diff --git a/CHANGELOG.md b/CHANGELOG.md
index e1deb6eaac..b78f14626d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,23 @@
 
 All notable changes to this project will be documented in this file.
 
+## [4.3.18] - 2026-01-20
+
+### Security
+
+- Fix missing limits on various federated properties [GHSA-gg8q-rcg7-p79g](https://github.com/mastodon/mastodon/security/advisories/GHSA-gg8q-rcg7-p79g)
+- Fix remote user suspension bypass [GHSA-5h2f-wg8j-xqwp](https://github.com/mastodon/mastodon/security/advisories/GHSA-5h2f-wg8j-xqwp)
+- Fix missing length limits on some user-provided fields [GHSA-6x3w-9g92-gvf3](https://github.com/mastodon/mastodon/security/advisories/GHSA-6x3w-9g92-gvf3)
+- Fix missing access check for push notification settings update [GHSA-f3q8-7vw3-69v4](https://github.com/mastodon/mastodon/security/advisories/GHSA-f3q8-7vw3-69v4)
+
+### Fixed
+
+- Fix `FeedManager#filter_from_home` error when handling a reblog of a deleted status (#37486 by @ClearlyClaire)
+- Fix needlessly complicated SQL query in status batch removal (#37469 by @ClearlyClaire)
+- Fix `Vary` parsing in cache control enforcement (#37426 by @MegaManSec)
+- Fix thread-unsafe ActivityPub activity dispatch (#37423 by @MegaManSec)
+- Fix SignatureParser accepting duplicate parameters in HTTP Signature header (#37375 by @shleeable)
+
 ## [4.3.17] - 2026-01-07
 
 ### Security
diff --git a/FEDERATION.md b/FEDERATION.md
index 2819fa935a..c98f5924cf 100644
--- a/FEDERATION.md
+++ b/FEDERATION.md
@@ -47,3 +47,22 @@ Mastodon requires all `POST` requests to be signed, and MAY require `GET` reques
 ### Additional documentation
 
 - [Mastodon documentation](https://docs.joinmastodon.org/)
+
+## Size limits
+
+Mastodon imposes a few hard limits on federated content.
+These limits are intended to be very generous and way above what the Mastodon user experience is optimized for, so as to accomodate future changes and unusual or unforeseen usage patterns, while still providing some limits for performance reasons.
+The following table attempts to summary those limits.
+
+| Limited property                                              | Size limit | Consequence of exceeding the limit |
+| ------------------------------------------------------------- | ---------- | ---------------------------------- |
+| Serialized JSON-LD                                            | 1MB        | **Activity is rejected/dropped**   |
+| Profile fields (actor `PropertyValue` attachments) name/value | 2047       | Field name/value is truncated      |
+| Number of profile fields (actor `PropertyValue` attachments)  | 50         | Fields list is truncated           |
+| Poll options (number of `anyOf`/`oneOf` in a `Question`)      | 500        | Items list is truncated            |
+| Account username (actor `preferredUsername`) length           | 2048       | **Actor will be rejected**         |
+| Account display name (actor `name`) length                    | 2048       | Display name will be truncated     |
+| Account note (actor `summary`) length                         | 20kB       | Account note will be truncated     |
+| Account `attributionDomains`                                  | 256        | List will be truncated             |
+| Account aliases (actor `alsoKnownAs`)                         | 256        | List will be truncated             |
+| Custom emoji shortcode (`Emoji` `name`)                       | 2048       | Emoji will be rejected             |
diff --git a/app/controllers/activitypub/inboxes_controller.rb b/app/controllers/activitypub/inboxes_controller.rb
index 49cfc8ad1c..3d910b4e79 100644
--- a/app/controllers/activitypub/inboxes_controller.rb
+++ b/app/controllers/activitypub/inboxes_controller.rb
@@ -3,6 +3,7 @@
 class ActivityPub::InboxesController < ActivityPub::BaseController
   include JsonLdHelper
 
+  before_action :skip_large_payload
   before_action :skip_unknown_actor_activity
   before_action :require_actor_signature!
   skip_before_action :authenticate_user!
@@ -16,6 +17,10 @@ class ActivityPub::InboxesController < ActivityPub::BaseController
 
   private
 
+  def skip_large_payload
+    head 413 if request.content_length > ActivityPub::Activity::MAX_JSON_SIZE
+  end
+
   def skip_unknown_actor_activity
     head 202 if unknown_affected_account?
   end
diff --git a/app/controllers/api/web/push_subscriptions_controller.rb b/app/controllers/api/web/push_subscriptions_controller.rb
index 167d16fc4d..4dacfd6dbb 100644
--- a/app/controllers/api/web/push_subscriptions_controller.rb
+++ b/app/controllers/api/web/push_subscriptions_controller.rb
@@ -55,7 +55,7 @@ class Api::Web::PushSubscriptionsController < Api::Web::BaseController
   end
 
   def set_push_subscription
-    @push_subscription = ::Web::PushSubscription.find(params[:id])
+    @push_subscription = ::Web::PushSubscription.where(user_id: active_session.user_id).find(params[:id])
   end
 
   def subscription_params
diff --git a/app/lib/activitypub/activity.rb b/app/lib/activitypub/activity.rb
index f5b55fff6a..f86bd3211a 100644
--- a/app/lib/activitypub/activity.rb
+++ b/app/lib/activitypub/activity.rb
@@ -5,6 +5,7 @@ class ActivityPub::Activity
   include Redisable
   include Lockable
 
+  MAX_JSON_SIZE = 1.megabyte
   SUPPORTED_TYPES = %w(Note Question).freeze
   CONVERTED_TYPES = %w(Image Audio Video Article Page Event).freeze
 
diff --git a/app/lib/activitypub/activity/update.rb b/app/lib/activitypub/activity/update.rb
index 5185507bdc..3eac307d07 100644
--- a/app/lib/activitypub/activity/update.rb
+++ b/app/lib/activitypub/activity/update.rb
@@ -32,7 +32,8 @@ class ActivityPub::Activity::Update < ActivityPub::Activity
     @status = Status.find_by(uri: object_uri, account_id: @account.id)
 
     # Ignore updates for old unknown objects, since those are updates we are not interested in
-    return if @status.nil? && object_too_old?
+    # Also ignore unknown objects from suspended users for the same reasons
+    return if @status.nil? && (@account.suspended? || object_too_old?)
 
     # We may be getting `Create` and `Update` out of order
     @status ||= ActivityPub::Activity::Create.new(@json, @account, **@options).perform
diff --git a/app/lib/activitypub/parser/poll_parser.rb b/app/lib/activitypub/parser/poll_parser.rb
index 758c03f07e..d43eaf6cfb 100644
--- a/app/lib/activitypub/parser/poll_parser.rb
+++ b/app/lib/activitypub/parser/poll_parser.rb
@@ -3,6 +3,10 @@
 class ActivityPub::Parser::PollParser
   include JsonLdHelper
 
+  # Limit the number of items for performance purposes.
+  # We truncate rather than error out to avoid missing the post entirely.
+  MAX_ITEMS = 500
+
   def initialize(json)
     @json = json
   end
@@ -48,6 +52,6 @@ class ActivityPub::Parser::PollParser
   private
 
   def items
-    @json['anyOf'] || @json['oneOf']
+    (@json['anyOf'] || @json['oneOf'])&.take(MAX_ITEMS)
   end
 end
diff --git a/app/models/account.rb b/app/models/account.rb
index 893c2ea396..b5cf8f91f1 100644
--- a/app/models/account.rb
+++ b/app/models/account.rb
@@ -76,6 +76,13 @@ class Account < ApplicationRecord
   DISPLAY_NAME_LENGTH_LIMIT = (ENV['MAX_DISPLAY_NAME_CHARS'] || 30).to_i
   NOTE_LENGTH_LIMIT = (ENV['MAX_BIO_CHARS'] || 500).to_i
 
+  # Hard limits for federated content
+  USERNAME_LENGTH_HARD_LIMIT = 2048
+  DISPLAY_NAME_LENGTH_HARD_LIMIT = 2048
+  NOTE_LENGTH_HARD_LIMIT = 20.kilobytes
+  ATTRIBUTION_DOMAINS_HARD_LIMIT = 256
+  ALSO_KNOWN_AS_HARD_LIMIT = 256
+
   AUTOMATED_ACTOR_TYPES = %w(Application Service).freeze
 
   include Attachmentable # Load prior to Avatar & Header concerns
@@ -103,7 +110,7 @@ class Account < ApplicationRecord
   validates_with UniqueUsernameValidator, if: -> { will_save_change_to_username? }
 
   # Remote user validations, also applies to internal actors
-  validates :username, format: { with: USERNAME_ONLY_RE }, if: -> { (!local? || actor_type == 'Application') && will_save_change_to_username? }
+  validates :username, format: { with: USERNAME_ONLY_RE }, length: { maximum: USERNAME_LENGTH_HARD_LIMIT }, if: -> { (!local? || actor_type == 'Application') && will_save_change_to_username? }
 
   # Remote user validations
   validates :uri, presence: true, unless: :local?, on: :create
diff --git a/app/models/custom_emoji.rb b/app/models/custom_emoji.rb
index df4696c9ce..ce02cb9e45 100644
--- a/app/models/custom_emoji.rb
+++ b/app/models/custom_emoji.rb
@@ -27,6 +27,8 @@ class CustomEmoji < ApplicationRecord
   LOCAL_LIMIT = (ENV['MAX_EMOJI_SIZE'] || 256.kilobytes).to_i
   LIMIT       = [LOCAL_LIMIT, (ENV['MAX_REMOTE_EMOJI_SIZE'] || 256.kilobytes).to_i].max
   MINIMUM_SHORTCODE_SIZE = 2
+  MAX_SHORTCODE_SIZE = 128
+  MAX_FEDERATED_SHORTCODE_SIZE = 2048
 
   SHORTCODE_RE_FRAGMENT = '[a-zA-Z0-9_]{2,}'
 
@@ -48,7 +50,8 @@ class CustomEmoji < ApplicationRecord
   validates_attachment :image, content_type: { content_type: IMAGE_MIME_TYPES }, presence: true
   validates_attachment_size :image, less_than: LIMIT, unless: :local?
   validates_attachment_size :image, less_than: LOCAL_LIMIT, if: :local?
-  validates :shortcode, uniqueness: { scope: :domain }, format: { with: SHORTCODE_ONLY_RE }, length: { minimum: MINIMUM_SHORTCODE_SIZE }
+  validates :shortcode, uniqueness: { scope: :domain }, format: { with: SHORTCODE_ONLY_RE }, length: { minimum: MINIMUM_SHORTCODE_SIZE, maximum: MAX_FEDERATED_SHORTCODE_SIZE }
+  validates :shortcode, length: { maximum: MAX_SHORTCODE_SIZE }, if: :local?
 
   scope :local, -> { where(domain: nil) }
   scope :remote, -> { where.not(domain: nil) }
diff --git a/app/models/custom_filter.rb b/app/models/custom_filter.rb
index bacf158261..10925ffd95 100644
--- a/app/models/custom_filter.rb
+++ b/app/models/custom_filter.rb
@@ -30,6 +30,8 @@ class CustomFilter < ApplicationRecord
 
   EXPIRATION_DURATIONS = [30.minutes, 1.hour, 6.hours, 12.hours, 1.day, 1.week].freeze
 
+  TITLE_LENGTH_LIMIT = 256
+
   include Expireable
   include Redisable
 
@@ -41,6 +43,7 @@ class CustomFilter < ApplicationRecord
   accepts_nested_attributes_for :keywords, reject_if: :all_blank, allow_destroy: true
 
   validates :title, :context, presence: true
+  validates :title, length: { maximum: TITLE_LENGTH_LIMIT }
   validate :context_must_be_valid
 
   normalizes :context, with: ->(context) { context.map(&:strip).filter_map(&:presence) }
diff --git a/app/models/custom_filter_keyword.rb b/app/models/custom_filter_keyword.rb
index 112798b10a..1abec4ddc4 100644
--- a/app/models/custom_filter_keyword.rb
+++ b/app/models/custom_filter_keyword.rb
@@ -17,7 +17,9 @@ class CustomFilterKeyword < ApplicationRecord
 
   belongs_to :custom_filter
 
-  validates :keyword, presence: true
+  KEYWORD_LENGTH_LIMIT = 512
+
+  validates :keyword, presence: true, length: { maximum: KEYWORD_LENGTH_LIMIT }
 
   alias_attribute :phrase, :keyword
 
diff --git a/app/models/list.rb b/app/models/list.rb
index cd01774539..d5dc04b395 100644
--- a/app/models/list.rb
+++ b/app/models/list.rb
@@ -17,6 +17,7 @@ class List < ApplicationRecord
   include Paginable
 
   PER_ACCOUNT_LIMIT = 50
+  TITLE_LENGTH_LIMIT = 256
 
   enum :replies_policy, { list: 0, followed: 1, none: 2 }, prefix: :show
 
@@ -26,7 +27,7 @@ class List < ApplicationRecord
   has_many :accounts, through: :list_accounts
   has_many :active_accounts, -> { merge(ListAccount.active) }, through: :list_accounts, source: :account
 
-  validates :title, presence: true
+  validates :title, presence: true, length: { maximum: TITLE_LENGTH_LIMIT }
 
   validate :validate_account_lists_limit, on: :create
 
diff --git a/app/services/activitypub/process_account_service.rb b/app/services/activitypub/process_account_service.rb
index e516582d43..0d7f58a15a 100644
--- a/app/services/activitypub/process_account_service.rb
+++ b/app/services/activitypub/process_account_service.rb
@@ -6,6 +6,7 @@ class ActivityPub::ProcessAccountService < BaseService
   include Redisable
   include Lockable
 
+  MAX_PROFILE_FIELDS = 50
   SUBDOMAINS_RATELIMIT = 10
   DISCOVERIES_PER_REQUEST = 400
 
@@ -122,15 +123,15 @@ class ActivityPub::ProcessAccountService < BaseService
 
   def set_immediate_attributes!
     @account.featured_collection_url = valid_collection_uri(@json['featured'])
-    @account.display_name            = @json['name'] || ''
-    @account.note                    = @json['summary'] || ''
+    @account.display_name            = (@json['name'] || '')[0...(Account::DISPLAY_NAME_LENGTH_HARD_LIMIT)]
+    @account.note                    = (@json['summary'] || '')[0...(Account::NOTE_LENGTH_HARD_LIMIT)]
     @account.locked                  = @json['manuallyApprovesFollowers'] || false
     @account.fields                  = property_values || {}
-    @account.also_known_as           = as_array(@json['alsoKnownAs'] || []).map { |item| value_or_id(item) }
+    @account.also_known_as           = as_array(@json['alsoKnownAs'] || []).take(Account::ALSO_KNOWN_AS_HARD_LIMIT).map { |item| value_or_id(item) }
     @account.discoverable            = @json['discoverable'] || false
     @account.indexable               = @json['indexable'] || false
     @account.memorial                = @json['memorial'] || false
-    @account.attribution_domains     = as_array(@json['attributionDomains'] || []).map { |item| value_or_id(item) }
+    @account.attribution_domains     = as_array(@json['attributionDomains'] || []).take(Account::ATTRIBUTION_DOMAINS_HARD_LIMIT).map { |item| value_or_id(item) }
   end
 
   def set_fetchable_key!
@@ -251,7 +252,10 @@ class ActivityPub::ProcessAccountService < BaseService
   def property_values
     return unless @json['attachment'].is_a?(Array)
 
-    as_array(@json['attachment']).select { |attachment| attachment['type'] == 'PropertyValue' }.map { |attachment| attachment.slice('name', 'value') }
+    as_array(@json['attachment'])
+      .select { |attachment| attachment['type'] == 'PropertyValue' }
+      .take(MAX_PROFILE_FIELDS)
+      .map { |attachment| attachment.slice('name', 'value') }
   end
 
   def mismatching_origin?(url)
diff --git a/app/services/fan_out_on_write_service.rb b/app/services/fan_out_on_write_service.rb
index 3ad57cbea3..2ee08bedfc 100644
--- a/app/services/fan_out_on_write_service.rb
+++ b/app/services/fan_out_on_write_service.rb
@@ -14,6 +14,8 @@ class FanOutOnWriteService < BaseService
     @account   = status.account
     @options   = options
 
+    return if @status.proper.account.suspended?
+
     check_race_condition!
     warm_payload_cache!
 
diff --git a/docker-compose.yml b/docker-compose.yml
index d8460fae6f..6f6358180a 100644
--- a/docker-compose.yml
+++ b/docker-compose.yml
@@ -59,7 +59,7 @@ services:
   web:
     # You can uncomment the following line if you want to not use the prebuilt image, for example if you have local code changes
     # build: .
-    image: ghcr.io/glitch-soc/mastodon:v4.3.17
+    image: ghcr.io/glitch-soc/mastodon:v4.3.18
     restart: always
     env_file: .env.production
     command: bundle exec puma -C config/puma.rb
@@ -83,7 +83,7 @@ services:
     # build:
     #   dockerfile: ./streaming/Dockerfile
     #   context: .
-    image: ghcr.io/glitch-soc/mastodon-streaming:v4.3.17
+    image: ghcr.io/glitch-soc/mastodon-streaming:v4.3.18
     restart: always
     env_file: .env.production
     command: node ./streaming/index.js
@@ -102,7 +102,7 @@ services:
   sidekiq:
     # You can uncomment the following line if you want to not use the prebuilt image, for example if you have local code changes
     # build: .
-    image: ghcr.io/glitch-soc/mastodon:v4.3.17
+    image: ghcr.io/glitch-soc/mastodon:v4.3.18
     restart: always
     env_file: .env.production
     command: bundle exec sidekiq
diff --git a/lib/mastodon/version.rb b/lib/mastodon/version.rb
index bd10ef1ad1..eccf7b1b9b 100644
--- a/lib/mastodon/version.rb
+++ b/lib/mastodon/version.rb
@@ -13,7 +13,7 @@ module Mastodon
     end
 
     def patch
-      17
+      18
     end
 
     def default_prerelease