Firo bug forces urgent upgrade to stop forged FIRO

A flaw in Firo’s Spark private-transaction protocol could allow forged coins and unlimited token creation. Node operators must upgrade to v0.14.18.0 within hours before block 1,371,000 activates; older software will become incompatible when the fork enforces the new rules.

Firo scheduled the hard fork to activate at block 1,371,000, which the project estimated would arrive around 10:00 UTC on Sept. 4. The chain reached block 1,370,847 early on Sept. 4, leaving only hours for exchanges, miners, full-node and masternode operators, and wallet providers to install the release. The fork is tied to block height rather than a clock time, so nodes that do not upgrade will be unable to validate the upgraded chain after activation.

The bug affected Spark’s handling of multi-input private spends. Firo disclosed on Aug. 13 that, under certain conditions, a flaw in multi-input Spark transactions could be exploited to forge coins and inflate the supply. The project noted the issue did not expose private keys, did not let an attacker remove coins from existing addresses, and did not impact single-input spends.

In August the team issued v0.14.17.2 as a temporary patch that limited Spark transactions to a single input. That restriction prevented further inflation but reduced privacy: larger payments had to be split into multiple single-input transactions, which are easier to correlate than multi-input private spends. Developers kept the stopgap in place while preparing a permanent fix.

Release v0.14.18.0 implements a versioned Chaum V2 proof and updates the transaction format to repair the vulnerability and restore multi-input Spark spending. Updated wallets will automatically resume normal multi-input behavior at the fork block, and the software will continue to validate historical Spark transactions. Existing Spark coins, balances, addresses and wallet keys remain valid and require no migration or reminting.

Firo reported that the researcher who disclosed the flaw generated roughly 200 FIRO on mainnet in a controlled test and that there was no evidence of other inflation at the time of the Aug. 13 notice. The project urged all operators and service providers to upgrade immediately to avoid being cut off from the network after the fork.

The mandatory upgrade applies to anyone running full nodes, masternodes, mining software, wallets that relay transactions and services such as exchanges. Passive holders whose funds remain untouched do not need to take action to preserve balances. Firo plans to publish a full technical disclosure and a post-mortem after the fork activates, and the release notes contain the upgrade steps for operators.