Memory-Exhaustion Attack on the Blocklace Byzantine-Repelling Conflict-Free Replicated Data Type
Source: arXiv:2607.15185 · Published 2026-07-16 · By Erick Lavoie
TL;DR
This paper analyzes a memory exhaustion attack on the blocklace, a Byzantine-repelling Conflict-Free Replicated Data Type (CRDT) based on authenticated directed acyclic graphs encoding causal relationships between updates issued by nodes. The blocklace protocol limits replication of updates to only those that either provide new evidence of Byzantine nodes or contain signatures from presumed-correct nodes along with incriminating evidence on known Byzantine nodes, theoretically bounding the total replicated updates. However, the author demonstrates that an attacker can generate arbitrarily many new identities and produce self-incriminating evidence for each, causing correct nodes to replicate an unbounded and growing number of useless updates. This can exhaust memory resources at correct nodes, effectively a form of Sybil attack exploiting the acceptance policy. The paper discusses how this attack does not invalidate the blocklace’s core properties but renders it impractical without additional identifier management constraints. To mitigate the attack, the author proposes restricting accepted identifiers using mechanisms akin to interest-driven replication and social graph filtering, leaving detailed design for future work.
Key findings
- The blocklace acceptance policy uses a disjunctive condition: replicated updates must either incriminate a new Byzantine node or be signed by a seemingly correct node and provide evidence covering all locally known Byzantine nodes (Def. 3.1).
- An attacker can generate new identities endlessly, each proving their own Byzantine behavior, causing correct nodes to accept an unbounded number of incriminating updates.
- This memory exhaustion attack can fill up local storage of correct nodes, preventing storage of new valid updates from genuine clients.
- The attack applies regardless of whether byzantine nodes or colluders perform it, as the acceptance policy's first disjunct does not verify the author’s identity.
- The finite bound on damage guaranteed by the original blocklace paper (Theorem 1) holds only once attackers stop introducing new identifiers.
- Mitigation requires restrictions on the set of accepted identifiers to limit unbounded resource consumption.
- The attack can also be amplified by attackers embedding large payloads in updates, making each update costly.
- The problem is analogous to a Sybil attack where identities are cheaply created to overwhelm the system.
Threat model
The adversary is a powerful Byzantine actor who can create arbitrary new identities (Sybil attack) and produce updates incriminating themselves to force replication of arbitrary data by correct nodes. They cannot forge signatures of correct nodes but leverage the system’s acceptance policy that eagerly accepts any incriminating evidence about new Byzantine nodes. The adversary exploits the protocol’s eagerness to replicate any proof of misbehavior, overwhelming resource-limited correct nodes.
Methodology — deep read
Threat Model and Assumptions: The adversary consists of Byzantine nodes capable of arbitrary behavior, including producing invalid or equivocating updates, and colluding nodes who produce only valid updates but selectively omit incriminating evidence. The adversary can generate new identities arbitrarily. The system assumes authenticated updates via node signatures and a known finite identifier space, but nodes eagerly replicate updates revealing new Byzantine nodes or incriminating evidence per the blocklace’s acceptance invariant. The attacker cannot forge signatures of correct nodes but can fake identities by generating fresh keys.
Data and Setup: The paper is theoretical and analyses the replication behavior of sets of authenticated updates (blocks). It relies on earlier formal definitions of blocklaces and Byzantine-repelling predicates from Almeida & Shapiro (2025). No empirical dataset is used; the analysis is conceptual.
Blocklace Acceptance Architecture: The blocklace is a directed acyclic graph where updates (blocks) are linked to their causal ancestors. Replication behavior hinges on Definition 3.1 (Byzantine Repellance - Acceptance Invariant): A new update’s causal history H(b) is merged iff (1) the set of Byzantine nodes increases, or (2) the update is signed by a node not known Byzantine and its evidence includes all byzantine nodes known locally. This ensures only incriminating evidence or trusted updates covering known misbehavior are accepted.
Attack Procedure: The adversary creates many new node identities in the identifier space, each producing an update incriminating itself (e.g. invalid or equivocating update). These updates satisfy disjunct (1) and are accepted and replicated by correct nodes. By generating arbitrarily many such identities, the adversary forces correct nodes to grow their replicated state infinitely, exhausting memory.
Evaluation Protocol: The analysis explores the theoretical consequences of this attack on the acceptance criteria. The only metric is memory growth or resource consumption over time in a theoretical deployment. It qualitatively shows the attack circumvents the finite harm guarantee by exhausting the finite identifier space.
Reproducibility: No experimental code or datasets are released as this is a formal analysis and attack description. The paper builds on previously published blocklace definitions and theorems from Almeida & Shapiro’s prior work.
Concrete example end-to-end: An adversary creates a new keypair (new identity) and signs an invalid update incriminating itself. According to Def. 3.1 (disjunct 1), a correct node will replicate this update and increase its set of known Byzantine nodes. The attacker repeats with new identities, forcing correct nodes to keep many such incriminating updates, exhausting their memory.
Technical innovations
- Identification and formal demonstration of a memory exhaustion attack exploiting the blocklace’s acceptance criteria allowing unbounded replication of self-incriminating identities.
- Insight that the finite bound on damage holds only after the finite identifier space is exhausted or the attacker stops creating new identities.
- Highlighting that disjunct (1) of the acceptance invariant enables accepting arbitrary updates incriminating new Byzantine nodes without verifying author legitimacy.
- Proposal to mitigate via restricting accepted identifiers, akin to interest-driven or social-graph-based replication policies.
Limitations
- The analysis is theoretical and does not provide experimental validation or simulation of the attack’s impact on real-world deployments.
- No concrete design or implementation of the proposed mitigation (identifier restrictions) is presented.
- Assumes availability of a large enough identifier space to instantiate the attack; size and practical limits of such spaces not quantified.
- Does not explore defense strategies beyond restricting identifier acceptance or evaluating combined mitigations like proof-of-work.
- No evaluation under partial network conditions, network churn, or impact on liveness and availability.
Open questions / follow-ons
- How to design and integrate a practical identity acceptance restriction mechanism that balances openness with resource limits?
- What are the trade-offs of social graph or interest-driven replication policies in terms of consistency, availability, and fairness?
- Can cryptographic puzzles or proof-of-work mechanisms effectively bound the rate and quantity of updates to mitigate memory exhaustion?
- How would the attack behave in dynamic networks with churn and intermittent connectivity?
Why it matters for bot defense
This work illustrates a systemic vulnerability in distributed, authenticated data structures that rely on unconditional acceptance of incriminating evidence about arbitrary identities. For bot-defense and CAPTCHA practitioners, it highlights the critical importance of identity vetting and resource allocation policies to prevent memory or state exhaustion attacks by Sybil adversaries. Analogous to rate-limiting or challenge-response mechanisms that limit interaction from untrusted clients, decentralized systems must constrain which identities contribute state to avoid exhaustion.
Practitioners designing robust decentralized bot-defense systems or CRDTs can learn from this analysis that acceptance policies that do not limit identity introduction can be exploited to cause denial-of-service. Incorporating social trust, interest-driven replication, or computational cost mechanisms could be necessary to harden such systems. The paper’s insights reinforce that pure cryptographic authentication alone does not prevent Sybil-style resource attacks without complementary policy controls.
Cite
@article{arxiv2607_15185,
title={ Memory-Exhaustion Attack on the Blocklace Byzantine-Repelling Conflict-Free Replicated Data Type },
author={ Erick Lavoie },
journal={arXiv preprint arXiv:2607.15185},
year={ 2026 },
url={https://arxiv.org/abs/2607.15185}
}