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appservice: stop sending plaintext into encrypted rooms

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Without /sync nothing backfills the state store, so a room the bot had
not yet heard from looked unencrypted -- IsEncrypted answers false with
no error -- and any message the bot started on its own (ambient DMs,
expedition beats, briefings) went out in the clear. Under /sync the
cryptohelper registered StateStoreSyncHandler and the initial sync
carried every joined room's state, so the store was warm before the bot
ever spoke; the appservice port replicated the crypto plumbing but not
that backfill.

Resolve encryption and membership from the server on first read instead,
and refuse the send when they cannot be resolved: a message that fails is
recoverable, a plaintext one that has landed is not. Members are resolved
the same way, since an unfetched room shares the group session with
nobody.
This commit is contained in:
prosolis
2026-07-12 17:27:28 -07:00
parent da398cf674
commit 2482433896
3 changed files with 322 additions and 25 deletions
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33
internal/bot/appservice.go
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@@ -7,7 +7,6 @@ import (
"log/slog"
"os"
"path/filepath"
"sync"
"time"
"github.com/rs/zerolog"
@@ -107,7 +106,15 @@ func newAppserviceSession(cfg Config) (*Session, error) {
return nil, fmt.Errorf("registration sender_localpart resolves to %s but BOT_USER_ID is %s", as.BotMXID(), userID)
}
// Resolve room state (is it encrypted, who is in it) from the server on first
// use, since there is no /sync to backfill it. Must be installed before the
// first BotClient() call: makeClient copies as.StateStore into the client, and
// caches the client.
store := newLazyStateStore(as.StateStore)
as.StateStore = store
client := as.BotClient() // as_token auth + SetAppServiceUserID (?user_id=) assertion
store.client = client
// Assert the device via ?org.matrix.msc3202.device_id= on E2EE requests, and
// satisfy cryptohelper.Init's syncer check: with this set, Init permits a nil
// Syncer (we drive the crypto machine from transactions, not /sync). The param
@@ -172,29 +179,6 @@ func newAppserviceSession(cfg Config) (*Session, error) {
mach.HandleMemberEvent(ctx, evt)
})
// Without /sync there is no state backfill, so the client's StateStore starts
// empty. Before we hand off a decrypted event (whose reply may need to be
// encrypted), resolve the room once: mark it encrypted and populate its member
// list. Otherwise outbound sends go plaintext (IsEncrypted=false) and, worse,
// the group session is shared to nobody (GetRoomJoinedOrInvitedMembers empty)
// so recipients can't decrypt the bot's replies.
var resolved sync.Map // roomID -> struct{}, resolved once
resolveRoom := func(ctx context.Context, roomID id.RoomID) {
if _, done := resolved.LoadOrStore(roomID, struct{}{}); done {
return
}
var enc event.EncryptionEventContent
if err := client.StateEvent(ctx, roomID, event.StateEncryption, "", &enc); err == nil && enc.Algorithm != "" {
_ = as.StateStore.SetEncryptionEvent(ctx, roomID, &enc)
}
if members, err := client.Members(ctx, roomID); err == nil {
_ = as.StateStore.ReplaceCachedMembers(ctx, roomID, members.Chunk)
} else {
slog.Warn("appservice: failed to fetch room members; will retry", "room", roomID, "err", err)
resolved.Delete(roomID) // allow a later event to retry
}
}
// recoverSession handles an event whose megolm session we don't have yet. The
// keys are often merely in flight (a to-device m.room_key racing the room
// event), so wait briefly; if they never land, ask the sender to re-share and
@@ -236,7 +220,6 @@ func newAppserviceSession(cfg Config) (*Session, error) {
// Decrypt inbound encrypted room events and re-dispatch the plaintext so the
// normal message/reaction handlers fire (mirrors cryptohelper.HandleEncrypted).
ep.On(event.EventEncrypted, func(ctx context.Context, evt *event.Event) {
resolveRoom(ctx, evt.RoomID)
decrypted, err := ch.Decrypt(ctx, evt)
if err != nil {
if errors.Is(err, cryptohelper.NoSessionFound) {