CVE-2026-109365 in Zebra
Summary
by MITRE • 10/12/2026
Zebra before 6.4.0 contains an uncontrolled resource consumption vulnerability that allows unauthenticated peers to force full proof verification by serving forged block bodies with valid header hashes. Attackers can flip authorizing-only bytes such as a coinbase scriptSig, wasting Sapling, Orchard, and Ironwood proof verification without penalty and stalling sync for 67 seconds.
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Analysis
by VulDB Data Team • 10/12/2026
The vulnerability identified in Zebra versions prior to 6.4.0 represents a critical uncontrolled resource consumption flaw within the blockchain synchronization subsystem. This issue specifically targets the consensus layer's handling of block headers and bodies, allowing an unauthenticated network peer to exploit the protocol's proof verification mechanisms. By serving forged block bodies that contain valid header hashes but maliciously altered content in specific fields such as the coinbase scriptSig, an attacker can trigger a disproportionate computational load on validating nodes. The core technical flaw lies in the lack of sufficient validation checks before initiating expensive cryptographic operations, particularly those related to zero-knowledge proof verification for Sapling, Orchard, and Ironwood consensus layers.
From a technical perspective, the attack vector relies on manipulating authorizing-only bytes within the block structure. These fields are typically used for transaction authorization data but do not affect the validity of the block header hash itself. Because Zebra prioritizes maintaining network synchronization speed, it proceeds to verify proofs associated with these blocks upon receipt if the header is valid. The vulnerability allows an adversary to flip specific bits in the coinbase scriptSig or similar fields, creating a state where the proof verification process must run through its full computational cycle without any early rejection criteria being met. This results in the node wasting significant CPU and memory resources on verifying invalid or malformed proofs that would otherwise be discarded if proper pre-validation checks were implemented.
The operational impact of this vulnerability is severe for network stability and node availability. When a node processes these forged blocks, it becomes stalled during the proof verification phase, which can last up to 67 seconds per block depending on system performance and load. During this window, the node cannot process new incoming transactions or synchronize with other peers effectively, leading to a denial of service condition for that specific validator. If multiple such requests are sent in rapid succession by an attacker controlling several network nodes, it can cause widespread synchronization delays across the network, potentially disrupting consensus formation and reducing overall network throughput. This type of resource exhaustion attack undermines the reliability and performance guarantees expected from robust blockchain infrastructure.
In terms of industry standards classification, this vulnerability aligns with CWE-400 Uncontrolled Resource Consumption, as the system fails to limit the amount of resources consumed by a single input or request sequence. It also relates to CWE-285 Improper Authorization, since unauthenticated peers are able to trigger expensive operations without proper access controls or rate limiting mechanisms in place. From an ATT&CK perspective for blockchain systems, this behavior mirrors techniques associated with resource exhaustion attacks where adversaries aim to degrade service availability by forcing computational bottlenecks within the consensus engine. The exploitation does not require authentication but relies on network-level interaction and protocol compliance manipulation rather than direct code execution or privilege escalation.
Mitigation strategies must focus on implementing stricter pre-validation checks before initiating proof verification processes. Developers should introduce early rejection criteria for block bodies that contain anomalies in authorizing-only fields, ensuring that malformed structures are discarded before expensive cryptographic operations begin. Additionally, implementing rate limiting and connection throttling mechanisms can help mitigate the impact of such attacks by restricting the number of blocks a single peer can submit within a given timeframe. Upgrading to Zebra version 6.4.0 or later is essential as it addresses these flaws through enhanced validation logic that prevents unauthenticated peers from forcing full proof verification on invalid data structures, thereby restoring normal synchronization performance and protecting node resources from malicious exhaustion attempts.