In `@app/assets/javascripts/embed.js`:
- Around line 5-12: The code assumes the DOM element with id
'discourse-comments' exists and calls comments.appendChild(iframe), which will
throw if comments is null; update the logic where comments is obtained (variable
comments referencing document.getElementById('discourse-comments')) to guard
against null by checking if comments is truthy before creating/appending the
iframe (or return/exit early), so only create iframe and set
iframe.src/attributes and call comments.appendChild(iframe) when comments is
present.
- Around line 15-24: postMessageReceived currently uses
discourseUrl.indexOf(e.origin) which is vulnerable to host-substring bypasses;
change the origin check to compare origins exactly by using the URL origin
comparison (e.g. compute new URL(discourseUrl).origin and compare it ===
e.origin) or maintain a whitelist array of allowed origins and check e.origin
against that list with strict equality; update the validation in
postMessageReceived to use e.origin === allowedOrigin (or
allowedOrigins.includes(e.origin)) and leave the rest of the message handling
(e.data.type === 'discourse-resize' and iframe.height assignment) unchanged.

In `@app/jobs/scheduled/poll_feed.rb`:
- Around line 24-38: The poll_feed method lacks network/feed error handling and
calls i.content.scrub without nil checks; wrap the feed fetch and parse
(open(SiteSetting.feed_polling_url) and SimpleRSS.parse) in a begin/rescue to
catch and log exceptions, and skip processing on failure, and before calling
.scrub on i.content ensure i.content is present (e.g., use a safe fallback like
an empty string or i.description) so TopicEmbed.import is never called with nil
content; log any errors with context (feed URL and exception) and continue
processing remaining items.

In `@app/models/topic_embed.rb`:
- Line 13: The HTML injection risk comes from interpolating the raw url into the
I18n string in topic_embed.rb; change the construction so the URL is
HTML-escaped before embedding (e.g., use ERB::Util.html_escape or CGI.escapeHTML
on the url) and ensure the href uses a validated/sanitized http(s) URI (parse
and reject non-http(s) schemes) before escaping; update the line that builds
contents (the interpolation of url inside I18n.t('embed.imported_from', link:
...)) to use the escaped/sanitized value so the rendered <a href='...'> and link
text cannot break out or inject XSS.
- Around line 44-53: The import_remote method currently calls open(url) and
Readability::Document without validation or error handling; replace the direct
open call in TopicEmbed.import_remote with a safe fetch: validate and parse the
URL (only allow http/https), resolve the host and reject private/internal IPs,
enforce a max redirect count, use a HTTP client (e.g., Net::HTTP) with explicit
open/read timeouts and a maximum allowed content-length, and limit the HTML size
read; wrap the fetch and Readability::Document creation in a rescue block to
handle timeouts, network errors, invalid content, and oversized responses and
return or raise a controlled error instead of crashing, keeping the call site
TopicEmbed.import(user, url, opts[:title] || doc.title, doc.content) unchanged.

In `@app/views/embed/best.html.erb`:
- Around line 3-5: The two link_to calls that open in a new tab (the ones
calling I18n.t('embed.title') and I18n.t('embed.start_discussion') with target:
'_blank' and using `@topic_view.topic.url`) need a rel attribute to prevent
reverse-tabnabbing; update each link_to invocation to include rel: 'noopener
noreferrer' alongside the existing class and target options.
- Around line 16-17: The avatar <img> lacks alt text; update the tag that uses
post.user.small_avatar_url to include an alt attribute (e.g., using
post.user.username or a fallback like "#{post.user.username} avatar") so screen
readers receive a meaningful description; ensure you use the same
post.user.username symbol for the alt and handle nil/blank username with a
sensible default.

In `@app/views/layouts/embed.html.erb`:
- Around line 9-12: The postMessage call currently uses request.referer as the
targetOrigin which is unreliable and unsafe; update the embed HTML so
parent.postMessage uses a validated origin instead—prefer reading
SiteSetting.embeddable_host (or a derived safeOrigin constant) and fall back to
a safe default only if explicitly configured; ensure you validate/normalize the
origin before passing it to parent.postMessage and avoid using raw
request.referer directly in the targetOrigin parameter (refer to the
parent.postMessage usage and request.referer in embed.html.erb).

In `@config/routes.rb`:
- Around line 245-246: The project is using Rails 3.2.12 (Gemfile entry for
rails) which is vulnerable to CVE-2014-0130; update the Rails dependency in the
Gemfile to a secure version (preferably upgrade the app to Rails 4.x as the
codebase already supports it) then run bundle update to refresh gems, run the
test suite and fix any deprecations/migration issues (routes like the new get
'embed/best' => 'embed#best' may need no change but verify routing and globbing
behavior), and ensure all security-related tests/passive scans are re-run before
merging.

In `@lib/post_revisor.rb`:
- Around line 85-86: In update_post, capture the boolean result of
`@post.save`(validate: !@opts[:skip_validations]) and handle a failure the same
way PostCreator and TopicCreator do: if save returns false (or exception occurs)
raise a runtime error (or log and raise) to prevent subsequent
save_reply_relationships from running and creating inconsistent state; reference
update_post, `@post.save`(validate: !@opts[:skip_validations]), and
save_reply_relationships when making the change so the save outcome is checked
and failures are surfaced consistently.

In `@lib/topic_retriever.rb`:
- Around line 48-50: The fetch_http method calls
SiteSetting.embed_by_username.downcase which will raise if the setting is nil;
update fetch_http to guard the setting before calling downcase (e.g., check
SiteSetting.embed_by_username.blank? or use to_s/safe navigation) and return
early if unset, then proceed to lookup User.where(username_lower: ...) so that
User lookup only runs when embed_by_username is present; reference fetch_http,
SiteSetting.embed_by_username, and User.where to locate the change.
- Around line 27-30: The current two-step throttle uses
$redis.setnx(retrieved_key, "1") followed by $redis.expire(retrieved_key, 60),
which can leave the key permanent on a crash; replace this with a single atomic
Redis SET that uses NX and EX options (i.e., use the Redis client's SET with NX
and a 60s TTL) and remove the separate expire call so the key is created only
when absent and always has the expiration.

In `@spec/jobs/poll_feed_spec.rb`:
- Around line 18-27: Make the specs deterministic by stubbing
SiteSetting.feed_polling_enabled? to true in each example and ensuring only the
intended setting is nil; for the "requires feed_polling_url" test stub
SiteSetting.feed_polling_enabled? => true and stub SiteSetting.embed_by_username
to a valid non-nil value before calling poller.execute, and for the "requires
embed_by_username" test stub SiteSetting.feed_polling_enabled? => true and stub
SiteSetting.feed_polling_url to a valid non-nil value so poller.execute only
fails due to the specifically tested missing setting (references:
poller.execute, poller.expects(:poll_feed).never,
SiteSetting.stubs(:feed_polling_url), SiteSetting.stubs(:embed_by_username),
SiteSetting.stubs(:feed_polling_enabled?)).