CVE-2026-104426 in Zebra
Summary
by MITRE • 10/02/2026
Zebra before 6.1.0 contains an inefficient algorithmic complexity vulnerability in remaining_transaction_value that clones the entire block-level spent-UTXO map per transaction during contextual verification. Attackers can mine or seed the mempool with roughly 26,000 minimal single-input transactions in one block, stalling every validating node for over 52 seconds.
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Analysis
by VulDB Data Team • 10/02/2026
The vulnerability identified in Zebra versions prior to 6.1.0 represents a critical inefficiency in algorithmic complexity within the blockchain validation logic, specifically affecting the remaining_transaction_value function during contextual verification. This flaw stems from an architectural decision where the system clones the entire block-level spent-UTXO map for every single transaction processed. In standard blockchain operations, verifying that inputs have not been previously spent requires checking against a set of already consumed outputs. However, by creating a full copy of this state for each individual transaction rather than maintaining a shared or incremental reference, the computational overhead scales quadratically relative to the number of transactions in a block. This design choice transforms what should be an efficient lookup operation into a resource-intensive cloning process that consumes significant CPU cycles and memory bandwidth with every new transaction added to the mempool or validated within a block.
The operational impact of this inefficiency is severe, as it allows for a denial-of-service attack vector through computational exhaustion. An adversary can exploit this by mining or seeding the mempool with approximately 26,000 minimal single-input transactions bundled into a single block. Because each transaction triggers a full clone of the spent-UTXO map, the cumulative processing time escalates dramatically as more such transactions are included in the same block structure. This specific payload has been demonstrated to stall every validating node for over 52 seconds during verification. In a decentralized network where timely block propagation and validation are essential for consensus and security, such delays can disrupt synchronization, potentially leading to chain splits or reduced network throughput, thereby degrading the overall reliability and performance of the Zebra implementation.
From a classification perspective, this vulnerability aligns with CWE-400, which describes Uncontrolled Resource Consumption due to inefficient algorithmic complexity. The attack mechanism leverages the system's own validation logic against it, forcing nodes to perform excessive work for relatively small data payloads. This behavior is consistent with ATT&CK technique T1496, Resource Hijacking, where an attacker uses resources in a way that degrades performance or causes denial of service without necessarily crashing the application outright but rather by inducing significant latency and resource exhaustion. The specific context involves manipulating the mempool to force nodes into a high-latency state during block validation, which is a sophisticated form of protocol-level abuse targeting consensus mechanisms.
To mitigate this vulnerability, it is imperative for Zebra users to upgrade immediately to version 6.1.0 or later, where the underlying algorithmic complexity has been addressed and the inefficient cloning behavior corrected. For network operators unable to patch instantly, implementing strict mempool filtering policies can help reduce exposure. This includes limiting the number of small transactions allowed per block or adjusting validation timeouts to prevent nodes from hanging indefinitely on maliciously constructed blocks. Additionally, monitoring node performance metrics for unusual spikes in CPU usage during contextual verification can serve as an early warning system for such attacks. Ensuring that validator nodes are configured with appropriate resource limits and rate-limiting rules will further harden the network against this specific class of algorithmic complexity exploits until a full patch is deployed across all participating nodes.