VMware vCenter Server rhttproxy URI Normalization Bypass Vulnerability (CVE-2021-22017) Technical Analysis and Hardening Guide
Executive & SecOps Summary
SECOPS ASSESSMENTVMware vCenter Server versions 6.5, 6.7, and 7.0 without the VMSA-2021-0020 patch applied
An unauthenticated attacker with network access to port 443 can bypass proxy restrictions via improper URI normalization, gaining unauthorized access to internal endpoints
Immediately upgrade vCenter Server to the patched versions recommended in VMware Advisory VMSA-2021-0020
Affected Products & Version Scope
Remediation & Live Workaround Commands
PATCH & WORKAROUND1. Official Patch & Version Upgrade Commands
To remediate this vulnerability, connect to the vCenter Server Appliance (VCSA) via SSH and execute the following commands to stage and install the official security patches.
# Stage and install the latest security updates from the VMware repository
software-packages stage --url --acceptEulas
software-packages install --staged
# Restart all vCenter services to apply changes and verify status
service-control --stop --all
service-control --start --all
service-control --status2. Temporary Workaround & Hardening Configuration
If immediate patching is not feasible, restrict access to port 443 on the vCenter Server using firewall rules, limiting access only to trusted administrative subnets.
# Example: Restrict port 443 access to a specific admin IP (e.g., 192.168.10.50) using iptables
iptables -A INPUT -p tcp -s 192.168.10.50 --dport 443 -j ACCEPT
iptables -A INPUT -p tcp --dport 443 -j DROP3. Log Audit & IoC Verification Commands
Inspect the rhttproxy logs for suspicious URI patterns or potential bypass attempts.
# Scan rhttproxy logs for directory traversal or abnormal URI normalization bypass attempts
grep -Ei '(/\.\./|//|\.\./\.\.)' /var/log/vmware/rhttproxy/rhttproxy.log
# Verify the current vCenter Server build version
vcentercfg --versionStop Exposing VPN Ports to the Internet — Switch to Zero-Trust Access
Eliminate public-facing VPN concentrators and firewall management ports with identity-first Zero-Trust Network Access.
Technical Root Cause & Exploitation Mechanics
1. Vulnerability Overview & Root Cause
This vulnerability (CVE-2021-22017) resides in rhttproxy, the reverse proxy component of VMware vCenter Server. The root cause is an improper implementation of URI normalization within rhttproxy when processing incoming HTTP requests. When routing traffic to internal services, the proxy fails to correctly parse and sanitize specific URI structures, allowing path traversal or malformed URI patterns to bypass access control filters.
2. Exploitation Mechanics & Attack Scenario
A malicious actor with network access to the vCenter Server's HTTPS port (TCP 443) can craft a request containing non-normalized URI paths (such as double slashes or directory traversal sequences). The rhttproxy validation logic misinterprets the path as a public-facing endpoint and forwards it. However, when the backend processes the request, it resolves the normalized path, granting the attacker direct access to restricted internal endpoints and administrative APIs without authentication. This vulnerability is actively exploited in the wild and is listed in the CISA KEV catalog.
3. Infrastructure & Business Impact
vCenter Server is the central management hub for enterprise virtualized environments. Successful exploitation of this vulnerability allows attackers to bypass perimeter controls and access internal APIs. This can be chained with other local vulnerabilities to achieve remote code execution (RCE), leading to full compromise of the hypervisor infrastructure, unauthorized virtual machine access, and potential deployment of ransomware across the entire enterprise network.
Official Vendor Advisories & References
Rhttproxy as used in vCenter Server contains a vulnerability due to improper implementation of URI normalization. A malicious actor with network access to port 443 on vCenter Server may exploit this issue to bypass proxy leading to internal endpoints being accessed.
Frequently Asked SecOps Questions (CVE-2021-22017)
FAQQ1. How critical is CVE-2021-22017 and is it actively exploited in the wild?
CVE-2021-22017 is rated CVSS 5.3 (MEDIUM) with a 30-day EPSS exploitation probability of 49.2%. It is listed in the CISA Known Exploited Vulnerabilities (KEV) catalog, requiring immediate emergency remediation.
Q2. Which vendors and products are affected by CVE-2021-22017?
Primary impacted vendors include vmware across vcenter_server. Scope summary: VMware vCenter Server 7.0 (prior to 7.0 U3c), 6.7 (prior to 6.7 U3p), and 6.5 (prior to 6.5 U3r)
Q3. What temporary mitigations or workarounds exist for CVE-2021-22017 before patching?
[Mitigation] Immediately upgrade vCenter Server to the patched versions recommended in VMware Advisory VMSA-2021-0020 Follow the CLI remediation commands and interactive SecOps checklist on this page to restrict exposure prior to your maintenance window.
Generate an Internal Engineering Notice or CISO Report for CVE-2021-22017
Instantly format this advisory into an executive CISO summary or engineering patch ticket.
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