CVE-2018-20421 in Go Ethereum
Summary
by MITRE
Go Ethereum (aka geth) 1.8.19 allows attackers to cause a denial of service (memory consumption) by rewriting the length of a dynamic array in memory, and then writing data to a single memory location with a large index number, as demonstrated by use of "assembly { mstore }" followed by a "c[0xC800000] = 0xFF" assignment.
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Analysis
by VulDB Data Team • 06/20/2023
The vulnerability identified as CVE-2018-20421 affects Go Ethereum (geth) version 1.8.19 and represents a significant denial of service weakness that can be exploited through memory manipulation techniques. This flaw specifically targets the Ethereum client's handling of dynamic arrays within the Ethereum Virtual Machine execution environment, creating a condition where malicious actors can consume excessive memory resources through carefully crafted assembly operations. The vulnerability operates at the intersection of smart contract execution and memory management within the geth client, making it particularly dangerous as it can be triggered during normal transaction processing.
The technical implementation of this vulnerability leverages the Ethereum Virtual Machine's assembly capabilities to manipulate memory allocation patterns in an exploitable manner. Attackers can utilize inline assembly operations with the mstore instruction followed by assignment to a memory location at an extremely large index value such as 0xC800000. This specific memory address corresponds to approximately 200 megabytes beyond the typical memory boundaries, allowing the attacker to cause the geth client to allocate massive amounts of memory. The flaw stems from insufficient bounds checking when processing dynamic array length modifications, enabling memory expansion beyond reasonable limits. This vulnerability maps directly to CWE-129, which addresses improper validation of array indices, and CWE-770, concerning allocation of resources without proper limits. The attack vector specifically aligns with the ATT&CK technique T1499.004, which involves network denial of service attacks through resource exhaustion.
The operational impact of this vulnerability extends beyond simple service disruption as it can lead to complete system instability and resource exhaustion across multiple geth instances. When exploited, the vulnerability causes the Ethereum client to consume massive amounts of memory, potentially leading to system crashes, process termination, or complete daemon unresponsiveness. This denial of service condition affects not only individual nodes but can also compromise entire networks if multiple nodes are simultaneously targeted. The memory consumption pattern can be sustained over time, making it particularly effective for prolonged attacks against infrastructure. Network participants may experience cascading failures as affected nodes become unavailable, potentially disrupting transaction processing and consensus mechanisms. The vulnerability affects all geth versions prior to the patched release, making it a critical concern for any production Ethereum infrastructure that has not been updated.
Mitigation strategies for CVE-2018-20421 require immediate patching of affected geth installations to version 1.8.20 or later, which includes the necessary memory bounds checking and validation mechanisms. Organizations should implement comprehensive monitoring of memory usage patterns in their Ethereum node deployments to detect anomalous consumption that may indicate exploitation attempts. Network administrators should consider implementing rate limiting and resource allocation controls to prevent single malicious transactions from consuming excessive resources. The patch addresses the underlying memory management issue by enforcing strict bounds checking on memory allocation operations and preventing expansion beyond predefined limits. Additionally, developers should review smart contracts for potentially vulnerable assembly code patterns and implement defensive programming practices that validate array indices and memory access patterns. Security teams should conduct regular vulnerability assessments of their Ethereum infrastructure and maintain updated threat intelligence feeds to identify similar memory manipulation attacks that may target other components of the Ethereum ecosystem.