CVE-2018-3737 in sshpk
Summary
by MITRE
sshpk is vulnerable to ReDoS when parsing crafted invalid public keys.
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Analysis
by VulDB Data Team • 02/17/2020
The vulnerability identified as CVE-2018-3737 affects the sshpk library, which is a fundamental component in Node.js applications for handling SSH public key operations. This library serves as a critical interface for cryptographic operations within numerous enterprise systems and development environments, making its security implications particularly significant. The vulnerability manifests as a Regular Expression Denial of Service (ReDoS) flaw that specifically occurs during the parsing of malformed or crafted public key inputs. When an attacker provides specially crafted invalid public keys to applications using sshpk, the library's regular expression patterns become susceptible to catastrophic backtracking behaviors.
The technical root cause of this vulnerability lies in the implementation of regular expressions within the sshpk library's key parsing logic. These patterns are designed to validate and parse various formats of SSH public keys including rsa, dsa, ecdsa, and ed25519 types. However, certain regex patterns contain nested quantifiers that create exponential time complexity when processing maliciously crafted inputs. The vulnerability is particularly insidious because it can be triggered through seemingly benign public key parsing operations that applications perform during authentication workflows, key exchange processes, or certificate validation procedures. Attackers can craft input strings that cause the regular expression engine to spend enormous amounts of computational time, effectively creating a denial of service condition that can crash applications or render systems unresponsive.
The operational impact of CVE-2018-3737 extends beyond simple service disruption to potentially compromise entire application availability and system stability. Applications that rely on sshpk for SSH key management, authentication services, or cryptographic operations become vulnerable to this attack vector. When exploited, the vulnerability can cause applications to consume excessive CPU resources, leading to system resource exhaustion and potential system crashes. This is particularly concerning in high-traffic environments where multiple concurrent requests could be processed simultaneously, amplifying the denial of service impact. The vulnerability affects any Node.js application that utilizes the sshpk library for public key parsing operations, making it widespread across the Node.js ecosystem and potentially impacting enterprise infrastructure, cloud services, and development toolchains that depend on SSH key management functionality.
Mitigation strategies for CVE-2018-3737 require immediate action to update the sshpk library to versions that address the regular expression vulnerability. The fix typically involves replacing vulnerable regex patterns with more efficient alternatives that do not exhibit catastrophic backtracking behavior. Organizations should conduct comprehensive vulnerability assessments to identify all applications and services using affected versions of sshpk, particularly in environments where SSH key operations are frequently performed. System administrators should implement monitoring and alerting mechanisms to detect unusual CPU usage patterns that may indicate exploitation attempts. Additionally, input validation should be strengthened at application boundaries to prevent malformed public key data from reaching the vulnerable sshpk library components. The vulnerability aligns with CWE-400, which specifically addresses Regular Expression Denial of Service, and can be mapped to ATT&CK technique T1499.004 for resource exhaustion attacks. Organizations should also consider implementing rate limiting and input sanitization measures to reduce the attack surface and prevent exploitation through automated attack vectors.