A flaw in Monero's network protection

Monero, the cryptocurrency known for its anonymity, is facing a new security flaw that allows the revelation of the IP address of nodes responsible for transactions. A team of researchers has published a paper on arXiv (reference 2607.07062) demonstrating that the integration of the Tor network into Monero's infrastructure introduces a fundamental vulnerability for deanonymization.

Their work, titled “Deanonymizing Monero Transactions in Tor Network”, was conducted by Ruisheng Shi, Shihan Zhang, Yulian Ge, Lina Lan, Qingfeng Zhang, and Qin Wang.

The heart of the problem: routing via Tor

Monero deploys the Dandelion++ protocol to obscure the origin of transactions at the network level. This mechanism operates in two phases: a “stem” phase where the transaction passes through a random chain of nodes, and a “fluff” phase where it is broadcast to the entire network. The goal is to prevent an observer from correlating an IP address with a transaction.

However, the researchers discovered that when a Monero node uses Tor, its originating transactions are exclusively transferred to two outgoing Tor hidden service nodes (proxy nodes) before being propagated to the clearnet. This predictable structure allows an attacker to capture originating transactions by occupying the outgoing connections of the targeted node.

ProxyMark: a three-step deanonymization framework

The researchers designed ProxyMark, a three-step attack framework:

  1. Identifying the roles of nodes: determine which nodes act as outgoing Tor proxies.
  2. Identifying originating transactions: locate transactions that come directly from the target node.
  3. Deanonymizing the node's location: correlate the collected data to identify the actual IP address.

The experiments were conducted on the Tor production network, the Monero mainnet, and the testnet, demonstrating the practical effectiveness of ProxyMark in deanonymizing transactions originating from Monero nodes via Tor.

A known issue in the community

This vulnerability occurs within a broader context of surveillance of the Monero network. The website monero.fail, which lists public Monero nodes, warns its users:

“There has been an influx of malicious nodes hosted by chain analysis companies and government agencies that spy on Monero users.”

The site also specifies that safeguards such as Dandelion++ intended to mitigate this problem are not foolproof.

Blockchain analysis companies like Chainalysis have exploited a similar vulnerability by leveraging “super-nodes” — nodes connecting to a large number of peers — to log which IP address broadcasts each transaction first. While the details of the transactions remain hidden on the blockchain, correlating an IP address with a transaction timestamp can be devastating for privacy.

Protection recommendations

In light of these threats, several measures are recommended:

  • Run Tor and I2P in proxy alongside one’s own Monero node with the --tx-proxy option enabled.
  • Always use your own node for any wallet expenditure.
  • Use a wallet with coin control like Feather Wallet to exclude potentially contaminated outputs.
  • Consolidate contaminated outputs by sending them to oneself multiple times to reduce traceability probability.

A typical command to run a secure node looks like this:

monerod \
  --tx-proxy tor,127.0.0.1:9050,disable_noise \
  --tx-proxy i2p,127.0.0.1:4444,disable_noise \
  --enable-dns-blocklist \
  --ban-list ~/ban_list.txt

An attack that adds to Eclipse Attacks

This discovery adds to another publication presented at the NDSS 2025 symposium, titled “Eclipse Attacks on Monero's Peer-to-Peer Network”, which demonstrates that an attacker can isolate a Monero node from the rest of the network to manipulate its view of the blockchain. Together, these two attack vectors illustrate that the network layer remains the weak link in Monero's anonymity, despite its robust on-chain cryptographic guarantees (ring signatures, stealth addresses, RingCT).

Users concerned about their privacy should thus adopt a defense-in-depth approach, combining Tor, I2P, a personal node, and constant vigilance against attempts to contaminate their wallet.