ZEBRA: Permanent Block Discovery Halt via Gossip Queue Saturation and Syncer Poisoning

CISA KEV is observed exploitation evidence. FIRST EPSS is a dated prediction of exploitation in the next 30 days. Missing scores are unknown. This collector requests predictions only for explicit CVE aliases in the public NuGet archive; Microsoft release views retain their existing prediction scope.

No collected risk observation for these identifiers. This does not establish absence of exploitation or a zero probability.

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.

Source published

8 May 2026 15:11:24 UTC

Source modified

8 May 2026 17:23:27 UTC

Observed here

23 Oct 2026 15:48:27 UTC

Source terms

CVE Program Terms of Use

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: cve-program · CVE Program Terms of Use. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 4.0 · 8.7 CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N

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.

Source published

May 8, 2026, 3:11:24 PM (timezone not stated)

Source modified

May 8, 2026, 5:23:27 PM (timezone not stated)

Observed here

7 Nov 2026 10:31:10 UTC

Source terms

ENISA EUVD · source terms and attribution apply

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: euvd · ENISA EUVD · source terms and attribution apply. Source text is retained; field grouping and change labels are provided by ChangeIntel.

EUVD CVSS 4.0 · 8.7 CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N

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.

Source published

8 May 2026 16:16:13 UTC

Source modified

26 Oct 2026 18:10:00 UTC

Observed here

27 Oct 2026 11:12:17 UTC

Source terms

NIST NVD usage terms; underlying source attribution retained

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: nvd · NIST NVD usage terms; underlying source attribution retained. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 4.0 · [email protected] · Secondary · 8.7 CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

## 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 with empty `inv` to `FindBlocks` requests and with `NotFound` to block download requests. Both are valid protocol responses that carried zero misbehavior penalty. The attacker's connection was never banned and never disconnected, allowing the degradation to persist indefinitely. Combining these two vectors, an attacker could suppress both block discovery paths simultaneously from a single connection, causing the node to fall permanently behind the chain tip. ## Impact **Denial of Service** * **Attack Vector:** Network, unauthenticated. Requires only a single TCP peer connection. * **Effect:** Permanent halt of block discovery. The targeted node falls behind the chain tip and never recovers without operator intervention. * **Scope:** Any Zebra node reachable by the attacker over the peer-to-peer network. ## Fixed Versions This issue is fixed in Zebra 4.4.0. The fix drops connections that send empty responses to `FindBlocks` and `FindHeaders` messages, preventing attackers from degrading the syncer path without consequence. ## Mitigation Users should upgrade to Zebra 4.4.0 or later immediately. There are no known workarounds for this issue. Immediate upgrade is the only way to protect against this attack. ## Credits Zebra the researcher who reported this issue through the coordinated disclosure process.

Source published

8 May 2026 19:12:35 UTC

Source modified

8 May 2026 19:41:27 UTC

Observed here

24 Oct 2026 13:19:54 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · GitHub Advisory Database · CC-BY-4.0. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS_V4 · CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N

Publisher rating · HIGH