CVE-2023-27522: Apache HTTP Server Smuggling Vulnerability

CVE-2023-27522 Overview

CVE-2023-27522 is an HTTP Response Smuggling vulnerability in Apache HTTP Server via the mod_proxy_uwsgi module. This issue affects Apache HTTP Server versions from 2.4.30 through 2.4.55. Special characters in the origin response header can truncate or split the response forwarded to the client, potentially enabling attackers to manipulate HTTP responses.

Critical Impact

Attackers can exploit this vulnerability to manipulate HTTP responses, potentially leading to cache poisoning, session hijacking, or delivery of malicious content to end users through response splitting attacks.

Affected Products

• Apache HTTP Server versions 2.4.30 through 2.4.55
• Debian Linux 10.0 (using affected Apache versions)
• uWSGI deployments behind affected Apache HTTP Server versions

Discovery Timeline

• 2023-03-07 - CVE-2023-27522 published to NVD
• 2025-05-01 - Last updated in NVD database

Technical Details for CVE-2023-27522

Vulnerability Analysis

This vulnerability is classified under CWE-444 (Inconsistent Interpretation of HTTP Requests), which relates to HTTP Request/Response Smuggling. The flaw exists in the mod_proxy_uwsgi module, which is responsible for proxying requests to uWSGI application servers.

When Apache HTTP Server forwards responses from a uWSGI backend through the mod_proxy_uwsgi module, it fails to properly sanitize special characters in the origin response headers. This improper handling allows malicious backend responses containing crafted header values to truncate or split the HTTP response stream.

The vulnerability can be exploited without authentication and requires no user interaction, making it accessible to network-based attackers. The primary impact is on the integrity of HTTP communications, as attackers can manipulate how responses are interpreted by clients or intermediate proxies.

Root Cause

The root cause of this vulnerability lies in insufficient input validation within the mod_proxy_uwsgi module. When processing response headers from uWSGI backend servers, the module does not adequately filter or escape special characters such as carriage returns (\r) and line feeds (\n). These characters have special meaning in the HTTP protocol and can be used to inject additional headers or terminate the header section prematurely.

This allows a malicious or compromised backend server to craft responses that, when forwarded by Apache, result in ambiguous or multiple HTTP responses being sent to the client.

Attack Vector

The attack vector is network-based and can be exploited when an attacker has control over or can influence responses from a uWSGI backend server. The attack scenario typically involves:

1. An attacker sends a request to the Apache HTTP Server that is proxied to a uWSGI backend
2. The backend responds with specially crafted headers containing control characters
3. Apache's mod_proxy_uwsgi forwards these headers without proper sanitization
4. The client or intermediate proxy interprets the response differently than intended, potentially receiving injected content or having the response split into multiple messages

This can lead to cache poisoning attacks where malicious content is cached and served to other users, or session hijacking through header injection.

Detection Methods for CVE-2023-27522

Indicators of Compromise

• Unusual HTTP response patterns in web server access logs showing unexpected header values
• Cache entries containing unexpected or malicious content that differs from origin server responses
• Backend uWSGI server responses containing \r\n sequences or other control characters in header values
• Anomalous user complaints about receiving incorrect or malicious content from legitimate URLs

Detection Strategies

• Monitor Apache access and error logs for malformed responses or proxy errors related to mod_proxy_uwsgi
• Implement deep packet inspection to detect HTTP response headers containing embedded control characters
• Deploy web application firewalls (WAF) with rules to detect HTTP response smuggling patterns
• Review cache contents periodically for unexpected or suspicious entries that may indicate cache poisoning

Monitoring Recommendations

• Enable verbose logging for the mod_proxy_uwsgi module to capture detailed proxy transaction information
• Implement HTTP response validation at the edge to detect split or truncated responses
• Set up alerts for unusual response sizes or header counts that may indicate response smuggling attempts
• Monitor for integrity mismatches between expected and actual cached content

How to Mitigate CVE-2023-27522

Immediate Actions Required

• Upgrade Apache HTTP Server to version 2.4.56 or later, which contains the fix for this vulnerability
• If immediate upgrade is not possible, consider disabling mod_proxy_uwsgi until patching is complete
• Review and harden uWSGI backend configurations to prevent injection of malicious response headers
• Implement network segmentation to limit exposure of vulnerable proxy configurations

Patch Information

Apache has released security patches addressing this vulnerability in Apache HTTP Server version 2.4.56. Organizations should upgrade to this version or later as soon as possible. For detailed patch information and download links, refer to the Apache HTTP Server Vulnerabilities page.

Additional vendor-specific patches are available:

• Debian LTS Security Announcement
• Gentoo GLSA 2023-09-01

Workarounds

• Disable the mod_proxy_uwsgi module if uWSGI proxying functionality is not required in your environment
• Implement strict input/output validation at both the Apache proxy and uWSGI backend layers
• Deploy a reverse proxy or WAF in front of Apache to filter potentially malicious response headers
• Use alternative proxying methods such as mod_proxy_http with HTTP-based communication to uWSGI if feasible

# Disable mod_proxy_uwsgi temporarily until patch is applied
# For Debian/Ubuntu systems:
sudo a2dismod proxy_uwsgi
sudo systemctl restart apache2

# For RHEL/CentOS systems, comment out the LoadModule directive:
# Edit /etc/httpd/conf.modules.d/00-proxy.conf
# Comment out: LoadModule proxy_uwsgi_module modules/mod_proxy_uwsgi.so
sudo systemctl restart httpd