ZEBRA is a Zcash node written entirely in Rust. Prior to version 4.4.0, a composite denial-of-service vulnerability in Zebra's block discovery pipeline allows an unauthenticated remote attacker to permanently halt all new block discovery on a targeted node. The attack exploits three independent weaknesses in the gossip, syncer, and download subsystems — all exercisable from a single TCP connection — to create a monotonically growing block deficit that never self-heals. This issue has been patched in version 4.4.0.
Affected: crates.io: zebrad
## Summary
A composite denial-of-service vulnerability in Zebra's block discovery pipeline allows an unauthenticated remote attacker to permanently halt all new block discovery on a targeted node. The attack exploits three independent weaknesses in the gossip, syncer, and download subsystems — all exercisable from a single TCP connection — to create a monotonically growing block deficit that never self-heals.
## Severity
**Critical** — This is a Denial of Service vulnerability that requires no authentication, no special privileges, and only a single peer connection. The halt is permanent: the node will never recover without operator intervention.
## Affected Versions
All Zebra versions prior to 4.4.0.
## Description
Zebra discovers new blocks through two complementary paths: a gossip path (peers announce blocks via `inv` messages, triggering individual block downloads) and a syncer path (Zebra periodically queries peers with `FindBlocks`/`FindHeaders` to discover chains of missing blocks). Both paths must function for normal operation.
The gossip path was vulnerable because there was no per-connection rate limit on `inv` messages. A single connection could send enough sequential `inv` messages with fake block hashes to fill the entire gossip download queue in under a millisecond. The `FullQueue` return value was silently ignored, so legitimate block announcements from honest peers were dropped with no warning.
The syncer backup path could be degraded by responding w…