LDK Patches Reconnect Vulnerability to Prevent Bitcoin Theft

Impact of the Vulnerability

The vulnerability in LDK allows a malicious channel peer to steal the value of a forwarded payment by lying after reconnecting. This can happen when a channel peer acknowledges an update, then reconnects and pretends it never received it. Before the fix, this false claim could cause LDK to sign a conflicting commitment transaction, which represents a channel's agreed state and can be used to settle it on Bitcoin's blockchain.

How the Attack Works

The malicious sender could confirm the transaction on-chain and let the payment settle with the next recipient. It could then reclaim the incoming payment contract when it expired, even though the forwarding node knew the secret normally used to claim payment. The forwarding application would have paid downstream without recovering the corresponding incoming funds.

Fixes and Updates

The October 1-dated v0.2.7 and v0.1.13 security releases address the LDK reconnect vulnerability on the 0.2 and 0.1 branches, respectively. Bitcoin Optech described the fixes in its Oct. 9 newsletter. The fixes permit retransmission only while the peer's acknowledgment remains outstanding and force-close the channel when the peer claims an already-acknowledged update was missed. Additionally, the v0.2.7 update addresses a different theft path involving LSPS2 just-in-time payments, where a liquidity service opens a channel as part of handling a payment. An intercepted payment could misrepresent its amount, causing the service to open a channel and forward more Bitcoin than the incoming payment supplied.

Recommendations for Developers

LDK’s architecture documentation explains that the SDK is compiled and executed inside applications. Developers must incorporate the relevant patched library code into deployed software. For LSPS2 integrations, the PR 5042 commit explanation flags that payment contracts queued by a prior version retain unvalidated amounts; teams need to account for those pending contracts as well as updating the library.