CVE-2020-26895 in Lightning Network Daemon
Summary
by MITRE • 10/21/2020
Prior to 0.10.0-beta, LND (Lightning Network Daemon) would have accepted a counterparty high-S signature and broadcast tx-relay invalid local commitment/HTLC transactions. This can be exploited by any peer with an open channel regardless of the victim situation (e.g., routing node, payment-receiver, or payment-sender). The impact is a loss of funds in certain situations.
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Analysis
by VulDB Data Team • 11/22/2020
The vulnerability identified as CVE-2020-26895 affects the Lightning Network Daemon (LND) software version prior to 0.10.0-beta, representing a critical flaw in the cryptographic signature validation process within the Lightning Network protocol implementation. This weakness stems from the daemon's improper handling of elliptic curve digital signature algorithm (ECDSA) signatures, specifically failing to reject high-S signatures that are mathematically valid but non-standard according to the protocol's security requirements. The Lightning Network protocol relies on ECDSA signatures for transaction validation and commitment chain management, where each participant maintains a commitment transaction that can be broadcast to the network under specific conditions. When a high-S signature is accepted, it creates a scenario where the signature remains valid but violates the expected signature format that the protocol enforces for security purposes.
The technical flaw manifests when LND processes commitment transactions and HTLC (Hashed Time-Locked Contract) transactions from counterparties, particularly during channel operations where the daemon must validate signatures for potential transaction broadcasts. In standard ECDSA implementations, signatures consist of two components r and s, where the s component can be represented in two equivalent forms due to the mathematical properties of elliptic curves. The protocol typically requires signatures to use the low-S form for consistency and security, but LND's pre-0.10.0-beta implementation failed to enforce this requirement, allowing high-S signatures to pass validation. This vulnerability enables an attacker to manipulate the signature validation process, creating conditions where invalid commitment transactions could be broadcast to the network.
The operational impact of this vulnerability extends across all participants in the Lightning Network, affecting routing nodes, payment receivers, and payment senders equally since any peer with an open channel can exploit this weakness. The attack vector does not require the victim to be in a specific operational state or role within the network, making it particularly dangerous as it can be executed by any malicious actor with access to a channel. The potential for fund loss arises in scenarios where the attacker can force the broadcasting of invalid commitment transactions, potentially leading to situations where funds are lost due to transaction malleability or invalid transaction states that the network cannot properly resolve. This vulnerability represents a direct violation of the security principles outlined in the CWE-310 Cryptographic Issues category, specifically addressing weak cryptographic implementation where the system fails to properly validate signature formats against protocol requirements.
The exploitability of CVE-2020-26895 aligns with ATT&CK technique T1552.001 for unsecured credentials and T1071.004 for application layer protocol traffic, as malicious actors can leverage this weakness to manipulate network transactions and potentially gain unauthorized access to funds. The vulnerability essentially creates a window where the security model of the Lightning Network is compromised, allowing attackers to potentially double-spend funds or create invalid transaction states that can be exploited through the network's transaction relay mechanisms. Network participants must understand that this vulnerability impacts the fundamental trust model of the Lightning Network, where the proper validation of cryptographic signatures is essential for preventing unauthorized transaction broadcasts and maintaining the integrity of the commitment chain. The impact is particularly severe because it affects the core security mechanisms that protect against transaction manipulation and unauthorized fund access, making it a critical concern for all network participants regardless of their specific role or operational context within the Lightning Network infrastructure.