diff --git a/lib/open_project/text_formatting/filters/markdown_filter.rb b/lib/open_project/text_formatting/filters/markdown_filter.rb
index 39ee6061744..1fcb8aa68fb 100644
--- a/lib/open_project/text_formatting/filters/markdown_filter.rb
+++ b/lib/open_project/text_formatting/filters/markdown_filter.rb
@@ -63,11 +63,11 @@ module OpenProject::TextFormatting
       ##
       # Extensions to the default CommonMarker operation
       def commonmark_extensions
-        # Disable all the extension enabled by default by commonmarker.
+        enabled = %i[table strikethrough]
+
         %i[strikethrough tagfilter table autolink tasklist shortcodes]
           .index_with(false)
-          # But enable those that the context has enabled explicitly
-          .merge(context.fetch(:commonmarker_extensions, %i[table strikethrough tagfilter]).index_with(true))
+          .merge(enabled.index_with(true))
       end
     end
   end
diff --git a/spec/lib/open_project/text_formatting/markdown/security_spec.rb b/spec/lib/open_project/text_formatting/markdown/security_spec.rb
new file mode 100644
index 00000000000..19c4f62ec08
--- /dev/null
+++ b/spec/lib/open_project/text_formatting/markdown/security_spec.rb
@@ -0,0 +1,201 @@
+# frozen_string_literal: true
+
+#-- copyright
+# OpenProject is an open source project management software.
+# Copyright (C) the OpenProject GmbH
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License version 3.
+#
+# OpenProject is a fork of ChiliProject, which is a fork of Redmine. The copyright follows:
+# Copyright (C) 2006-2013 Jean-Philippe Lang
+# Copyright (C) 2010-2013 the ChiliProject Team
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License
+# as published by the Free Software Foundation; either version 2
+# of the License, or (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+#
+# See COPYRIGHT and LICENSE files for more details.
+#++
+require "spec_helper"
+
+# These specs exist because cmark-gfm's :tagfilter extension is intentionally
+# NOT enabled in MarkdownFilter (it is extremely slow on large inputs such as long tables).
+# The same protection is provided by SanitizationFilter, which strips any tag
+# not on its allowlist.
+# This specs serves as an additional measure to test the specific tags part of tagfilter.
+RSpec.describe "Markdown security (tagfilter-equivalent protections)" do # rubocop:disable RSpec/DescribeClass
+  def to_html(markdown)
+    OpenProject::TextFormatting::Renderer.format_text(markdown).to_s
+  end
+
+  # The 9 tags that cmark-gfm's :tagfilter extension targets.
+  # https://github.github.com/gfm/#disallowed-raw-html-extension-
+  %w[
+    title textarea style xmp iframe noembed noframes script plaintext
+  ].each do |tag|
+    describe "raw <#{tag}> in markdown input" do
+      it "does not produce a live <#{tag}> element in the output" do
+        html = to_html("before <#{tag}>payload</#{tag}> after")
+        expect(html).not_to match(/<#{tag}\b/i)
+        expect(html).not_to match(%r{</#{tag}>}i)
+      end
+
+      it "does not produce a live <#{tag}> in an HTML block" do
+        markdown = <<~MD
+          paragraph one
+
+          <#{tag}>payload</#{tag}>
+
+          paragraph two
+        MD
+        html = to_html(markdown)
+        expect(html).not_to match(/<#{tag}\b/i)
+        expect(html).not_to match(%r{</#{tag}>}i)
+      end
+
+      it "strips uppercase <#{tag.upcase}> just the same" do
+        html = to_html("x <#{tag.upcase}>payload</#{tag.upcase}> y")
+        expect(html).not_to match(/<#{tag}\b/i)
+      end
+
+      it "strips <#{tag}> with attributes" do
+        html = to_html(%(x <#{tag} id="a" class="b" data-x="c">payload</#{tag}> y))
+        expect(html).not_to match(/<#{tag}\b/i)
+      end
+    end
+  end
+
+  describe "classic XSS payloads" do
+    it "neutralises an inline <script> alert" do
+      html = to_html("hello <script>alert('xss')</script> world")
+      expect(html).not_to match(/<script\b/i)
+      expect(html).not_to include("alert('xss')")
+    end
+
+    it "neutralises a <script src=...> external loader" do
+      html = to_html(%(<script src="https://evil.example.com/x.js"></script>))
+      expect(html).not_to match(/<script\b/i)
+      expect(html).not_to include("evil.example.com")
+    end
+
+    it "neutralises an <iframe> pointing at javascript:" do
+      html = to_html(%(<iframe src="javascript:alert(1)"></iframe>))
+      expect(html).not_to match(/<iframe\b/i)
+      expect(html).not_to match(/javascript:/i)
+    end
+
+    it "neutralises <iframe srcdoc=...> HTML injection" do
+      html = to_html(%(<iframe srcdoc="<script>alert(1)</script>"></iframe>))
+      expect(html).not_to match(/<iframe\b/i)
+      expect(html).not_to match(/<script\b/i)
+    end
+
+    it "neutralises <style> CSS injection with url() data-exfil" do
+      # The <style> element must be removed; the CSS `url()` cannot fire without
+      # a surviving <style> wrapper. Any leftover URL text is no longer a CSS rule.
+      html = to_html(%(<style>body { background: url('https://evil.example.com/?'); }</style>))
+      expect(html).not_to match(/<style\b/i)
+      expect(html).not_to match(%r{</style>}i)
+    end
+
+    it "neutralises <plaintext> document takeover" do
+      markdown = <<~MD
+        safe paragraph before
+
+        <plaintext>everything from here should be hidden
+
+        but we render a trailing paragraph too
+      MD
+      html = to_html(markdown)
+      expect(html).not_to match(/<plaintext\b/i)
+      # The trailing paragraph must still be present and rendered as normal content.
+      expect(html).to include("trailing paragraph")
+    end
+
+    it "neutralises <title> document-title takeover" do
+      html = to_html("<title>Attacker Title</title>")
+      expect(html).not_to match(/<title\b/i)
+    end
+
+    it "neutralises <textarea> that would otherwise swallow following content" do
+      markdown = <<~MD
+        before
+
+        <textarea>swallowed</textarea>
+
+        after
+      MD
+      html = to_html(markdown)
+      expect(html).not_to match(/<textarea\b/i)
+      expect(html).to include("after")
+    end
+
+    it "neutralises <xmp> raw-text takeover" do
+      markdown = <<~MD
+        before
+
+        <xmp><script>alert(1)</script></xmp>
+
+        after
+      MD
+      html = to_html(markdown)
+      expect(html).not_to match(/<xmp\b/i)
+      expect(html).not_to match(/<script\b/i)
+      expect(html).to include("after")
+    end
+
+    it "strips <img onerror=...> attribute-based XSS" do
+      html = to_html(%(<img src="x" onerror="alert(1)">))
+      expect(html).not_to include("onerror")
+      expect(html).not_to include("alert(1)")
+    end
+
+    it "strips event-handler attributes on allowed tags" do
+      html = to_html(%(<a href="https://example.com" onclick="alert(1)">link</a>))
+      expect(html).to match(/<a\b/i)
+      expect(html).not_to include("onclick")
+      expect(html).not_to include("alert(1)")
+    end
+
+    it "strips javascript: protocol on <a href>" do
+      html = to_html(%([click](javascript:alert(1))))
+      expect(html).not_to match(/href="javascript:/i)
+    end
+
+    it "strips <object> and <embed> which are not on the allowlist" do
+      html = to_html(%(<object data="https://evil.example.com/x.swf"></object><embed src="x.swf">))
+      expect(html).not_to match(/<object\b/i)
+      expect(html).not_to match(/<embed\b/i)
+      expect(html).not_to include("evil.example.com")
+    end
+
+    it "strips inline SVG <script>" do
+      html = to_html(%(<svg><script>alert(1)</script></svg>))
+      expect(html).not_to match(/<script\b/i)
+      expect(html).not_to include("alert(1)")
+    end
+  end
+
+  describe "regression: disabling tagfilter does not regress existing behaviour" do
+    it "still escapes a bare '<script>' text token in inline flow" do
+      html = to_html("this is a <script>")
+      expect(html).not_to match(/<script\b/i)
+    end
+
+    it "preserves non-dangerous HTML passthrough (e.g. <strong>)" do
+      html = to_html("a <strong>bold</strong> word")
+      expect(html).to match(%r{<strong>bold</strong>})
+    end
+  end
+end