Monero Privacy Faces New Threat with MAP Decoder Attack - Here’s What You Need to Know

A new vulnerability in Monero, one of the leading privacy-focused coins, has come to light. Initially dubbed the "Rucknium Ratio Attack," it’s now known as the Maximum A Posteriori (MAP) Decoder attack. The proposed solution, called OSPEAD, aims to address this issue, but the revelation has sparked plenty of discussion.

Monero’s privacy model relies on hiding your real transaction among 15 decoys, creating a total ring size of 16. Sounds solid, right? Well, research suggests otherwise. Using standard statistical methods, that ring size of 16 shrinks to an average of 4.2, slashing Monero’s privacy by about 75% compared to what was previously assumed. It’s still leagues ahead of Bitcoin’s transparency, but this is a wake-up call for XMR users.

For everyday Monero users, the big question is how to protect your privacy in light of this news. You might think "churning"—sending funds to yourself—would help. Unfortunately, it’s not that simple. Churning could do nothing, cost you fees, or even weaken your privacy if done carelessly. Skip it unless you’re crystal clear on why you’re doing it.

Instead, try this recommendation: use two wallets—one for incoming funds and one for outgoing. Many Monero wallet apps let you manage multiple "buckets" of funds within the same interface, so it’s not as clunky as it sounds. Here’s the idea. Only share stealth addresses (those starting with an 8) from your Incoming wallet, and make a fresh one for every person or service—don’t reuse addresses across exchanges, for instance. Your Outgoing wallet is for sending funds only; never share its addresses with anyone. The Incoming wallet’s sole job is to top up the Outgoing one—no third-party sends allowed. If your wallet only offers a single address starting with a 4, ditch it—it’s not up to snuff.

This two-wallet setup adds a layer of separation that boosts your privacy beyond what we thought Monero offered before this attack surfaced.

Why Two Wallets Make a Difference

So why does this trick work? It’s all about adding a hop in the transaction chain, muddying the waters between incoming and outgoing funds.

The MAP Decoder attack alone can pinpoint the real spend 23.5% of the time—not catastrophic by itself, but it gets dicey when paired with other attacks like poisoned outputs or flooding. Take Bob, for example. He buys Monero from an exchange, withdraws it to his wallet, and later sends it back to a different account on the same exchange—or one that shares data with it. Normally, the exchange sees a 6.25% chance those funds connect, based on a ring size of 16. With the MAP attack, that jumps to 23.5%—a 1-in-4.2 shot. But if Bob uses two wallets, that probability drops to 5.7%, better than the pre-attack baseline.

Now imagine a flooding attack, where an attacker floods the network with outputs they control, shrinking the effective ring size from 16 to 5.5, as Rucknium noted in October 2024. In extreme cases, some transactions hit an effective ring size of 1—meaning the real spend is exposed. Combine flooding with the MAP attack, and the ring size could plummet to between 1 and 2, leaving your privacy hanging by a thread.

The bigger fix lies in OSPEAD and, eventually, Full Chain Membership Proofs to replace ring signatures entirely. That’s still a year or two out, so for now, Monero devs are eyeing a hard fork with OSPEAD and a ring size bump to 32 or 64. It’ll mean a heftier blockchain and slower validation, but the privacy payoff is worth it. This whole saga started with a HackerOne report 3.5 years ago, and while the delay in going public raised eyebrows, it’s out now—likely a smart move, since secretive three letter agencies probably already knew. Privacy isn’t free, but with a little effort and two wallets, you can stay ahead of the game.