CVE-2020-26896 in Lightning Network Daemon
Summary
by MITRE • 10/21/2020
Prior to 0.11.0-beta, LND (Lightning Network Daemon) had a vulnerability in its invoice database. While claiming on-chain a received HTLC output, it didn't verify that the corresponding outgoing off-chain HTLC was already settled before releasing the preimage. In the case of a hash-and-amount collision with an invoice, the preimage for an expected payment was instead released. A malicious peer could have deliberately intercepted an HTLC intended for the victim node, probed the preimage through a colluding relayed HTLC, and stolen the intercepted HTLC. The impact is a loss of funds in certain situations, and a weakening of the victim's receiver privacy.
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Analysis
by VulDB Data Team • 11/22/2020
The vulnerability identified as CVE-2020-26896 represents a critical flaw in the Lightning Network Daemon (LND) software that existed prior to version 0.11.0-beta. This issue specifically targets the invoice database management system within LND, creating a scenario where the software fails to properly validate the state of related transactions before releasing cryptographic preimages. The vulnerability stems from insufficient validation logic that should have ensured proper transaction sequencing and state management between on-chain and off-chain payment channels. According to CWE-284 access control vulnerability classification, this represents a failure in proper authorization checks within the payment processing pipeline, where the system incorrectly grants access to sensitive cryptographic materials without verifying transaction integrity.
The technical exploitation of this vulnerability occurs through a sophisticated attack pattern involving hash-and-amount collisions within the Lightning Network's payment routing system. When an LND node processes an on-chain HTLC (Hashed Time-Locked Contract) output, it should verify that the corresponding off-chain HTLC has been properly settled before releasing the preimage that unlocks the payment. However, the flaw allows for a malicious actor to intercept an HTLC intended for the victim node, then use a colluding relay node to probe for the preimage through a separate HTLC with the same hash but different amount. This creates a scenario where the victim node releases the preimage for a payment that was never actually settled, effectively enabling the attacker to steal the intercepted funds. The attack leverages the fundamental trust model of the Lightning Network where nodes must verify transaction states before releasing sensitive cryptographic information.
The operational impact of CVE-2020-26896 extends beyond simple financial loss to include significant privacy implications for Lightning Network participants. The vulnerability creates a scenario where an attacker can systematically target specific nodes to gain information about their payment channels and transaction patterns, effectively weakening the privacy protections that the Lightning Network is designed to provide. This attack pattern aligns with techniques described in the MITRE ATT&CK framework under T1583 for acquisition of resources and T1046 for network service scanning, as the malicious peer must actively probe and manipulate the payment routing system to identify vulnerable nodes. The financial loss occurs when legitimate payments are diverted to attackers who exploit the preimage release mechanism, while the privacy degradation creates a persistent risk for node operators who may be targeted for future attacks based on their transaction history.
Mitigation strategies for CVE-2020-26896 require immediate software updates to LND versions 0.11.0-beta or later, which implement proper validation checks for HTLC settlement states before releasing preimages. Network operators should also implement additional monitoring for unusual payment patterns and hash collisions that might indicate attempted exploitation. The fix addresses the core validation logic by ensuring that all on-chain HTLC claims are verified against corresponding off-chain transaction states, preventing premature preimage release. Organizations should also consider implementing network-level security measures such as rate limiting for HTLC processing and enhanced transaction monitoring to detect potential exploitation attempts. The vulnerability demonstrates the importance of proper state management in distributed payment systems and highlights the need for comprehensive testing of transaction validation logic in cryptocurrency implementations.