analyzer/analyzer.rb
@@ -0,0 +1,80 @@
+require "active_storage"
+require "active_storage/analyzer"
+require "nokogiri"
+require "zip"
+
+module ExcelAnalyzer
+  ##
+  # The Analyzer class is responsible for analyzing Excel files uploaded through
+  # Active Storage. It checks for various features within the Excel file such as
+  # hidden rows, columns, sheets, pivot caches, and external links.
+  #
+  # The class uses rubyzip and Nokogiri for reading and parsing the contents of
+  # the Excel (.xlsx) files.
+  #
+  class Analyzer < ActiveStorage::Analyzer
+    XLSX_CONTENT_TYPE =
+      "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet"
+
+    def self.accept?(blob)
+      blob.content_type == XLSX_CONTENT_TYPE
+    end
+
+    def metadata
+      { excel: excel_metadata }
+    end
+
+    private
+
+    def excel_metadata
+      download_blob_to_tempfile(&method(:probe))
+    end
+
+    def probe(tempfile)
+      Zip::File.open(tempfile.path) do |zip_file|
+        {
+          pivot_cache: zip_file.glob("xl/pivotCache/*").any?,
+          external_links: zip_file.glob("xl/externalLinks/*").any?,
+          hidden_rows: hidden_rows?(zip_file),
+          hidden_columns: hidden_columns?(zip_file),
+          hidden_sheets: hidden_sheets?(zip_file)
+        }
+      end
+
+    rescue StandardError => ex
+      { error: ex.message }
+    end
+
+    def namespace
+      { "ns" => "http://schemas.openxmlformats.org/spreadsheetml/2006/main" }
+    end
+
+    def hidden?(object)
+      object.attr("hidden") == "true" ||
+        object.attr("hidden") == "1" ||
+        object.attr("state") == "hidden"
+    end
+
+    def hidden_rows?(zip_file)
+      zip_file.glob("xl/worksheets/*.xml").any? do |worksheet_file|
+        doc = Nokogiri::XML(worksheet_file.get_input_stream.read)
+        doc.xpath("//ns:row", namespace).any?(&method(:hidden?))
+      end
+    end
+
+    def hidden_columns?(zip_file)
+      zip_file.glob("xl/worksheets/*.xml").any? do |worksheet_file|
+        doc = Nokogiri::XML(worksheet_file.get_input_stream.read)
+        doc.xpath("//ns:col", namespace).any?(&method(:hidden?))
+      end
+    end
+
+    def hidden_sheets?(zip_file)
+      workbook_file = zip_file.glob("xl/workbook.xml").first
+      return false unless workbook_file
+
+      doc = Nokogiri::XML(workbook_file.get_input_stream.read)
+      doc.xpath("//ns:sheet", namespace).any?(&method(:hidden?))
+    end
+  end
+end
diff --git a/gems/excel_analyzer/lib/excel_analyzer/railtie.rb b/gems/excel_analyzer/lib/excel_analyzer/railtie.rb
new file mode 100644
index 0000000000..6c92d905d9
--- /dev/null
+++ b/gems/excel_analyzer/lib/excel_analyzer/railtie.rb
@@ -0,0 +1,12 @@
+require "rails"
+require "active_storage"
+
+module ExcelAnalyzer
+  ##
+  # This Railtie integrates the gem with Rails by extending ActiveStorage's
+  # Analyzers with the custom ExcelAnalyzer::Analyzer.
+  #
+  class Railtie < Rails::Railtie
+    config.active_storage.analyzers.prepend ExcelAnalyzer::Analyzer
+  end
+end
diff --git a/gems/excel_analyzer/spec/excel_analyzer/analyzer_spec.rb b/gems/excel_analyzer/spec/excel_analyzer/analyzer_spec.rb
new file mode 100644
index 0000000000..bdce8360b9
--- /dev/null
+++ b/gems/excel_analyzer/spec/excel_analyzer/analyzer_spec.rb
@@ -0,0 +1,91 @@
+# frozen_string_literal: true
+
+require "spec_helper"
+
+RSpec.describe ExcelAnalyzer::Analyzer do
+  describe ".accept?" do
+    subject { ExcelAnalyzer::Analyzer.accept?(blob) }
+
+    context "when the blob is an Excel file" do
+      let(:blob) do
+        fake_blob(content_type: ExcelAnalyzer::Analyzer::XLSX_CONTENT_TYPE)
+      end
+
+      it { is_expected.to eq true }
+    end
+
+    context "when the blob is not an Excel file" do
+      let(:blob) { fake_blob(content_type: "text/plain") }
+      it { is_expected.to eq false }
+    end
+  end
+
+  describe "#metadata" do
+    let(:metadata) { ExcelAnalyzer::Analyzer.new(blob).metadata }
+
+    context "when the blob is an Excel file with hidden data" do
+      let(:blob) do
+        fake_blob(io: File.open(File.join(__dir__, "../fixtures/suspect.xlsx")),
+                  content_type: ExcelAnalyzer::Analyzer::XLSX_CONTENT_TYPE)
+      end
+
+      it "detects pivot cache" do
+        expect(metadata[:excel][:pivot_cache]).to eq true
+      end
+
+      it "detects external links" do
+        expect(metadata[:excel][:external_links]).to eq true
+      end
+
+      it "detects hidden rows" do
+        expect(metadata[:excel][:hidden_rows]).to eq true
+      end
+
+      it "detects hidden columns" do
+        expect(metadata[:excel][:hidden_columns]).to eq true
+      end
+
+      it "detects hidden sheets" do
+        expect(metadata[:excel][:hidden_sheets]).to eq true
+      end
+    end
+
+    context "when the blob is an Excel file without hidden data" do
+      let(:blob) do
+        fake_blob(io: File.open(File.join(__dir__, "../fixtures/data.xlsx")),
+                  content_type: ExcelAnalyzer::Analyzer::XLSX_CONTENT_TYPE)
+      end
+
+      it "does not detect hidden data" do
+        expect(metadata[:excel]).to eq(
+          pivot_cache: false,
+          external_links: false,
+          hidden_rows: false,
+          hidden_columns: false,
+          hidden_sheets: false
+        )
+      end
+    end
+
+    context "when the blob is not an Excel file" do
+      let(:blob) do
+        fake_blob(io: File.open(File.join(__dir__, "../fixtures/plain.txt")),
+                  content_type: "text/plain")
+      end
+
+      it "returns an error metadata" do
+        expect(metadata[:excel]).to eq(
+          error: "Zip end of central directory signature not found"
+        )
+      end
+    end
+  end
+
+  private
+
+  def fake_blob(io: nil, content_type:)
+    dbl = double(content_type: content_type)
+    allow(dbl).to receive(:open).and_yield(io)
+    dbl
+  end
+end
diff --git a/gems/excel_analyzer/spec/fixtures/data.xlsx b/gems/excel_analyzer/spec/fixtures/data.xlsx
new file mode 100644
index 0000000000..132ff364f8
Binary files /dev/null and b/gems/excel_analyzer/spec/fixtures/data.xlsx differ
diff --git a/gems/excel_analyzer/spec/fixtures/plain.txt b/gems/excel_analyzer/spec/fixtures/plain.txt
new file mode 100644
index 0000000000..a496efee84
--- /dev/null
+++ b/gems/excel_analyzer/spec/fixtures/plain.txt
@@ -0,0 +1 @@
+This is a text file
diff --git a/gems/excel_analyzer/spec/fixtures/suspect.xlsx b/gems/excel_analyzer/spec/fixtures/suspect.xlsx
new file mode 100644
index 0000000000..d2e01fcc30
Binary files /dev/null and b/gems/excel_analyzer/spec/fixtures/suspect.xlsx differ
diff --git a/gems/excel_analyzer/spec/spec_helper.rb b/gems/excel_analyzer/spec/spec_helper.rb
new file mode 100644
index 0000000000..5fe1b720c1
--- /dev/null
+++ b/gems/excel_analyzer/spec/spec_helper.rb
@@ -0,0 +1,16 @@
+# frozen_string_literal: true
+
+require "bundler/setup"
+require "excel_analyzer/analyzer"
+
+RSpec.configure do |config|
+  # Enable flags like --only-failures and --next-failure
+  config.example_status_persistence_file_path = ".rspec_status"
+
+  # Disable RSpec exposing methods globally on `Module` and `main`
+  config.disable_monkey_patching!
+
+  config.expect_with :rspec do |c|
+    c.syntax = :expect
+  end
+end