Bitcoin Core Reveals Tor Bug in Thousands of Nodes
The bug mainly affected mainnet nodes with Tor and whitelist settings; a mining bug in Bitcoin Core 28.0 could also cause block templates to freeze until a restart.
Key Takeaways
- Bitcoin Core published a security advisory about a Tor bug that caused thousands of Bitcoin nodes to treat incoming Tor traffic from unknown sources as trusted.
- The bug was patched in October 2025 and mainly affected mainnet nodes with whitelist settings; nodes without a whitelist were not affected.
- A second bug could block a mining node until restart, while a third bug could cause nodes to freeze for 10 minutes while processing new blocks.
Bitcoin Core published a security advisory about a bug that caused thousands of Bitcoin nodes to treat traffic from unknown sources over Tor as trusted. The bug was patched back in October 2025, but only made public this week. At the same time, a second, more serious bug also came to light that could block a mining node until the operator restarted the system.
How the Tor Bug Worked
A node checks and relays Bitcoin transactions and keeps its own copy of the blockchain. Normally, this kind of computer limits how much traffic it accepts so it does not get overloaded. In Bitcoin Core, a whitelist can relax those limits for computers the operator trusts.
The problem happened in setups where Bitcoin Core worked together with Tor, the service that hides internet addresses. Incoming Tor connections then appeared to come from the local Tor software. If that address fell within a trusted range, outsiders got the same permissions. According to the advisory, that caused thousands of reachable mainnet nodes to whitelist all incoming Tor connections.
Martin Zumsande and Antoine Poinsot reported the bug in September 2025. Bitcoin Core classified the issue as medium severity. Nodes without a whitelist setting were not affected.
Mining Bug and Another Flaw
The second bug was in the way a node handles competing versions of the blockchain. If the network drops one version, transactions from those rejected blocks can return to the mempool. A rule in Bitcoin Core 28.0 could then remove a restored transaction while dependent transactions stayed in place. That could cause a mining node to try building block templates with missing inputs, after which its own checks would reject those blocks.
An affected mining node could then no longer make valid block templates until a restart. Niklas Goegge reported this on September 26, 2025. The advisory does not mention any attacks. A third bug, also medium severity, could cause a node to freeze for 10 minutes while processing new blocks. If an attacker controlled all three selected connections for fast block relay, that failure could be triggered again and again.
Why This Matters
For European Bitcoin users, this shows how important updates for node software are. Bitcoin Core normally waits to share details until the last affected release is out of support, so administrators can install patches first. That policy is meant to keep vulnerabilities from becoming widely exploitable too early, while still giving operators time to update their nodes. Earlier security issues around Bitcoin Core also show how quickly node software can come under pressure when vulnerabilities turn out to be broadly exploitable.