diff --git a/CHANGELOG.md b/CHANGELOG.md
index 8c5ec67d85..39e975479e 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,33 @@
 
 All notable changes to this project will be documented in this file.
 
+## [4.5.5] - 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)
+
+### Changed
+
+- Skip tombstone creation on deleting from 404 (#37533 by @ClearlyClaire)
+
+### Fixed
+
+- Fix potential duplicate handling of quote accept/reject/delete (#37537 by @ClearlyClaire)
+- 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 `quote_approval_policy` being reset to user defaults when omitted in status update (#37436 and #37474 by @mjankowski and @shleeable)
+- Fix `Vary` parsing in cache control enforcement (#37426 by @MegaManSec)
+- Fix missing URI scheme test in `QuoteRequest` handling (#37425 by @MegaManSec)
+- Fix thread-unsafe ActivityPub activity dispatch (#37423 by @MegaManSec)
+- Fix URI generation for reblogs by accounts with numerical ActivityPub identifiers (#37415 by @oneiros)
+- Fix SignatureParser accepting duplicate parameters in HTTP Signature header (#37375 by @shleeable)
+- Fix emoji with variant selector not being rendered properly (#37320 by @ChaosExAnima)
+- Fix mobile admin sidebar displaying under batch table toolbar (#37307 by @diondiondion)
+
 ## [4.5.4] - 2026-01-07
 
 ### Security
diff --git a/FEDERATION.md b/FEDERATION.md
index 03ea5449de..eb91d9545f 100644
--- a/FEDERATION.md
+++ b/FEDERATION.md
@@ -48,3 +48,22 @@ 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 ced68d39fc..2edd92dbc7 100644
--- a/app/controllers/api/web/push_subscriptions_controller.rb
+++ b/app/controllers/api/web/push_subscriptions_controller.rb
@@ -62,7 +62,7 @@ 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 d07d1c2f24..eab345ce45 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/accept.rb b/app/lib/activitypub/activity/accept.rb
index 144ba9645c..92a8190c03 100644
--- a/app/lib/activitypub/activity/accept.rb
+++ b/app/lib/activitypub/activity/accept.rb
@@ -46,7 +46,7 @@ class ActivityPub::Activity::Accept < ActivityPub::Activity
 
   def accept_quote!(quote)
     approval_uri = value_or_id(first_of_value(@json['result']))
-    return if unsupported_uri_scheme?(approval_uri) || quote.quoted_account != @account || !quote.status.local?
+    return if unsupported_uri_scheme?(approval_uri) || quote.quoted_account != @account || !quote.status.local? || !quote.pending?
 
     # NOTE: we are not going through `ActivityPub::VerifyQuoteService` as the `Accept` is as authoritative
     # as the stamp, but this means we are not checking the stamp, which may lead to inconsistencies
diff --git a/app/lib/activitypub/activity/delete.rb b/app/lib/activitypub/activity/delete.rb
index 3e77f9b955..f606d9520f 100644
--- a/app/lib/activitypub/activity/delete.rb
+++ b/app/lib/activitypub/activity/delete.rb
@@ -56,7 +56,7 @@ class ActivityPub::Activity::Delete < ActivityPub::Activity
   end
 
   def revoke_quote
-    @quote = Quote.find_by(approval_uri: object_uri, quoted_account: @account)
+    @quote = Quote.find_by(approval_uri: object_uri, quoted_account: @account, state: [:pending, :accepted])
     return if @quote.nil?
 
     ActivityPub::Forwarder.new(@account, @json, @quote.status).forward! if @quote.status.present?
diff --git a/app/lib/activitypub/activity/update.rb b/app/lib/activitypub/activity/update.rb
index d94f876761..e22bea2c64 100644
--- a/app/lib/activitypub/activity/update.rb
+++ b/app/lib/activitypub/activity/update.rb
@@ -30,7 +30,8 @@ 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 eb0faa9e64..1c29263d37 100644
--- a/app/models/account.rb
+++ b/app/models/account.rb
@@ -80,6 +80,13 @@ 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
@@ -112,7 +119,7 @@ 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: -> { (remote? || actor_type_application?) && will_save_change_to_username? }
+  validates :username, format: { with: USERNAME_ONLY_RE }, length: { maximum: USERNAME_LENGTH_HARD_LIMIT }, if: -> { (remote? || 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 206bb80be4..5394f9322f 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 07bbfd4373..1151c7de98 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 8fd1953ab3..49ead642ac 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, validate: true
 
@@ -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/models/quote.rb b/app/models/quote.rb
index e81d427089..4ad393e3a5 100644
--- a/app/models/quote.rb
+++ b/app/models/quote.rb
@@ -51,9 +51,9 @@ class Quote < ApplicationRecord
 
   def reject!
     if accepted?
-      update!(state: :revoked)
+      update!(state: :revoked, approval_uri: nil)
     elsif !revoked?
-      update!(state: :rejected)
+      update!(state: :rejected, approval_uri: nil)
     end
   end
 
diff --git a/app/services/activitypub/fetch_remote_status_service.rb b/app/services/activitypub/fetch_remote_status_service.rb
index 0473bb5939..e08f82f7d9 100644
--- a/app/services/activitypub/fetch_remote_status_service.rb
+++ b/app/services/activitypub/fetch_remote_status_service.rb
@@ -92,7 +92,6 @@ class ActivityPub::FetchRemoteStatusService < BaseService
       existing_status = Status.remote.find_by(uri: uri)
       if existing_status&.distributable?
         Rails.logger.debug { "FetchRemoteStatusService - Got 404 for orphaned status with URI #{uri}, deleting" }
-        Tombstone.find_or_create_by(uri: uri, account: existing_status.account)
         RemoveStatusService.new.call(existing_status, redraft: false)
       end
     end
diff --git a/app/services/activitypub/process_account_service.rb b/app/services/activitypub/process_account_service.rb
index eb67daf7e8..be71b0b645 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
 
@@ -123,15 +124,15 @@ 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!
@@ -252,7 +253,10 @@ 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 46b9245be5..1c469f3763 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 3615b745f1..bcda267f57 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:v4.5.4
+    image: ghcr.io/glitch-soc/mastodon:v4.5.5
     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:v4.5.4
+    image: ghcr.io/glitch-soc/mastodon-streaming:v4.5.5
     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:v4.5.4
+    image: ghcr.io/glitch-soc/mastodon:v4.5.5
     restart: always
     env_file: .env.production
     command: bundle exec sidekiq
diff --git a/lib/mastodon/version.rb b/lib/mastodon/version.rb
index c00efc1ef2..8fa37f2258 100644
--- a/lib/mastodon/version.rb
+++ b/lib/mastodon/version.rb
@@ -13,7 +13,7 @@ module Mastodon
     end
 
     def patch
-      4
+      5
     end
 
     def default_prerelease
diff --git a/spec/requests/api/web/push_subscriptions_spec.rb b/spec/requests/api/web/push_subscriptions_spec.rb
index 21830d1b1c..88c0302f86 100644
--- a/spec/requests/api/web/push_subscriptions_spec.rb
+++ b/spec/requests/api/web/push_subscriptions_spec.rb
@@ -163,9 +163,10 @@ RSpec.describe 'API Web Push Subscriptions' do
   end
 
   describe 'PUT /api/web/push_subscriptions/:id' do
-    before { sign_in Fabricate :user }
+    before { sign_in user }
 
-    let(:subscription) { Fabricate :web_push_subscription }
+    let(:user) { Fabricate(:user) }
+    let(:subscription) { Fabricate(:web_push_subscription, user: user) }
 
     it 'gracefully handles invalid nested params' do
       put api_web_push_subscription_path(subscription), params: { data: 'invalid' }