CVE-2021-40438: Resf Rocky Linux SSRF Vulnerability

CVE-2021-40438 Overview

CVE-2021-40438 is a critical Server-Side Request Forgery (SSRF) vulnerability in Apache HTTP Server's mod_proxy module. A crafted request URI-path can cause mod_proxy to forward the request to an origin server chosen by a remote attacker. This vulnerability affects Apache HTTP Server versions 2.4.48 and earlier, making it a significant threat to a vast number of web servers deployed worldwide.

This vulnerability allows unauthenticated remote attackers to manipulate proxy requests, potentially enabling them to access internal services, bypass access controls, exfiltrate sensitive data, or pivot to attack other systems within the network. Due to the widespread use of Apache HTTP Server as a reverse proxy, the attack surface is extensive.

Critical Impact

This vulnerability is listed in CISA's Known Exploited Vulnerabilities (KEV) catalog, indicating active exploitation in the wild. Attackers can leverage this SSRF to access internal services, cloud metadata endpoints, and sensitive resources that should not be externally accessible.

Affected Products

• Apache HTTP Server 2.4.48 and earlier
• Red Hat Enterprise Linux (versions 7.x and 8.x)
• Red Hat JBoss Core Services 1.0
• Debian Linux (versions 9.0, 10.0, 11.0)
• Fedora 34 and 35
• Oracle HTTP Server (12.2.1.3.0, 12.2.1.4.0)
• NetApp Clustered Data ONTAP, StorageGRID, Cloud Backup
• Siemens SINEMA Server, SINEC NMS, RUGGEDCOM NMS
• Tenable.sc
• F5 F5OS
• Broadcom Brocade Fabric Operating System

Discovery Timeline

• 2021-09-16 - CVE-2021-40438 published to NVD
• 2025-10-27 - Last updated in NVD database

Technical Details for CVE-2021-40438

Vulnerability Analysis

This SSRF vulnerability resides within Apache HTTP Server's mod_proxy module, which is commonly used to configure the server as a reverse proxy. The flaw exists in how mod_proxy processes and validates request URI paths before forwarding them to backend servers.

When Apache HTTP Server is configured with mod_proxy enabled, an attacker can craft malicious HTTP requests with specially constructed URI paths that trick the proxy into forwarding requests to arbitrary destinations. This bypasses the intended proxy configuration, allowing the attacker to specify their own target server. The vulnerability is particularly dangerous because it requires no authentication and can be exploited remotely over the network, though it does require specific conditions to be met for successful exploitation.

The impact of successful exploitation includes the ability to access internal services not intended for external access, reach cloud provider metadata endpoints (such as AWS IMDSv1), scan internal networks, and potentially exfiltrate sensitive information from protected resources.

Root Cause

The root cause of CVE-2021-40438 is improper input validation in the mod_proxy module's URI parsing logic. The module fails to adequately sanitize and validate the request URI-path before using it to construct the forwarded proxy request. Specifically, the vulnerability stems from CWE-918 (Server-Side Request Forgery), where the application fails to properly verify that user-supplied input represents the intended destination for proxied requests.

The flawed logic allows attackers to inject characters or sequences in the URI that manipulate the proxy destination, causing mod_proxy to forward requests to attacker-controlled or otherwise unintended servers rather than the configured backend.

Attack Vector

The attack is conducted remotely over the network without requiring authentication. An attacker sends a specially crafted HTTP request to a vulnerable Apache HTTP Server instance that has mod_proxy enabled. The crafted URI-path contains malicious elements that exploit the parsing flaw, causing the server to forward the request to an arbitrary origin server specified by the attacker.

Attack scenarios include:

• Cloud Metadata Access: Targeting cloud instance metadata services (e.g., http://169.254.169.254/) to retrieve credentials, tokens, and instance configuration
• Internal Service Discovery: Scanning and accessing internal services, databases, and APIs that are not exposed to the internet
• Network Pivoting: Using the compromised proxy as a pivot point to attack other systems within the internal network
• Data Exfiltration: Retrieving sensitive information from internal resources through the proxied requests

The vulnerability can be triggered through standard HTTP requests, making it easily weaponizable. For detailed technical analysis and exploitation mechanics, refer to the Apache HTTPD Vulnerabilities Overview and the Cisco Security Advisory for Apache HTTPD.

Detection Methods for CVE-2021-40438

Indicators of Compromise

• Unusual outbound connections from the Apache HTTP Server to internal IP ranges (e.g., 10.x.x.x, 172.16.x.x, 192.168.x.x) or cloud metadata endpoints (169.254.169.254)
• HTTP access logs showing requests with malformed or suspicious URI paths containing unexpected characters or encoding
• Proxy error logs indicating connections to unexpected backend destinations
• Network traffic from the web server to internal services that should not receive external requests

Detection Strategies

• Monitor Apache access logs for requests with unusually long or malformed URI paths, especially those containing encoded characters or internal IP addresses
• Implement network-level monitoring to detect outbound connections from web servers to sensitive internal services or cloud metadata endpoints
• Deploy intrusion detection system (IDS) rules to identify SSRF attack patterns targeting mod_proxy
• Utilize SentinelOne's behavioral AI detection to identify anomalous proxy behavior and unauthorized network connections from web server processes

Monitoring Recommendations

• Enable verbose logging for mod_proxy to capture detailed information about proxied requests and destinations
• Configure network segmentation monitoring to alert on web server traffic to sensitive internal subnets
• Implement application-layer firewall rules to inspect and block suspicious URI patterns in incoming requests
• Regularly review Apache error logs for proxy-related errors that may indicate exploitation attempts

How to Mitigate CVE-2021-40438

Immediate Actions Required

• Upgrade Apache HTTP Server to version 2.4.49 or later immediately, as this version contains the security fix for CVE-2021-40438
• If immediate patching is not possible, consider temporarily disabling mod_proxy if it is not essential for operations
• Implement network segmentation to limit the web server's ability to reach sensitive internal services
• Apply vendor-specific patches from Red Hat, Debian, Oracle, and other affected distributions

Patch Information

Apache has addressed this vulnerability in Apache HTTP Server version 2.4.49 and later. Organizations should update to the latest available version to receive all security fixes. For detailed patch information, consult the Apache HTTPD Vulnerabilities Overview.

Multiple vendors have released security advisories and patches:

• Oracle January 2022 CPU Security Alerts
• Oracle April 2022 CPU Security Alerts
• Debian Security Advisory DSA-4982
• Siemens Security Advisory SSA-685781
• NetApp Security Advisory NTAP-20211008-0004

Workarounds

• Restrict mod_proxy to only forward requests to explicitly defined backend servers using the ProxyPass directive with specific target URLs
• Implement strict URL validation at the application or WAF layer to reject requests with suspicious URI patterns
• Use firewall rules to block outbound connections from the Apache server to unauthorized destinations, particularly internal IP ranges and cloud metadata endpoints
• Consider deploying a web application firewall (WAF) with SSRF protection rules as an additional defense layer

# Example Apache configuration to restrict proxy destinations
# Only allow proxying to specific trusted backends
<VirtualHost *:80>
    ServerName example.com
    
    # Disable general forward proxy functionality
    ProxyRequests Off
    
    # Only allow proxying to specific backend servers
    ProxyPass /api http://internal-api-server:8080/api
    ProxyPassReverse /api http://internal-api-server:8080/api
    
    # Deny access to proxy for all other requests
    <Proxy *>
        Require all denied
    </Proxy>
    
    # Allow only the defined proxy paths
    <Proxy http://internal-api-server:8080/*>
        Require all granted
    </Proxy>
</VirtualHost>