From 146709a2dddb78fd46f11ef8591b1cb59672440b Mon Sep 17 00:00:00 2001
From: Kim Alvefur <zash@zash.se>
Date: Thu, 23 Dec 2021 14:08:20 +0100
Subject: mod_tombstones: Remember deleted accounts #1307

Presence subscriptions are normally revoked on account deletion, which
informs the contact. Sometimes this notification gets lost e.g. due to
s2s problems. The accounts JID may also be present e.g. in MUC
affiliations, chat group member lists, pubsub subscriptions or other
systems. These may grant privileges which would fall to someone who
creates the same account again, which this module is meant to prevent.
---
 CHANGES                    |  1 +
 plugins/mod_tombstones.lua | 75 ++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 76 insertions(+)
 create mode 100644 plugins/mod_tombstones.lua

diff --git a/CHANGES b/CHANGES
index 4d1945db..f6d77f35 100644
--- a/CHANGES
+++ b/CHANGES
@@ -16,6 +16,7 @@ TRUNK
 -   mod_cron: One module to rule all the periodic tasks
 -   mod_admin_shell: New home of the Console admin interface
 -   mod_admin_socket: Enable secure connections to the Console
+-   mod_tombstones: Prevent registration of deleted accounts
 
 ### Security and authentication
 
diff --git a/plugins/mod_tombstones.lua b/plugins/mod_tombstones.lua
new file mode 100644
index 00000000..1707c9dd
--- /dev/null
+++ b/plugins/mod_tombstones.lua
@@ -0,0 +1,75 @@
+-- TODO warn when trying to create an user before the tombstone expires
+-- e.g. via telnet or other admin interface
+local datetime = require "util.datetime";
+local errors = require "util.error";
+local jid_split = require"util.jid".split;
+local st = require "util.stanza";
+
+-- Using a map store as key-value store so that removal of all user data
+-- does not also remove the tombstone, which would defeat the point
+local graveyard = module:open_store(nil, "map");
+
+local ttl = module:get_option_number("user_tombstone_expiry", nil);
+-- Keep tombstones forever by default
+--
+-- Rationale:
+-- There is no way to be completely sure when remote services have
+-- forgotten and revoked all memberships.
+
+module:hook_global("user-deleted", function(event)
+	if event.host == module.host then
+		local ok, err = graveyard:set(nil, event.username, os.time());
+		if not ok then module:log("error", "Could store tombstone for %s: %s", event.username, err); end
+	end
+end);
+
+-- Public API
+function has_tombstone(username)
+	local tombstone, err = graveyard:get(nil, username);
+
+	if err or not tombstone then return tombstone, err; end
+
+	if ttl and tombstone + ttl < os.time() then
+		module:log("debug", "Tombstone for %s created at %s has expired", username, datetime.datetime(tombstone));
+		graveyard:set(nil, username, nil);
+		return nil;
+	end
+	return tombstone;
+end
+
+module:hook("user-registering", function(event)
+	local tombstone, err = has_tombstone(event.username);
+
+	if err then
+		event.allowed, event.error = errors.coerce(false, err);
+		return true;
+	elseif not tombstone then
+		-- Feel free
+		return;
+	end
+
+	module:log("debug", "Tombstone for %s created at %s", event.username, datetime.datetime(tombstone));
+	event.allowed = false;
+	return true;
+end);
+
+module:hook("presence/bare", function(event)
+	local origin, presence = event.origin, event.stanza;
+
+	-- We want to undo any left-over presence subscriptions and notify the former
+	-- contact that they're gone.
+	--
+	-- FIXME This leaks that the user once existed. Hard to avoid without keeping
+	-- the contact list in some form, which we don't want to do for privacy
+	-- reasons.  Bloom filter perhaps?
+	if has_tombstone(jid_split(presence.attr.to)) then
+		if presence.attr.type == "probe" then
+			origin.send(st.error_reply(presence, "cancel", "gone", "User deleted"));
+			origin.send(st.presence({ type = "unsubscribed"; to = presence.attr.from; from = presence.attr.to }));
+		elseif presence.attr.type == nil or presence.attr.type == "unavailable" then
+			origin.send(st.error_reply(presence, "cancel", "gone", "User deleted"));
+			origin.send(st.presence({ type = "unsubscribe"; to = presence.attr.from; from = presence.attr.to }));
+		end
+		return true;
+	end
+end, 1);
-- 
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