The developers of Monero (XMR) have reportedly fixed a bug in the crypto platform’s codebase that could have potentially led to its users losing their funds or double spending. Referred to as the “burning bug”, it would have allowed an attacker to send the privacy-oriented cryptocurrency to a stealth (for one-time only use) address numerous times.

“Burning Bug”

Should this have happened, the digital funds would not be usable, or spendable, ever again. The Monero development team said if the attacker used a normal (permanent) address, the cryptocurrency sent would have been “burned” or destroyed.

As noted in a September 25th blog post by the Monero team, crypto exchange operated wallets would have been vulnerable to the now patched “burning bug.” The vulnerability’s post mortem report explained that exchanges are set up so that they quickly deposit XMR to wallets so users can issue buy or sell orders.

However, the deposited funds cannot be spent as the digital currency they’ve received may not be usable for an outgoing transaction. As described in the Monero team’s blog, the bug “entails the wallet not providing a warning” if it is sent “burnt output.”

“Means To Indirectly Benefit”

This vulnerability could potentially have allowed a bad actor to burn funds belonging to an organization’s crypto wallet, the blog explained. The attacker would not gain any monetary benefit from this malicious activity and they would also have to pay small transaction fees. However, Monero team’s blog said “there are probably means to indirectly benefit.”

As mentioned, the bug would have allowed XMR to be transferred to the same stealth address multiple times. As a result, “multiple duplicate key images” would be generated but the blockchain network will reject a key image if it was previously found on its ledger – as this would be seen as “an attempt” to double spend.

“Unspendable/Burnt Outputs”

So, this particular bug could have been exploited as the stealth address would allow a user to only spend from it once. In other words, it would only allow one valid output (or outgoing) transaction to be issued “by selecting the largest denomination” (default) from all the transactions.

“The remainder of the outputs would be unspendable/burnt”, the Monero team wrote. They also explained how the attacker could have “practically” exploited the bug had it not been fixed:

An attacker first generates a random private transaction key. Thereafter, they modify the code to merely use this particular private transaction key, which ensures multiple transactions to the same public address (e.g. an exchange's hot wallet) are sent to the same stealth address.

Monero's Developers

Employing this strategy, the bad actor could “send a thousand transactions of 1 XMR to an exchange. Because the exchange’s wallet does not warn for this particular abnormality … the exchange will … credit the attacker with 1000 XMR.”

Using the XMR received, the attacker could exchange it for Bitcoin (BTC) and then withdraw his or her funds – resulting in the exchange being “left with 999 unspendable / burnt outputs of 1 XMR.”