The Hidden Risks of Sub-1 sat/vB Transactions

While sub-1 sat/vB transactions might seem appealing due to lower fees, they introduce serious risks that every user should understand before using them. These risks can lead to stuck transactions, financial losses, and poor user experience.

#1 Stuck Transactions:

• Most mining pools currently do not mine transactions with 0.1 sat/vB or similar ultra-low fee rates

• Your transaction may remain unconfirmed for days, weeks, or indefinitely

• Funds become inaccessible - you can't spend the same UTXOs until the transaction confirms

• Particularly problematic for time-sensitive operations such as mint or trade

#2 Dust UTXO Attacks

How the Attack Works:

• Malicious actors send low-fee-rate UTXOs to your address

• Later, your wallet automatically includes this UTXO when constructing a new transaction

• The entire transaction inherits the low fee rate, making it unconfirmable

• You must pay expensive RBF (Replace-By-Fee) acceleration costs to fix the issue

• Why it’s sneaky:

  • Our wallet blocks UTXOs <600 sats, but attacks over 600 sats bypass this.

  • Merging these UTXOs (via wallet tools) spreads the poison to new transactions.

#3: Transaction Chain Limitations

The Technical Problem:

• Bitcoin's mempool has a 25-transaction ancestor limit

• Repeatedly spending 0.1 sat/vB transactions quickly approaches this limit

• Once reached, new transactions cannot be broadcast

• Regular users have no way to resolve this without technical tools

Resolution Challenges:

• Only solution is using RBF (Replace-By-Fee) acceleration tools to replace earlier transactions

• Current tool limitations - many RBF implementations have bugs and don't handle data cleanup properly

🎯 Recommendations for Users

• Avoid sub-1 sat/vB transactions unless you fully understand the risks

• Be Aware of the limited ecosystem support for handling sub-1 sat/vB transactions properly and potential inconsistent behavior across different wallet implementations

• Keep RBF tools available for emergency acceleration

How UniSat may support sub-1 sat/vB transactions

• We will do more research and aim to first implement in our explorer UniScan

• More experimental tools may be possible in the future

More on how Sub-1 sat/vB transactions work: https://docs.unisat.io/technical/understanding-sub-1-sat-vb-bitcoin-transactions