What Is a Sybil Attack? Why Airdrops Always Have Someone Gaming the Rules

How is a sybil attack different from ordinary scams like phishing sites?

The most fundamental difference is what gets targeted and how. Phishing targets a person, an attacker tries to trick a victim into revealing their Private Key, Seed Phrase, or personal information, directly stealing assets the victim already owned. A sybil attack targets a mechanism instead, the attacker is not stealing anything from anyone else, they are exploiting a loophole in the system rules to make themselves look like many independent participants, in order to claim far more than their fair share from a distribution mechanism that was designed to spread rewards evenly or give each person one share.

In other words, the victim of phishing is a specific targeted individual, while the victim of a sybil attack is the fairness of the entire distribution mechanism, along with the other genuine users who should have received a reasonable share but ended up with less because that share got diluted. This is also why sybil attacks often sit in a gray area legally and ethically, they do not involve stealing anyone else property, they exploit gaps in how the rules themselves were designed.

The article mentions a hacker targeting wallets already known to be eligible for an Airdrop. How does that attack actually work?

This attack exploits the Token approval mechanism, on most blockchains, if you want a Smart Contract to act on your behalf with a particular token, for example, trading on a decentralized exchange, you first have to sign an approval transaction granting that contract permission to move that token out of your wallet. An attacker targets a list of wallets already confirmed to be eligible for an upcoming airdrop, a list that is often possible to externally analyze once the snapshot timing is set but before the token actually launches, and uses stolen private keys or a malicious approval the victim was tricked into signing to pre-obtain permission to move whatever tokens those wallets are about to receive.

Once the airdropped tokens actually land in those wallets, the attacker can immediately use the previously obtained approval to transfer the tokens straight to an address they control, requiring no action from the victim at all, the victim may not even know their wallet has been compromised until the moment the airdropped tokens simply vanish. This illustrates something important: approval management does not only matter at the moment you are actively using a protocol, an old approval you granted and long forgot about can be exploited at some unexpected future point you never anticipated.

Why didn't Arbitrum's rule excluding clusters of more than 20 addresses catch more of the sybil addresses?

The logic behind that rule was: if a large group of addresses frequently transfer funds between each other, they are likely controlled by the same person and worth flagging for exclusion. But the rule had an obvious ceiling, it only caught clusters larger than 20 addresses, so an attacker only needed to split their operation into groups of 19 or fewer addresses to slip past the rule entirely, even if the same person was simultaneously running five or six such small clusters totaling hundreds of addresses behind the scenes.

The more fundamental problem is that the rule only looked at one signal, funds moving between addresses, and X-explore analysis found that genuinely sophisticated attackers avoided directly transferring funds between their addresses at all, deliberately spreading batch operations across different chains instead, for example running part of the operation on Ethereum mainnet and another part on Optimism, so that each individual chain looked like nothing more than scattered, independent activity. Anyone designing detection rules that only search for patterns within a single chain would completely miss this kind of cross-chain distributed footprint. This illustrates that a single-dimension rule, no matter how well-intentioned, can fundamentally only stop careless attackers, a genuinely determined attacker who studies the rules in detail will always find the gap the rules never covered.

LayerZero managed to filter out nearly 40% of wallets. Does that mean this problem has essentially been solved?

A high filter rate does not mean the problem has been solved, it just means that for that particular distribution, the detection resources and methods deployed were more comprehensive. The 40% figure itself is worth sitting with: it means nearly four in ten assessed wallets initially looked like they might be eligible participants but were ultimately judged to be sybils or disqualified, a proportion that reflects just how large-scale sybil farming operations have become, rather than demonstrating that defenders have gained a decisive upper hand.

More importantly, the relationship between detection methods and evasion techniques is fundamentally a dynamic arms race where both sides continuously learn from and escalate against each other, not a static problem solved once and for all. The graph analysis, funding-source tracing, and temporal analysis LayerZero deployed were genuinely more comprehensive than Arbitrum simple rules at the time, but these methods themselves have already been publicly discussed and studied by attackers, the next wave of attackers is likely already working out how to make their operation patterns look less like a cluster and their timing distribution closer to genuine human randomness. This means no single defensive method can be treated as a final solution, protocols need to expect that defensive rules themselves will require continuous updating as attack techniques evolve, rather than designing one rule set and assuming the problem is permanently solved.