CVE-2021-41592 in c-lightning
Summary
by MITRE • 10/04/2021
Blockstream c-lightning through 0.10.1 allows loss of funds because of dust HTLC exposure.
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Analysis
by VulDB Data Team • 10/09/2021
The vulnerability identified as CVE-2021-41592 affects Blockstream c-lightning versions 0.10.1 and earlier, presenting a critical financial risk through improper handling of dust HTLCs within the Lightning Network protocol implementation. This flaw specifically targets the channel closure mechanism where funds can be lost due to miscalculations in HTLC (Hashed Time-Locked Contract) dust values during the settlement process. The issue manifests when a channel is closed and the system fails to properly account for dust HTLCs that should be considered negligible but are instead treated as valid transactions within the closing protocol.
The technical root cause stems from how the c-lightning implementation processes HTLC outputs during channel closure, particularly when dealing with dust amounts that fall below the minimum transaction value threshold. This vulnerability creates a scenario where certain HTLC outputs are not properly excluded from the final settlement, leading to potential fund loss for channel participants. The flaw operates at the protocol level within the Lightning Network's channel management system, specifically in the logic that determines which HTLCs should be included in the closing transaction. When the system encounters dust HTLCs during the settlement phase, it incorrectly includes them in the final transaction, potentially causing funds to be lost or inaccessible due to the transaction structure not properly accounting for these minimal value outputs.
The operational impact of this vulnerability extends beyond simple fund loss to encompass broader network stability and participant confidence in the Lightning Network infrastructure. Channel participants who experience this vulnerability during channel closure may lose funds that were intended to be recovered through the normal settlement process, creating financial losses that could range from small amounts to significant portions depending on the channel's total capacity and the specific transaction details. The vulnerability affects the fundamental trust model of the Lightning Network where participants expect their funds to be properly secured and recoverable through standard channel closure procedures. Network operators and users must consider that this flaw could potentially be exploited by malicious actors to target specific channels or participants during settlement operations, though the primary risk comes from legitimate channel closures where the system's logic fails to properly handle dust HTLCs.
Mitigation strategies for CVE-2021-41592 primarily involve upgrading to c-lightning version 0.10.2 or later, which contains the necessary patches to properly handle dust HTLCs during channel closure. Users should also implement proper channel monitoring and regular security audits to identify any potential exposure to this vulnerability before upgrading. The fix addresses the underlying protocol implementation by ensuring that dust HTLCs are properly identified and excluded from settlement transactions, preventing the loss of funds that would otherwise occur due to the improper inclusion of these minimal value outputs. Organizations should also consider implementing additional transaction validation checks and monitoring for unusual settlement patterns that might indicate exposure to this vulnerability. This remediation aligns with best practices for maintaining secure Lightning Network implementations and follows the principles outlined in CWE-400 for proper resource management and transaction handling. The vulnerability demonstrates the critical importance of proper dust handling in cryptocurrency systems and highlights the need for thorough testing of edge cases in financial transaction protocols.