Apache HTTP Server 2.4.49 Path Traversal and Remote Code Execution Vulnerability (CVE-2021-41773) Security Advisory
Executive & SecOps Summary
SECOPS ASSESSMENTApache HTTP Server version 2.4.49.
Attackers can exploit this vulnerability using specially crafted URLs to access sensitive files outside the document root or execute arbitrary commands (RCE) if CGI scripts are enabled.
Immediately upgrade to the latest stable version (2.4.51 or higher) and enforce strict directory access controls ('Require all denied').
Affected Products & Version Scope
Remediation & Live Workaround Commands
PATCH & WORKAROUND1. Official Patch & Version Upgrade Commands
Immediately upgrade Apache HTTP Server to version 2.4.51 or later to fully resolve the vulnerability.
# For Debian/Ubuntu-based systems
sudo apt-get update
sudo apt-get install --only-upgrade apache2 -y
# For RedHat/CentOS-based systems
sudo yum update httpd -y
# Verify the installed version
apache2 -v || httpd -v2. Temporary Workaround & Hardening Configuration
If an immediate upgrade is not feasible, modify the 'httpd.conf' configuration file to restrict directory access and disable vulnerable modules.
# 1. Explicitly deny access to the root directory
<Directory />
AllowOverride none
Require all denied
</Directory>
# 2. Disable mod_cgi if it is not strictly required
# LoadModule cgi_module modules/mod_cgi.so
# 3. Test configuration and restart the service
sudo apachectl configtest
sudo systemctl restart apache2 || sudo systemctl restart httpd3. Log Audit & IoC Verification Commands
Scan your web server access logs to identify potential path traversal attempts and RCE exploitation patterns.
# Search for encoded path traversal patterns (.%2e) in access logs
grep -E '\.%2e|%2e%2e' /var/log/apache2/access.log /var/log/httpd/access_log
# Detect potential command execution attempts via cgi-bin
grep -i 'cgi-bin' /var/log/apache2/access.log | grep -E 'sh|bash|cmd'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
A critical flaw was introduced in Apache HTTP Server 2.4.49 due to a change made to the path normalization logic. Attackers can bypass standard path traversal protections by utilizing URL-encoded characters such as '%2e%2e' to represent dot-dot ('..') sequences. Because the server fails to properly normalize and validate these encoded paths, it allows requests to map to files outside the directories configured by Alias-like directives, leading to a Path Traversal vulnerability (CWE-22).
2. Exploitation Mechanics & Attack Scenario
An attacker sends a crafted HTTP GET or POST request containing encoded traversal sequences, such as '/cgi-bin/.%2e/.%2e/.%2e/.%2e/etc/passwd'.
- Arbitrary File Read: If the default configuration does not explicitly protect directories outside the document root with 'Require all denied', the request succeeds, allowing the attacker to read sensitive system files.
- Remote Code Execution (RCE): If CGI scripts ('mod_cgi') are enabled for these aliased paths and a shell binary (like '/bin/sh') is accessible, the attacker can pipe commands through the HTTP request body to execute arbitrary code under the privileges of the web server daemon.
3. Infrastructure & Business Impact
This vulnerability has a CVSS score of 9.8 (Critical) and is actively exploited in the wild, as confirmed by its inclusion in the CISA KEV catalog. With an EPSS score of 0.9999, the likelihood of exploitation is extremely high. Successful exploitation can lead to full system compromise, unauthorized data exfiltration, lateral movement within the internal network, and severe disruption of business operations.
Official Vendor Advisories & References
A flaw was found in a change made to path normalization in Apache HTTP Server 2.4.49. An attacker could use a path traversal attack to map URLs to files outside the directories configured by Alias-like directives. If files outside of these directories are not protected by the usual default configuration "require all denied", these requests can succeed. If CGI scripts are also enabled for these aliased pathes, this could allow for remote code execution. This issue is known to be exploited in the wild. This issue only affects Apache 2.4.49 and not earlier versions. The fix in Apache HTTP Server 2.4.50 was found to be incomplete, see CVE-2021-42013.
Frequently Asked SecOps Questions (CVE-2021-41773)
FAQQ1. How critical is CVE-2021-41773 and is it actively exploited in the wild?
CVE-2021-41773 is rated CVSS 9.8 (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-2021-41773?
Primary impacted vendors include apache across http_server. Scope summary: This issue specifically affects Apache HTTP Server version 2.4.49. Note that the fix introduced in version 2.4.50 was incomplete (tracked as CVE-2021-42013), making both 2.4.49 and 2.4.50 vulnerable in practice.
Q3. What temporary mitigations or workarounds exist for CVE-2021-41773 before patching?
[Mitigation] Immediately upgrade to the latest stable version (2.4.51 or higher) and enforce strict directory access controls ('Require all denied'). 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-2021-41773
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).
Adobe ColdFusion Path Traversal Remote Code Execution Vulnerability Urgent Advisory (CVE-2026-48282)
[Affected Scope] Adobe ColdFusion versions 2025.9, 2023.20 and earlier
Subscribe to Daily Vulnerability Briefings & KEV Alerts
Concise morning briefings for SecOps, DevSecOps, and SRE teams covering newly exploited CVEs and mitigation priorities.