CVE-2021-41272 in Besu
Summary
by MITRE • 12/14/2021
Besu is an Ethereum client written in Java. Starting in version 21.10.0, changes in the implementation of the SHL, SHR, and SAR operations resulted in the introduction of a signed type coercion error in values that represent negative values for 32 bit signed integers. Smart contracts that ask for shifts between approximately 2 billion and 4 billion bits (nonsensical but valid values for the operation) will fail to execute and hence fail to validate. In networks where vulnerable versions are mining with other clients or non-vulnerable versions this will result in a fork and the relevant transactions will not be included in the fork. In networks where vulnerable versions are not mining (such as Rinkeby) no fork will result and the validator nodes will stop accepting blocks. In networks where only vulnerable versions are mining the relevant transaction will not be included in any blocks. When the network adds a non-vulnerable version the network will act as in the first case. Besu 21.10.2 contains a patch for this issue. Besu 21.7.4 is not vulnerable and clients can roll back to that version. There is a workaround available: Once a transaction with the relevant shift operations is included in the canonical chain, the only remediation is to make sure all nodes are on non-vulnerable versions.
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Analysis
by VulDB Data Team • 12/16/2021
The vulnerability described in CVE-2021-41272 represents a critical implementation flaw in the Besu Ethereum client that affects the handling of bitwise shift operations. This issue specifically impacts versions 21.10.0 through 21.10.1 where changes to the SHL, SHR, and SAR operations introduced a signed type coercion error. The flaw manifests when smart contracts attempt to execute shift operations with values between approximately 2 billion and 4 billion bits, which while nonsensical from a practical standpoint, remain valid operations within the Ethereum Virtual Machine specification. This error creates a fundamental inconsistency in how negative 32-bit signed integer values are processed during bitwise operations, leading to execution failures that can cascade through the entire network.
The technical nature of this vulnerability aligns with CWE-191, which addresses integer underflow and overflow conditions, and more specifically relates to signed integer overflow scenarios where negative values are improperly handled during arithmetic operations. The flaw exists in the Ethereum client's virtual machine implementation where the signed type coercion fails to properly handle the conversion of large shift values, causing the execution to terminate prematurely. This represents a classic case of improper handling of signed integer operations that can lead to denial of service conditions within the blockchain network. The vulnerability affects the core consensus mechanism by creating divergent execution paths between vulnerable and non-vulnerable nodes, potentially leading to network forks or complete network paralysis depending on the deployment scenario.
The operational impact of CVE-2021-41272 extends beyond simple execution failures to threaten network stability and consensus integrity. When vulnerable versions participate in mining alongside non-vulnerable clients, the network experiences a fork where transactions involving problematic shift operations are excluded from the forked chain, creating a split in the canonical blockchain. In networks where vulnerable versions dominate mining operations, these transactions simply never get included in any blocks, effectively creating a denial of service condition for smart contracts utilizing such operations. The most severe impact occurs on networks where only vulnerable versions are mining, as the entire network becomes unable to process these specific transaction types. The vulnerability also affects network recovery mechanisms since the fix requires coordinated node updates, making the network susceptible to prolonged periods of inoperability until all nodes are upgraded to non-vulnerable versions.
Network resilience and security considerations are significantly compromised by this vulnerability, as it creates potential attack vectors for malicious actors seeking to exploit network instability. The ATT&CK framework's T1499.004 technique for network denial of service becomes applicable here, where adversaries could potentially cause network partitions or forks by deploying contracts that trigger the vulnerable shift operations. The recommended mitigation strategy involves immediate upgrade to Besu version 21.10.2 which contains the necessary patch, or rolling back to version 21.7.4 which was not affected by this issue. The workaround of ensuring all nodes are on non-vulnerable versions once a problematic transaction has been included in the canonical chain represents a critical operational procedure that must be followed to prevent network fragmentation. Additionally, network operators should implement monitoring systems to detect and prevent the execution of potentially vulnerable transactions, as well as maintain detailed upgrade procedures to ensure coordinated node updates across the entire network infrastructure.