Bitcoin developer Antoine Riard has disclosed two new bugs that affect wealthy node operators within the Lightning Network, a payments protocol with over $500 million worth of BTC capacity.

The transaction jamming attack exploits Bitcoin Core software’s transaction selection, announcement, and propagation mechanisms of Lightning Network-connected Bitcoin full nodes.

Dubbed “transaction relay throughput overflow attacks,” the bugs allow an assailant to steal bitcoin (BTC) from the wealthiest Lightning nodes. Although there’s no evidence that a thief has actually exploited these bugs, Lightning implementation providers Éclair and Core Lightning are already working on software patches.

Specifically, the cost- and time-intensive attack is only worth the effort for victims with more than roughly $130,000 worth of BTC and is best suited for nodes holding above half a million dollars.

Bitcoin Lightning transaction relay throughput overflow attacks

The attack would enable a thief to steal funds from the victim’s Lightning channel by preventing time-sensitive transactions such as justice transactions from propagating through the network. After jamming the node for 32 Bitcoin blocks (Core Lightning defaults) or 140 blocks (Éclair defaults), the robber could make off with an irrevocable bounty.

In regular clock time, that would mean approximately 5.5 hours to steal from a default Core Lightning node or 24 hours for a node running Éclair default software.

By default, nodes limit the number of unconfirmed transactions they transmit or accept at any given time to reduce the chance of various denial-of-service (DoS) attacks. The attacker can conduct a high overflow jamming attack that blocks the victim from sending a justice transaction by continuously overwhelming the node with high fee rate transactions. 

By default, a Bitcoin Core node will always choose to propagate the highest fee transactions first and queue lower fee transactions — even if one of those lower fee transactions is the nodes’ own Lightning Network justice transaction.

This is one bug that Core Lightning and Éclair are patching, thanks to Riard’s responsible disclosure.

Again, the high overflow jamming attack blocks the victim from sending an anti-theft transaction by continuously overbidding with higher fee transactions, hence the name “high overflow.”

For this reason, the attack is expensive — with initial estimates north of $130,000 throughout the hours of the attack.

In addition to this high overflow jamming attack, Riard explained another variation of the transaction jamming bug: low overflow.

A variation with thousands of low-fee transactions

The low overflow is a cheaper variant but less reliable for the attacker. Here, to save money, the attacker targets a victim trying to send a transaction to nodes with a maximum unrequested transactions queue of 5,000 per peer.

The attacker floods the victim with a large number of transactions using a minimum transaction fee rate. The victim then announces these transactions to its peers and the peers try to drain the queue by requesting those transactions. If the attacker can maintain a queue of over 5,000 transactions, the attack might be successful. 

Technically speaking, the low overflow attack leverages Lightning nodes’ interaction with Bitcoin Core’s MAX_PEER_TX_ANNOUNCEMENTS default, causing inbound transactions to overflow this threshold.

Read more: New Bitcoin Lightning Network bug: Unattributed payment routing

Patching the bug

Riard proposed several mitigations for Lightning Network node software implementations. These providers are working on patches, including random transaction rebroadcasting, more aggressive fee-rebroadcasting, limitation of identical finality time-sensitive transactions, and over-provisioning of transaction relay throughput with peer nodes.

He also proposed changes to Bitcoin Core itself to assist Lightning Network operators. However, changes to Bitcoin Core typically take far longer and need more reviews than Lightning software implementations.

Riard’s Critical Vulnerability Error (CVE) request number 178025 is tracking bug patches of his high and low transaction relay throughput overflow attacks.

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Swathes of Bitcoin’s history have been erased from the internet forum that hosted communications between developers for nearly a decade.

Today, the migration of the group email list commonly known as the ‘Bitcoin Mailing List‘ is complete and Bitcoin Core developers have purged its archives from their open-source home on the Linux Foundation forever.

Visitors to the former homepage of the archive are now simply greeted with an error message: “No such list bitcoin-dev.”

It is the end of an era of sorts for communicating about Bitcoin development.

From now on, the list will live on via a Google Group, supported by the $2 trillion tech giant’s infrastructure. The move was prompted, in part, by the Linux Foundation’s decision to stop hosting email lists as of year-end 2023.

Bitcoin developer Bryan Bishop has now uploaded historical records from Linux and all of the other record-keeping systems of bitcoin-dev, including SourceForge.net and OSUOSL, to the Google Group.

For visitors who encounter an error when trying to retrieve historical messages from Linux, it’s suggested they input the URL into Archive.org’s WayBack Machine which has logged many of its formerly hosted webpages that tracked emails between Bitcoin developers.

Read more: The main Bitcoin-dev mailing list might cease operating next month

Reflecting on 15 years of Bitcoin developer emails

With the transition to Google Groups underway, some people took the opportunity to reflect on the evolution of technical Bitcoin development. In the lead-up to the switch from Linux to Google, many developers admitted that their communication style had drifted over the years from email to a multi-channel approach.

For the first decade of Bitcoin communications starting in 2008, email was the near-exclusive venue for communicating about technical changes. For years, email remained the forum to discuss changes to Bitcoin Core, the world’s dominant software for nodes validating BTC transactions.

Nowadays, several developers say they prefer to speak via messengers, social media, private servers, or an assortment of other channels.

In short, bitcoin-dev is no longer the exclusive gathering place for Bitcoin devs. As technology has advanced, communication channels have proliferated that allow doxxed, pseudonymous, and truly anonymous communication among the most senior contributors to Bitcoin’s protocol.

With the Linux listserv sunsetted and Google Groups operational, developers will continue to write code and debate others’ code contributions for years to come.

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Bitcoin developers today disclosed details of another high-severity software bug. According to senior Core developers, over 13% of the home and business computers around the world that enforce Bitcoin’s rules are vulnerable to a remote shutdown.

The bug, named CVE-2024-35202, affects Bitcoin nodes running Core software prior to version 25.0. Nodes that have not updated to at least 25.0 allow an attacker to remotely exploit an assertion in the software logic that handles block transaction (‘blocktxn’) messages.

Specifically, the vulnerability stems from Core’s compact block protocol, which uses shortened transaction identifiers to reduce internet bandwidth use. An attacker can trigger a collision in these identifiers, causing the node to request a full block.

Although requesting a full, unabridged block is a safety precaution, software versions prior to 25.0 have a flaw in their handling logic of subsequent blocktxn messages. In short, the node can be forced into an invalid state through manipulating logic gates, causing it to crash entirely.

Read more: Bitcoin devs finally admitting to major mistakes in Core software

Bug patched since May 2023, but Bitcoin Core does not auto-update

Credit for discovering and disclosing the vulnerability goes to Niklas Gögge, who also provided the patch implemented in Bitcoin Core v25.0. He patched this bug in Bitcoin Core pull request number 26898 and other developers had merged it into production by May 26, 2023.

According to self-declared values declared by internet-accessible nodes tracked by BitNodes.io, 13.7% of the 18,843 nodes operating the Bitcoin network are vulnerable to the attack. Developers encourage all node operators to update their software to patch this vulnerability. The latest version of Bitcoin Core software is 28.0. 

Although quite serious, the bug has little financial benefit to an average attacker, as it requires sophisticated manipulation of the compact block protocol and does not allow for double-spending of bitcoin without coordinating a variety of other financial and social engineering schemes.

Nevertheless, it is a security vulnerability that could be exploited by a corporate or governmental actor who wants to disrupt the operations of Bitcoin for financially-deferred reasons.

The disclosure of this bug follows a recent trend of Bitcoin Core developers revealing serious vulnerabilities in older software versions. Because Core software does not automatically update by default, node operators must manually choose to download, verify, and update their software.

Unless Bitcoin node operators update their software, a portion of the network could be at risk of a shutdown.

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Bitcoin Core developers have issued a new, high-severity warning about a software bug on one in every six Bitcoin nodes.

On Thursday, workers at the open source Bitcoin Core Project who maintain the software running on over 98% of reachable full nodes, disclosed that there is a major security problem with the software running on 17% of the network.

Specifically, all software prior to Bitcoin Core version 24.0.1 is at risk. This denial-of-service bug affects approximately 3,330 of the 19,200 self-declared user agents of reachable Bitcoin full nodes, according to surveillance estimates from Bitnodes.

In pre-24.0.1 Bitcoin Core software, a malicious actor can spam nodes with low-difficulty header chains. By forcing nodes to download and store extremely long chains of headers, the attack could crash the node by overwhelming bandwidth or storage on the device.

Developers patched this bug in Bitcoin Core pull request (PR) number 25717 and merged that into production on December 12, 2022 with the release of v24.0.1. The current version of Bitcoin Core node software, now at 27.1, includes this and other bug fixes.

Although quite serious, few known exploits of this bug exist on the public record. The bug has little financial benefit to the attacker, as it’s quite expensive to generate and broadcast header chains to execute the denial-of-service.

Nevertheless, it is a security vulnerability that could be exploited by an extremely wealthy, powerful, or sophisticated actor — such as a nation — who wanted to disrupt the operations of Bitcoin for non-financial or financially-deferred reasons.

Why Bitcoin Core developers are disclosing this bug

In early June, developers agreed to disclose serious bugs in Bitcoin’s Core software that had been patched for at least 18 months. Initially, they disclosed bugs in versions 20 and below. (For context, today’s version is 27.1.)

Every few weeks, however, they disclosed more software bugs. To their credit, the releases were in the interest of transparency and to thank developers’ voluntary, responsible disclosures.

Read more: Bitcoin Core developer proposes new type of pruned node

As months have gone by, however, the Bitcoin Core Project has disclosed bugs affecting more and more recent versions. Thursday’s release describes significant risks to software versions 24 and prior – including software as recent as May 18, 2023.

As a result, this transparency roll-out by Bitcoin Core developers, which many observers initially dismissed as a historical curiosity, is quickly making a present-day impact.

Unless Bitcoin node operators update their software, up to 17% of the network could be at risk of a denial-of-service attack.

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Thanks to a culture of responsible disclosure, Bitcoin developers have managed to keep their major coding mistakes quiet. For years, senior developers simply patched security holes while keeping their mouths shut and this silence prevented hackers from exploiting those vulnerabilities.

However, a new movement toward transparency is unveiling a fascinating history of coding mistakes in Bitcoin.

On January 14, 2021, Aaron van Wirdum announced the release of Bitcoin Core 0.21.0, a major upgrade of Bitcoin Core software. Wladimir van der Laan, then Bitcoin’s lead maintainer and second successor to Satoshi Nakamoto, signed the software release that merged over 600 pull requests into production and over the following weeks, miners and node operators manually upgraded their full nodes.

Fast forward to today, three and a half years since developers recommended node operators upgrade to 0.21.0. The current version of Core is 27.1.

As so much time has elapsed, they’ve decided to reveal the truth about that upgrade which not only enabled a number of positive features but also patched major bugs — including bugs that could have allowed hackers to steal bitcoin.

Wisely, developers stayed quiet while most node operators upgraded to 0.21.0 or above.

Today, Core versions like 21.0 and prior are considered ‘end of life’ in developer-speak. That means that they’re no longer maintained and their use by node operators is de minimis. Indeed, over 90% of Bitcoin nodes run software version 0.21.1 or above. There are still approximately 400 reachable nodes that still run version 0.21.1 — only slightly above this week’s disclosure — and have refused to upgrade for years.

Read more: Is it illegal to operate a Bitcoin Lightning node?

A new vulnerability disclosure policy

Many Bitcoin Core developers have adopted a new policy of security vulnerability disclosures. In early June, many agreed that it’s safe to disclose major safety issues that have been patched for at least 1.5 years. That policy permits them to disclose security bugs all the way through Bitcoin Core version 24.

They are proceeding deliberately from the beginning, starting with this week’s disclosure of major bugs affecting version 20 and below.

This disclosure affects approximately 426 nodes that are reachable today on the public Bitcoin network. This curious cohort runs four-year-old Core version 0.20.1 and is affected by the recently unveiled security bugs.

Here are the 10 mistakes that Bitcoin developers have admitted this week.

• Remote code execution due to bug in miniupnpc, patched with Core 0.12.
• Node crash denial-of-service from multiple peers with large messages, patched with Core 0.10.1.
• Censorship of unconfirmed transactions, patched with Core 0.21.0.
• Unbound ban list CPU/memory denial-of-service, patched with Core 0.20.1.
• Netsplit from excessive time adjustment, patched with Core 0.21.0.
• CPU denial-of-service and node stalling from orphan handling, patched with Core 0.18.0.
• Memory denial-of-service from large ‘inv’ messages, patched with Core 0.20.0.
• Memory denial-of-service using low-difficulty headers, patched with Core 0.15.0.
• CPU-wasting denial-of-service due to malformed requests, patched with Core 0.20.0.
• Memory crash in parsing BIP72 URIs, patched with Core 0.20.0.

Read more: Bitcoin Optech celebrates years of major fixes to Bitcoin vulnerabilities

Old yet serious mistakes

Most of these bugs would, if a node ran old versions of Core software, allow direct theft of funds if that node had bitcoin on the Lightning network. For example, denial-of-service and transaction censorship attacks would allow a hacker to prevent a node from broadcasting a justice transaction, allowing the hacker to close a Lightning channel with that node and steal all its bitcoin.

One bug (netsplit from excessive time adjustment) was even more serious, as it could allow an attacker to hard-fork a node’s version of Bitcoin and, therefore, possibly introduce a double-spending problem.

Later this month, developers intend to disclose patched bugs prior to Bitcoin Core version 22.0 and in August will disclose patched bugs prior to Core v23.0.

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