Zebra Coinbase scriptSig Rewrite Causes Silent Block Drop Without Peer Penalty

Vulnerability Overview Title: Coinbase scriptSig rewrite drops a gossiped block on the inbound path without penalizing the supplying peer Description: This vulnerability involves a security issue in the Zebra (Rust) software, specifically where a rewritten Coinbase scriptSig causes a gossiped block to be dropped on the inbound path, without penalizing the peer that supplied the block. Scope of Impact Affected Versions: All versions up to v6.3.0 (latest release). Fixed Version: 6.4.2. Severity: Low. CVSS 3.1: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L. CWE: CWE-354 (Improper Verification of Cryptographic Signature); Secondary CWE-807 (Reliance on Untrusted Inputs in a Security Decision). Reporter: craftsoldier. GHSA: GHSA-4f6v-mj46-9xg3. Remediation Fixed Version: 6.4.2. Remediation Measures: A similar issue on the sync path was fixed in v6.3.0. Additionally, parent header lookup and a behind-tip error carrying the advertiser's address were added to penalize honest nodes serving old blocks. The inbound path was not fully fixed but was partially mitigated via a different mechanism (validator error downcast). Proof of Concept (PoC) Code Additional Information Impact: Delays in receiving the latest known block occur because peers are selected to provide that hash. Since the supplying peer is not penalized, the attacker remains a preferred peer. For nodes serving mining backends, a delayed tip may cause the backend to work on an outdated parent block, wasting hash power. Severity: This finding involves unauthenticated, default-configuration behavior that reliably drops the latest block when the attacker is selected. It can be repeated at no cost, but the effect is temporary, self-healing with delay, involves no total loss of availability, no state corruption, and no consensus divergence. PoC: A runtime PoC confirmed against Zebra v6.3.0 built with on regtest. The PoC fetches a block unseen by the victim, rewrites the Coinbase scriptSig from the true height to height 1 (OP_1 = 0x01) while maintaining block exchange (using ZIP-244 attributes), connects to the victim as a P2P peer, advertises the block hash via , and provides the forged block upon the victim's request. Debug Logs: Debug logs confirming the vulnerability path were obtained from the victim. Peer Scoring: The peer's remained at 0 throughout the process. Ping/pong succeeded after the drop, the connection remained available, and the peer was confirmed not to be banned, allowing the attack to be repeated. Workarounds: Limited. Prioritize connecting with trusted peers to reduce exposure to junk download races. Mining operators can monitor the age of the parent block in published templates and alert if the tip stops progressing. This is not a substitute for the fix. References: Several reference links are provided, including ZIP-244, ZIP-225, CWE-354, CWE-807, related advisories, and the fix patch. Summary This vulnerability involves a security issue in the Zebra software where a rewritten Coinbase scriptSig causes a gossiped block to be dropped on the inbound path without penalizing the supplying peer. Affected versions are all versions up to v6.3.0; the fixed version is 6.4.2. Severity is Low, with a CVSS score of CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L. Remediation includes fixing similar issues on the sync path in v6.3.0 and adding parent header lookup along with a behind-tip error carrying the advertiser's address. PoC code and debug logs are provided. Workarounds are limited; it is recommended to prioritize connections with trusted peers.

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Zebra Coinbase scriptSig Rewrite Causes Silent Block Drop Without Peer Penalty

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