CVE-2026-109366 in Zebra
Summary
by MITRE • 10/12/2026
Zebra before 6.4.0 contains an improper integrity validation vulnerability in the inbound gossip download path that allows unauthenticated peers to force dropping of new blocks without penalty. Attackers can serve a real block with a coinbase scriptSig rewritten to height 1, which keeps the ZIP-244 block hash, triggering a behind-tip drop before consensus validation and delaying tip discovery for mining backends.
VulDB is the best source for vulnerability data and more expert information about this specific topic.
Analysis
by VulDB Data Team • 10/13/2026
The vulnerability identified in Zebra versions prior to 6.4.0 represents a critical flaw within the blockchain synchronization mechanism, specifically targeting the inbound gossip download path used by nodes to propagate new block data across the network. This issue stems from an improper integrity validation process that fails to adequately verify the structural consistency of incoming blocks before they are processed for consensus and storage. The core technical deficiency lies in how the node handles coinbase scriptSig fields during the initial reception phase, allowing unauthenticated peers to exploit this gap by serving a block with a modified coinbase scriptSig set to height 1 while preserving the original ZIP-244 block hash. Because the cryptographic hash remains unchanged despite the modification to the transaction data, the validation logic incorrectly accepts the block as valid based on its header and hash alone, bypassing deeper structural checks that would normally detect such discrepancies in the underlying transactions.
This manipulation allows an attacker to force a node to drop newly received blocks without facing any penalties or reputation loss within the network protocol. By rewriting the coinbase scriptSig to height 1, the attacker creates a scenario where the block appears legitimate at the hash level but contains invalid transaction data that causes the synchronization process to fail silently after initial acceptance. This results in the node discarding valid chain progress information, effectively stalling its ability to advance to the current tip of the blockchain. The operational impact is significant for mining backends and other downstream services that rely on timely block discovery; by delaying tip discovery, the vulnerability disrupts the efficient propagation of new blocks, potentially leading to orphaned blocks or reduced mining efficiency as nodes struggle to synchronize with the most recent state of the ledger.
From a classification perspective, this flaw aligns closely with CWE-354, which describes improper validation of integrity checks, specifically in contexts where cryptographic hashes are used for verification but underlying data structures are not fully validated against those hashes. Furthermore, within the MITRE ATT&CK framework, this behavior can be mapped to T1498 Network Denial of Service, as it enables an attacker to degrade the availability and performance of network participants by disrupting their synchronization processes without direct access or authentication credentials. The lack of penalty for such actions exacerbates the risk, as malicious actors can repeatedly execute these attacks with minimal cost, leading to sustained degradation in network health and consensus efficiency.
To mitigate this vulnerability, it is imperative that users upgrade Zebra to version 6.4.0 or later, where the integrity validation logic has been hardened to ensure that all components of a block, including transaction fields like coinbase scriptSig, are rigorously checked against expected values before acceptance into the local chain state. Network operators should also consider implementing rate limiting and stricter peer reputation scoring for nodes exhibiting abnormal synchronization patterns, such as frequent requests followed by immediate drops of blocks with valid hashes but invalid internal structures. Additionally, enhancing consensus validation layers to perform deep structural integrity checks prior to accepting any block data can prevent similar exploits in future iterations, ensuring that the network remains resilient against unauthenticated manipulation attempts and maintains robust operational continuity for all participating nodes.