zebrad has unbounded memory leak in mempool download pipeline via timeout path cancel_handles retention

Am I affected

You are affected if:

1. You run zebrad up to and including v4.4.1.
2. Your node accepts inbound P2P connections (network.listen_addr is set, which is the default).
3. Your node's mempool is active (node is synced near the chain tip).

All default configurations are affected.

Summary

The mempool download pipeline's cancel_handles map retains entries for transactions whose verification times out at the outer RATE_LIMIT_DELAY (73-second) boundary. The tokio::time::error::Elapsed error carries no payload, so the transaction ID is unrecoverable and the corresponding cancel_handles entry (including the full Gossip::Tx(UnminedTx), up to ~2 MB) is never removed. Entries accumulate monotonically with no upper bound or garbage collection, leading to eventual out-of-memory process termination.

Details

Downloads::poll_next() at zebrad/src/components/mempool/downloads.rs:215-228 handles three terminal states for a verification task:

• Ok(Ok(...)): success. Calls cancel_handles.remove(&tx.transaction.id). Correct.
• Ok(Err(...)): verification error. Calls cancel_handles.remove(&hash). Correct.
• Err(elapsed): outer timeout. Returns Err(elapsed) without removing anything. Bug.

tokio::time::error::Elapsed has no payload, so the timed-out transaction's UnminedTxId is unrecoverable from the error. The consumer at zebrad/src/components/mempool.rs:663-672 explicitly acknowledges this gap with a TODO comment.

The only cleanup paths for cancel_handles are cancel(mined_ids) (removes entries matching mined transaction IDs; attacker transactions are never mined) and cancel_all() (clears everything on shutdown or chain reset). No periodic GC, no time-based eviction, and no count cap exists.

For direct tx pushes (Gossip::Tx), the retained entry holds the full deserialized transaction, which can be up to ~9 MB in memory for a transaction near the transparent-output extreme. Per-connection leak rate at worst case: ~685 KB/s (~2.4 GB/hour).

Patches

The fix preserves the UnminedTxId through the timeout error path: wrap the timeout future so the spawned task's outer error carries the txid (e.g., Err((txid, elapsed))). In Downloads::poll_next(), on the timeout arm, call cancel_handles.remove(&txid).

Workarounds

There is no configuration-level workaround. Restarting the node clears the accumulated entries. Operators running in memory-constrained environments (containers with cgroup limits) may see the process killed by the OOM killer before natural recovery.

Impact

Gradual, unbounded memory exhaustion of a Zebra node from unauthenticated P2P traffic. The leak is monotonic (entries are never freed under normal operation) but slow (~685 KB/s per connection worst case). An attacker must sustain traffic for hours to exhaust typical server memory. The node continues operating normally until memory pressure becomes critical, at which point the OS OOM killer terminates the process or the node degrades due to swap pressure. No consensus impact, no fund loss, no on-disk corruption.

Credit

Reported by @AnticsDecoded via a private GitHub Security Advisory submission. Working E2E reproduction on a live regtest node with staged parent/child transaction dependencies.