CVE-2021-23017 is a high-severity vulnerability identified in the F5 Nginx resolver. This vulnerability allows an attacker, capable of forging UDP packets from a DNS server, to cause a 1-byte memory overwrite. The potential impact includes crashes of the worker process, which can disrupt services relying on Nginx. The CVSS score for this vulnerability is 7.7, indicating a significant risk to organizations using affected versions.
Risk to organizations includes denial of service due to worker process crashes, and potential data integrity and confidentiality issues. It is crucial for organizations to assess their exposure to this vulnerability and take appropriate actions. Given the nature of the vulnerability, which can be exploited over the network, organizations should prioritize patching immediately.
The vulnerability was published on June 1, 2021, and has been marked as modified since its initial disclosure. As of now, there is no known exploit in the wild, but there is a proof of concept available on platforms such as GitHub. Organizations should not only patch but also monitor for any indicators of exploitation.
With an EPS score of 0.735, this vulnerability is positioned in the upper percentile of risk, indicating a high likelihood of exploitation. Organizations should consider this factor when determining their remediation strategies and timelines.
Given the high potential impact and the nature of the vulnerability, it is imperative for affected organizations to address this in their priority patch cycle.
Vulnerability Details
The official description of CVE-2021-23017 states: A security issue in nginx resolver was identified, which might allow an attacker who is able to forge UDP packets from the DNS server to cause 1-byte memory overwrite, resulting in worker process crash or potential other impact.
This vulnerability is classified under CWE-193, which pertains to the issue of 'Off-by-One Error'. The vulnerability affects several components, including nginx versions from 0.6.18 to 1.20.0, openresty, and various products from Oracle and Fedoraproject as detailed in the CVE information.
Technical Analysis
The root cause of this vulnerability lies in the nginx resolver's handling of DNS responses. Attackers may exploit this vulnerability by sending specially crafted DNS responses that forge UDP packets, leading to a memory overwrite. The attack vector is network-based, and due to the high attack complexity, it requires certain conditions to be successful, including the ability to send forged packets from the DNS server.
The attack complexity is rated as high, which means that while the vulnerability is serious, the prerequisites for exploitation are non-trivial. No privileges are required for an attacker to initiate this exploit, and user interaction is not needed. The impact on confidentiality and integrity is high, while the availability impact is low, primarily resulting in service interruptions.
Risk & Impact Analysis
In practical terms, the risk to organizations includes potential denial of service and possible unauthorized access to critical systems. Organizations utilizing affected versions of nginx should evaluate their exposure to this vulnerability, especially those in production environments where uptime is critical.
The blast radius of this vulnerability can extend significantly, given the widespread usage of nginx in various infrastructures. Organizations should consider the potential implications on their operational capabilities and data integrity. With an EPS score indicating a high likelihood of exploitation, immediate actions are warranted.
Given the CVSS score of 7.7 and the critical nature of the vulnerability, organizations should prioritize patching immediately. Regular security assessments and monitoring for unusual activities should be part of a comprehensive security posture.
Exploitation Status
Signal | Status |
|---|---|
Known Exploit | Yes |
Public PoC | Yes |
Actively Exploited | No |
Ransomware Use | No |
Affected Versions
The vulnerability affects multiple products including nginx versions 0.6.18 to 1.20.0, openresty, and several Oracle products. Specific affected versions include:
1. Nginx: 0.6.18 - 1.20.0 2. Openresty: All versions up to 1.19.3.2 3. Fedora: 33, 34 4. Various Oracle communications products and utilities.
Mitigation & Remediation
Organizations should apply the latest patches provided by vendors. For nginx, refer to the official advisory for patching instructions. In addition to patching, organizations can implement the following mitigations:
1. Review and update configurations to limit the use of the resolver directive if not necessary. 2. Implement network controls to restrict access to DNS servers. 3. Monitor logs for unusual DNS activity that could indicate attempts to exploit this vulnerability.
Organizations should consider engaging in penetration testing as part of their remediation strategy to identify any vulnerabilities in their systems.
Detection Guidance
Organizations should monitor for the following indicators to detect potential exploitation of CVE-2021-23017:
1. Unusual spikes in DNS query failures. 2. Unexpected service disruptions or crashes in nginx. 3. Log entries indicating DNS responses that may be malformed or unexpected.
AppSecure Threat Intelligence Insight
The long-term significance of CVE-2021-23017 lies in the critical nature of the DNS resolver in network infrastructures. As organizations increasingly rely on DNS for service discovery and communication, vulnerabilities such as this can have widespread ramifications.
This vulnerability exemplifies the need for comprehensive security practices including regular audits of configurations and implementation of security best practices. Security teams should enhance their defensive strategies to guard against similar vulnerabilities in the future.
Investing in continuous penetration testing will help identify weaknesses before they can be exploited.
A robust vulnerability management program is essential for ongoing security posture improvement.
Security teams should also stay informed about API security testing as part of their overall strategy.
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
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