CVE-2021-20325: Red Hat Enterprise Linux Regression Flaw

CVE-2021-20325 Overview

CVE-2021-20325 is a security regression vulnerability in the httpd package as shipped in Red Hat Enterprise Linux 8.5.0. This issue occurs because fixes for two previously addressed vulnerabilities (CVE-2021-40438 and CVE-2021-26691) were inadvertently missing from the RHEL 8.5.0 release. As a result, users who installed or upgraded to Red Hat Enterprise Linux 8.5.0 became vulnerable to these previously patched security flaws, even though they were properly fixed in Red Hat Enterprise Linux 8.4. This is a Red Hat-specific regression and does not affect upstream Apache httpd versions.

Critical Impact

This security regression re-exposes systems to Server-Side Request Forgery (SSRF) and heap buffer overflow vulnerabilities in Apache httpd that were previously patched, potentially allowing remote attackers to compromise affected Red Hat Enterprise Linux 8.5.0 systems.

Affected Products

• Red Hat Enterprise Linux 8.5.0
• httpd package as shipped in RHEL 8.5.0

Discovery Timeline

• 2022-02-18 - CVE-2021-20325 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-20325

Vulnerability Analysis

This vulnerability represents a security regression where critical patches for two separate vulnerabilities were omitted during the release process for Red Hat Enterprise Linux 8.5.0. The missing fixes correspond to CVE-2021-40438, a Server-Side Request Forgery (SSRF) vulnerability in mod_proxy, and CVE-2021-26691, a heap buffer overflow vulnerability in Apache httpd's mod_session component.

The regression occurs because the httpd package in RHEL 8.5.0 lacks the backported security fixes that were present in RHEL 8.4. This creates a situation where users who upgrade from RHEL 8.4 to 8.5.0 inadvertently downgrade their security posture. The CWE classifications (CWE-119: Improper Restriction of Operations within the Bounds of a Memory Buffer and CWE-787: Out-of-bounds Write) reflect the underlying memory corruption issues from the re-exposed CVE-2021-26691 vulnerability.

Root Cause

The root cause is a packaging and release process failure within Red Hat's software distribution pipeline. During the preparation of RHEL 8.5.0, the security patches for CVE-2021-40438 and CVE-2021-26691 that were successfully applied to RHEL 8.4 were not carried forward to the 8.5.0 release. This resulted in the httpd package in RHEL 8.5.0 containing older, vulnerable code despite fixes being available and previously deployed.

Attack Vector

The attack vector is network-based and requires no authentication or user interaction. An attacker can exploit the re-exposed vulnerabilities remotely:

1. CVE-2021-40438 (SSRF): Attackers can craft malicious requests to mod_proxy that cause the Apache server to make unintended requests to internal or external resources, potentially leading to information disclosure or access to internal services.

2. CVE-2021-26691 (Heap Overflow): Attackers can exploit the heap buffer overflow in mod_session to potentially achieve remote code execution or cause denial of service conditions.

The vulnerability mechanism involves memory corruption through improper boundary checks in the affected httpd modules. When processing specially crafted requests, the server may write data beyond allocated buffer boundaries, leading to potential code execution. For detailed technical analysis, refer to the Red Hat Bug Report #2017321.

Detection Methods for CVE-2021-20325

Indicators of Compromise

• Unexpected outbound connections from Apache httpd servers to internal or external hosts
• Unusual mod_proxy activity patterns in Apache access logs
• Evidence of memory corruption or Apache service crashes
• Anomalous session handling behavior in mod_session logs

Detection Strategies

• Verify the installed httpd package version against Red Hat's security advisory to confirm if vulnerable versions are present
• Monitor Apache httpd logs for requests containing unusual proxy targets or malformed session data
• Implement network monitoring to detect SSRF-style traffic patterns originating from web servers
• Use vulnerability scanners to identify RHEL 8.5.0 systems with unpatched httpd packages

Monitoring Recommendations

• Enable enhanced logging for mod_proxy and mod_session modules
• Configure intrusion detection systems to alert on known exploit patterns for CVE-2021-40438 and CVE-2021-26691
• Implement file integrity monitoring on httpd binaries and configuration files
• Monitor system resource usage for signs of memory exhaustion or corruption

How to Mitigate CVE-2021-20325

Immediate Actions Required

• Update the httpd package to the latest patched version provided by Red Hat
• Review and apply all available security updates for Red Hat Enterprise Linux 8
• Audit systems running RHEL 8.5.0 to identify vulnerable httpd installations
• Consider temporarily disabling mod_proxy and mod_session if not required until patches are applied

Patch Information

Red Hat has acknowledged this security regression and released updated httpd packages to address CVE-2021-20325. Administrators should consult the Red Hat Bug Report #2017321 for detailed patch information and apply the latest available updates using the standard yum update httpd or dnf update httpd commands. The updated packages restore the security fixes that were missing in the initial RHEL 8.5.0 release.

Workarounds

• Disable mod_proxy if forward proxy functionality is not required by removing or commenting out the proxy-related configuration
• Disable mod_session if session functionality is not needed to eliminate exposure to CVE-2021-26691
• Implement network segmentation to limit the impact of potential SSRF attacks
• Deploy a web application firewall (WAF) with rules to detect and block known exploit patterns

# Configuration example
# Temporarily disable mod_proxy and mod_session in httpd configuration
# Edit /etc/httpd/conf.modules.d/00-proxy.conf
# Comment out the following lines:
# LoadModule proxy_module modules/mod_proxy.so
# LoadModule proxy_http_module modules/mod_proxy_http.so

# Edit /etc/httpd/conf.modules.d/01-session.conf
# Comment out:
# LoadModule session_module modules/mod_session.so

# Restart httpd to apply changes
systemctl restart httpd