diff --git a/_plugins/safe_post_html.rb b/_plugins/safe_post_html.rb
new file mode 100644
index 0000000..6f1b598
--- /dev/null
+++ b/_plugins/safe_post_html.rb
@@ -0,0 +1,86 @@
+# frozen_string_literal: true
+#
+# Fails the build when a post's rendered body holds markup that would run code
+# in a reader's browser or load another document in the site's origin: an
+# active element (<script>, <object>, <base>, <meta>, <form>, ...), an event
+# handler or srcdoc/formaction attribute, or a URL attribute whose scheme is
+# not one a post links to (http(s), mailto, tel, a relative path or anchor, or
+# an inline image).
+#
+# It checks what kramdown produced, after conversion and before the layout, so
+# it sees every way Markdown can make an attribute (raw HTML, links, images,
+# attribute lists) as the browser will read it: entities decoded, attributes
+# split by an HTML5 parser (Nokogiri::HTML5, as browsers parse). Code blocks
+# are text by then and are not flagged. Posts are generated and then reviewed
+# as pull requests, so a failing build blocks the merge (and the deploy)
+# instead of silently rewriting the post.
+#
+# This repo builds via GitHub Actions (not the github-pages gem sandbox), so
+# custom _plugins are allowed.
+begin
+  require "nokogiri"
+rescue LoadError
+  # Plain `ruby test/run_all.rb` in CI: the gem is in the bundle's path.
+  require "bundler/setup"
+  require "nokogiri"
+end
+
+module SafePostHtml
+  ACTIVE_ELEMENTS = %w[
+    script style object embed applet base link meta form frame frameset noscript
+  ].freeze
+  ACTIVE_ATTRIBUTES = %w[srcdoc formaction].freeze
+  URL_ATTRIBUTES = %w[
+    href src action data poster background cite xlink:href srcset
+  ].freeze
+  SAFE_SCHEMES = %w[http https mailto tel].freeze
+
+  # Whether +url+ is one a post may carry. Spaces and control characters a
+  # browser ignores in a URL are ignored here too.
+  def self.safe_url?(url, element)
+    bare = url.to_s.gsub(/[\u0000- \u007f]/, "").downcase
+    scheme, colon, rest = bare.partition(":")
+    return true if colon.empty? || scheme.match?(%r{[/?#]})
+
+    return true if SAFE_SCHEMES.include?(scheme)
+
+    # An inline image: only where an image is loaded.
+    scheme == "data" && rest.start_with?("image/") && %w[img source].include?(element)
+  end
+
+  # The problems of a rendered post body, as "element attribute=value" lines.
+  def self.violations(html)
+    found = []
+    Nokogiri::HTML5::DocumentFragment.parse(html.to_s).traverse do |node|
+      next unless node.element?
+
+      name = node.name.downcase
+      found << "<#{name}>" if ACTIVE_ELEMENTS.include?(name)
+      node.attribute_nodes.each do |attr|
+        key = attr.name.downcase
+        if key.start_with?("on") || ACTIVE_ATTRIBUTES.include?(key)
+          found << "<#{name} #{key}>"
+        elsif URL_ATTRIBUTES.include?(key)
+          urls = key == "srcset" ? attr.value.split(",").map { |u| u.strip.split(/\s+/).first.to_s } : [attr.value]
+          urls.each do |url|
+            found << "<#{name} #{key}=\"#{url[0, 80]}\">" unless safe_url?(url, name)
+          end
+        end
+      end
+    end
+    found.uniq
+  end
+end
+
+# Guarded so test/ can require this file for SafePostHtml.violations without
+# Jekyll loaded, and without registering the hook.
+if defined?(Jekyll::Hooks)
+  Jekyll::Hooks.register :posts, :post_convert do |post|
+    problems = SafePostHtml.violations(post.content)
+    next if problems.empty?
+
+    raise Jekyll::Errors::FatalException,
+          "#{post.relative_path}: markup that runs code or loads a document " \
+          "(#{problems.first(5).join(', ')}). Remove it or put it in a code block."
+  end
+end
diff --git a/test/test_safe_post_html.rb b/test/test_safe_post_html.rb
new file mode 100644
index 0000000..1824031
--- /dev/null
+++ b/test/test_safe_post_html.rb
@@ -0,0 +1,56 @@
+# frozen_string_literal: true
+#
+# Unit tests for _plugins/safe_post_html.rb.
+#
+# The check runs over every post's rendered body and fails the build, so it must
+# catch each way markup can run code (as the browser reads it) and never flag
+# what posts legitimately carry: images, links, video embeds, code blocks.
+
+require "minitest/autorun"
+require_relative "../_plugins/safe_post_html"
+
+class TestSafePostHtml < Minitest::Test
+  ACTIVE = [
+    "<script>alert(1)</script>",
+    '<img src="x" onerror="alert(1)">',
+    "<svg onload=alert(1)></svg>",
+    '<a href="javascript:alert(1)">x</a>',
+    '<a href="java&#x73;cript:alert(1)">x</a>',
+    "<a href=\"java\tscript:alert(1)\">x</a>",
+    '<a href="JavaScript&colon;alert(1)">x</a>',
+    '<iframe srcdoc="&lt;script&gt;alert(1)&lt;/script&gt;"></iframe>',
+    '<iframe src="data:text/html,x"></iframe>',
+    "<object data=\"x.swf\"></object>",
+    '<form action="https://evil.example"><button>b</button></form>',
+    '<button formaction="javascript:alert(1)">b</button>',
+    '<meta http-equiv="refresh" content="0;url=https://evil.example">',
+    '<base href="https://evil.example/">',
+    "<p onmouseover=\"alert(1)\">p</p>"
+  ].freeze
+
+  def test_each_way_markup_can_run_code_is_reported
+    ACTIVE.each do |html|
+      refute_empty SafePostHtml.violations("<p>x</p>#{html}"), html
+    end
+  end
+
+  SAFE = [
+    '<img src="https://example.org/a.png" alt="그림">',
+    '<img src="/assets/img/a.png" srcset="/a.png 1x, /a@2x.png 2x">',
+    '<img src="data:image/png;base64,iVBORw0KGgo=" alt="점">',
+    '<a href="https://example.org/onboarding">안내</a>',
+    '<a href="#tldr">위로</a>',
+    '<a href="/posts/x/">글</a>',
+    '<a href="mailto:a@example.org">메일</a>',
+    '<a href="tel:+82-2-0000-0000">전화</a>',
+    '<iframe src="https://www.youtube.com/embed/x" allowfullscreen></iframe>',
+    '<pre><code>&lt;script&gt;alert(1)&lt;/script&gt; &lt;img onerror=x&gt;</code></pre>',
+    '<span data-on="x" title="based on=y">t</span>'
+  ].freeze
+
+  def test_what_posts_carry_is_not_reported
+    SAFE.each do |html|
+      assert_empty SafePostHtml.violations(html), html
+    end
+  end
+end