Apache HTTP Server is one of the most popular and fundamental web server technologies on the Internet, responsible for delivering content for millions of websites and applications. Its stability and performance make it a critical component of modern web infrastructure.

This vulnerability poses a high risk, as it allows an attacker to inject malicious content into responses sent to users, leading to attacks such as Cross-Site Scripting (XSS), web cache poisoning, and session theft. The primary attack vector requires the ability to influence headers generated by a backend application (e.g., PHP, Java, CGI scripts) that is routed through a vulnerable Apache instance.

Although there are no public reports of active exploitation for this specific CVE, HTTP Response Splitting is a classic and well-understood attack technique. Organizations using Apache HTTP Server as a reverse proxy for other applications are particularly at risk, especially if the backend applications do not strictly sanitize user-provided input before inserting it into response headers. A successful exploit could lead to widespread user account compromise or website defacement if a malicious response is cached by an intermediate proxy or CDN.

Technical Summary

The root cause of the CVE-2023-38709 vulnerability is a case of CWE-113: Improper Neutralization of CRLF Sequences in HTTP Headers (‘HTTP Response Splitting’). The vulnerability exists in how Apache HTTP Server handles responses generated by backend applications or content generators (such as CGI, PHP, or proxy application servers). Apache does not properly sanitize headers coming from these backend responses for carriage return (CR, \r) and line feed (LF, \n) characters.

The attack chain unfolds as follows:

1. An attacker identifies a vector to inject data into an HTTP response header generated by an application located behind the Apache server. This could be a parameter reflected in a Location header during a redirect or in a Set-Cookie header.
2. The attacker creates an input string containing the CRLF sequence (URL-encoded %0d%0a).
3. The backend application generates the response with the malicious header and passes it to Apache.
4. The vulnerable Apache server forwards the response to the client without removing the malicious CRLF sequence.
5. The client’s browser or an intermediate cache proxy interprets the CRLF sequence as the end of the legitimate server headers. The data following the CRLF sequence is then interpreted as a new, complete HTTP response under the attacker’s control.

A conceptual example of a malicious header generated by a backend application could be:

HTTP/1.1 302 Found
Location: /index.php?lang=en%0d%0aContent-Type:%20text/html%0d%0aContent-Length:%2025%0d%0a%0d%0a%3chtml%3eXSS-HERE%3c/html%3e

This allows an attacker to perform cache poisoning, Cross-Site Scripting (XSS) via the injection of a malicious HTML body, or session fixation attacks by injecting malicious Set-Cookie headers.

The vulnerability affects Apache HTTP Server versions 2.4.58 and earlier. A fix that properly sanitizes response headers from backend services is available in subsequent versions.

Recommendations

• Apply the patch immediately: Update all Apache HTTP Server instances to version 2.4.59 or higher to address this vulnerability.
• Defense-in-Depth: While patching Apache is critical, the exploit requires a vulnerable backend application. Audit the code of backend applications to ensure they perform strict input validation and do not reflect user-controllable data in HTTP response headers. This is the most effective security control for this class of vulnerability.
• Mitigation: If patching immediately is not possible, deploy a Web Application Firewall (WAF) with specific rules to detect and block CRLF injection attempts within HTTP requests that could be forwarded to backend systems.
• Detection and Monitoring: Actively monitor web server and application logs. Look for encoded carriage return (%0d) and line feed (%0a) characters in parameters known to be reflected in response headers. Monitor for anomalous response sizes or headers that may indicate a successful split.
• Incident Response: In case of suspected compromise, immediately flush all reverse proxy and CDN caches to eliminate any poisoned content. Invalidate active user sessions to mitigate potential account theft and rotate potentially compromised credentials.

[Callforaction-THREAT-Footer]