In `@app/assets/javascripts/embed.js`:
- Around line 5-12: The code assumes
document.getElementById('discourse-comments') always returns an element and that
discourseUrl ends with a slash; add a null-guard and normalize discourseUrl:
fetch the container via document.getElementById('discourse-comments') and if it
is null, no-op (or bail out) before creating/appending the iframe, and ensure
discourseUrl ends with '/' (append one if missing) before building iframe.src
using discourseEmbedUrl; update references:
document.getElementById('discourse-comments'), comments,
comments.appendChild(iframe), iframe.src, discourseUrl, and discourseEmbedUrl.
- Around line 15-24: The origin check in postMessageReceived is insecure because
it uses discourseUrl.indexOf(e.origin); replace that with a strict origin
comparison (e.origin === discourseUrl) or compare against a whitelist of allowed
origins, ensuring you reference the same discourseUrl and e.origin variables;
update the conditional in the postMessageReceived function to only accept exact
matches before processing e.data and setting iframe.height.

In `@app/jobs/scheduled/poll_feed.rb`:
- Around line 31-37: i.content can be nil causing a NoMethodError when calling
.scrub; update the RSS item handling in the loop so you normalize the item body
before scrubbing/using it — e.g. derive a string body by checking i.content,
then fallback to i.content_encoded or i.description (or empty string), call
.to_s.scrub on that result, and pass the cleaned string into TopicEmbed.import
along with url and i.title; ensure the nil-guarding happens before calling
.scrub and reference the loop handling where i.content is accessed and the
TopicEmbed.import call.
- Line 29: Replace the unsafe Kernel#open call by validating and fetching the
feed URL using URI and a direct HTTP client: parse SiteSetting.feed_polling_url
with URI.parse, ensure the scheme is "http" or "https", fetch the response body
with Net::HTTP (or a similar HTTP client) and then pass that body string to
SimpleRSS.parse; update the code around SimpleRSS.parse and
SiteSetting.feed_polling_url to perform the scheme validation and HTTP fetch and
add basic error handling for invalid URIs or network failures.

In `@app/models/topic_embed.rb`:
- Around line 44-53: The import_remote method uses open(url).read which is
unsafe (SSRF, no timeouts/size limits); replace it with a validated HTTP fetch
that only allows http/https, enforces open and read timeouts, and caps response
size before passing content to Readability::Document; update TopicEmbed.import
call to handle fetch errors (timeout, non-2xx, oversized body) by rescuing and
returning/logging a sensible failure, and reference the import_remote method and
Readability::Document when making these changes.
- Line 1: The file app/models/topic_embed.rb currently uses require_dependency
'nokogiri' which is incorrect for loading an external gem; replace the call to
require_dependency with a plain require 'nokogiri' so Nokogiri is loaded as a
gem (edit the top of topic_embed.rb where require_dependency 'nokogiri' appears
and change it to require 'nokogiri').
- Line 13: The code interpolates raw `url` into HTML via contents << ... which
risks XSS and also mutates the `contents` string; fix by HTML-escaping the `url`
before interpolation (e.g., use ERB::Util.html_escape or CGI.escapeHTML) and
avoid in-place mutation by building a new string (e.g., assign contents =
contents + ... or use a non-mutating concatenation) when calling
I18n.t('embed.imported_from', link: ...), so the link value is escaped and
`contents` is not mutated.

In `@app/views/embed/best.html.erb`:
- Line 16: The <img> tag rendering the avatar (src from
post.user.small_avatar_url) is missing an alt attribute; update the tag to
include a meaningful, escaped alt value such as the user's username (e.g., use
post.user.username or post.user.name) and provide a sensible fallback (empty
string) to ensure accessibility and avoid nil errors when the username is
missing.
- Line 6: The ERB template contains an invalid Ruby tag "end if" with no
condition; replace the dangling "end if" with a plain "end" ERB closing tag so
the conditional block is properly terminated (locate the "<%- end if %>"
occurrence in the template and change it to a simple "end" closing tag).

In `@app/views/embed/loading.html.erb`:
- Around line 6-11: The current IIFE uses setTimeout +
document.location.reload() with no stop condition, causing an unbounded reload
loop; modify the loader script to track retries (e.g., use a query param like
retry or a localStorage key such as "embed_retry_count"), increment it each
attempt inside the IIFE, and only call document.location.reload() if the count
is below a max (e.g., 5); when the max is reached, stop scheduling further
reloads and replace the page content with a friendly error message / retry
button so the embed shows an error state instead of continuously reloading.

In `@app/views/layouts/embed.html.erb`:
- Line 11: The embedded layout interpolates request.referer directly into
JavaScript (parent.postMessage) causing an XSS risk; change the interpolation to
escape the value using Rails' escape_javascript helper (use j(request.referer)
where request.referer is currently used in embed.html.erb) so the referer is
safely quoted, and add a server-side check before rendering this layout to
ensure request.referer (or derived origin) matches SiteSetting.embeddable_host
to restrict allowed target origins.

In `@db/migrate/20131217174004_create_topic_embeds.rb`:
- Line 3: The migration currently calls create_table :topic_embeds with force:
true which will silently drop the table if present; remove the force: true
option from the create_table call so the migration will raise an error if the
table already exists (preserving data) and run/update the migration tests to
ensure no further references expect destructive recreation of :topic_embeds.

In `@db/migrate/20131223171005_create_top_topics.rb`:
- Line 3: The migration currently uses create_table :top_topics, force: true
which will DROP the table if it exists and can cause silent data loss; remove
the force: true option from the create_table call and instead make the migration
idempotent by guarding the creation with a check for existence (e.g., use
table_exists?(:top_topics) or Rails' create_table ... if_not_exists option where
supported) or perform an explicit, intentional drop in the down/rollback path
only; update the migration that defines create_table :top_topics to use one of
these safer approaches so retries or replays won't destroy existing data.

In `@lib/topic_retriever.rb`:
- Around line 27-29: The current two-step lock (using
$redis.setnx(retrieved_key, "1") then $redis.expire(retrieved_key, 60)) is racy;
replace it with a single atomic Redis SET that applies NX and EX in one call
(use the Redis client SET with nx: true and ex: 60) so the key and TTL are
created atomically; adjust the conditional to check the SET return value (OK vs
nil) instead of setnx's truthiness and remove the separate expire call,
referencing retrieved_key, $redis.setnx and $redis.expire in the change.

In `@spec/controllers/embed_controller_spec.rb`:
- Around line 48-52: Create a concrete mock object for TopicView and make
TopicView.expects(:new).with(123, nil, {best: 5}) return that mock so the view
can call `@topic_view.posts` and `@topic_view.topic.url`; e.g., build a topic_view
mock, stub its posts to return an array (or an enumerable) and stub topic.url
(or topic to return a mock with url) before calling get :best so the template
can iterate and access nested attributes.

In `@spec/jobs/poll_feed_spec.rb`:
- Around line 12-28: The negative tests are not isolating the condition under
test because other preconditions may be falsy by default; update each `it` to
stub all other required SiteSetting values to valid values before calling
`poller.execute({})` so the test fails only when the targeted setting is
missing. Specifically, in the examples referencing `poller.execute`, ensure when
testing `feed_polling_url` you stub
`SiteSetting.stubs(:feed_polling_enabled?).returns(true)` and
`SiteSetting.stubs(:embed_by_username).returns("some_user")`; when testing
`embed_by_username` stub
`SiteSetting.stubs(:feed_polling_enabled?).returns(true)` and
`SiteSetting.stubs(:feed_polling_url).returns("http://example.com")`; and in the
`requires feed_polling_enabled?` case stub the other two settings to valid
values so each `poller.expects(:poll_feed).never` assertion only depends on the
single setting under test.