Zcash Zebra EUVD-2026-91433

Exploitation requires the victim zebrad node (before 6.3.0) to be actively downloading blocks via the sync path - i.e. during initial sync or while catching up to the tip - and to have the attacker present as a serving peer in its peer set; no authentication, user interaction, or non-default configuration is needed. The specific trigger is a peer returning a canonical block whose coinbase scriptSig is rewritten to claim height 1 while keeping the requested hash, abusing that V5 transaction IDs exclude the scriptSig. Limiting factors: the attacker must be an accepted block-supplying peer, and the impact is confined to delayed discovery of the newest block (no code execution, no data compromise, no persistent crash), bounding real-world severity below the raw CVSS score.

This is a denial-of-service/sync-integrity flaw in the Zebra (zebrad) Zcash node, scored CVSS 4.0 8.7 (High) with vector AV:N/AC:L/AT:N/PR:N/UI:N and impact confined to availability (VA:H; VC:N/VI:N), consistent with CWE-345 (insufficient verification of data authenticity). The network/no-auth/no-UI/low-complexity vector means any connected peer can trigger it, which raises the paper severity, but the real-world impact is bounded: the attacker only delays the victim node's discovery of the newest block by replaying a canonical block whose coinbase scriptSig falsely claims height 1 (V5 txids exclude the scriptSig, so the hash still matches). This is not RCE, data theft, or a hard crash - it degrades sync timeliness. No EPSS score, no CISA KEV listing, and no SSVC decision data were provided, and no public exploit is referenced, so there is no public exploit identified at time of analysis. Net: a legitimate correctness/robustness bug worth patching for node operators, but not a drop-everything emergency - the VA:H rating appears generous for what is effectively a sync-delay condition rather than a full availability outage (verify impact severity with the vendor advisory).

A malicious peer in the victim zebrad node's peer set responds to block-download requests with a canonical block whose coinbase scriptSig has been rewritten to claim height 1, while preserving the requested block hash. Because Zebra reads the height from the unvalidated scriptSig and drops blocks that appear far behind the tip before consensus validation - without penalizing the peer - the attacker can repeatedly stall the node's progress toward the chain tip. No public exploit is identified at time of analysis.