CVE-2023-27522 in HTTP Server
Summary
by MITRE • 03/07/2023
HTTP Response Smuggling vulnerability in Apache HTTP Server via mod_proxy_uwsgi. This issue affects Apache HTTP Server: from 2.4.30 through 2.4.55. Special characters in the origin response header can truncate/split the response forwarded to the client.
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Analysis
by VulDB Data Team • 01/04/2026
The CVE-2023-27522 vulnerability represents a critical HTTP response smuggling flaw within the Apache HTTP Server ecosystem, specifically impacting versions ranging from 2.4.30 through 2.4.55. This vulnerability manifests through the mod_proxy_uwsgi module, which serves as a proxy interface for communicating with uWSGI application servers. The flaw exploits how the server processes and forwards HTTP responses when special characters appear within origin response headers, creating a condition where response data can be inadvertently truncated or split during the forwarding process to end clients.
The technical mechanism behind this vulnerability stems from improper handling of special characters within HTTP header values during the proxy forwarding operation. When mod_proxy_uwsgi encounters certain special characters in response headers from upstream servers, the Apache server's internal parsing logic fails to properly manage the boundary conditions between response components. This mismanagement results in the HTTP response being fragmented or truncated, potentially allowing an attacker to inject malicious content or manipulate the response flow between the proxy server and the client. The vulnerability operates at the HTTP protocol level, specifically targeting the boundary conditions where response headers are processed and forwarded, making it particularly dangerous in environments where Apache serves as a reverse proxy for web applications.
The operational impact of this vulnerability extends beyond simple data corruption, as it creates potential attack vectors for various malicious activities including cache poisoning, cross-site scripting attacks, and session hijacking. An attacker could exploit this flaw to inject arbitrary content into responses, potentially redirecting users to malicious sites or injecting malicious scripts. The vulnerability is particularly concerning in production environments where Apache serves as a front-end proxy for multiple applications, as it could enable attackers to manipulate responses across various services. This issue falls under CWE-129, which addresses improper handling of HTTP headers, and aligns with ATT&CK technique T1190 for exploiting vulnerabilities in web applications. The vulnerability's classification as a response smuggling issue indicates its potential to disrupt the integrity of HTTP communication flows, making it a significant concern for organizations relying on Apache as their primary web server solution.
Mitigation strategies for CVE-2023-27522 require immediate attention through patching the affected Apache HTTP Server versions to the latest releases that contain the necessary fixes. Organizations should also implement network-level monitoring to detect anomalous HTTP response patterns that might indicate exploitation attempts. The recommended approach includes upgrading to Apache HTTP Server 2.4.56 or later, which contains the specific patches addressing the mod_proxy_uwsgi header handling issue. Additionally, implementing proper input validation and sanitization measures within the proxy configuration can help reduce the attack surface, though these should be considered supplementary to the primary patching strategy. Security teams should conduct thorough testing of the patched environment to ensure that the mitigation does not introduce compatibility issues with existing applications. Organizations utilizing mod_proxy_uwsgi should also review their current proxy configurations to identify any potential exposure and consider implementing additional security controls such as request/response inspection at the network level. The vulnerability's nature suggests that organizations should also implement comprehensive logging and monitoring of proxy operations to detect potential exploitation attempts.