CVE-2026-3234 Overview
A CRLF (Carriage Return Line Feed) injection vulnerability has been identified in mod_proxy_cluster, specifically within the decodeenc() function. This flaw allows remote attackers to bypass input validation by injecting CRLF sequences into cluster configuration data. Successful exploitation enables attackers to corrupt the response body of INFO endpoint responses. The vulnerability requires network access to the MCMP (Mod Cluster Management Protocol) protocol port but does not require authentication.
Critical Impact
Attackers with adjacent network access can manipulate cluster configuration responses without authentication, potentially leading to information corruption and integrity violations in load balancer configurations.
Affected Products
- mod_proxy_cluster (Apache HTTP Server module)
- Systems running MCMP protocol services
- Red Hat products using mod_proxy_cluster
Discovery Timeline
- 2026-03-12 - CVE CVE-2026-3234 published to NVD
- 2026-03-12 - Last updated in NVD database
Technical Details for CVE-2026-3234
Vulnerability Analysis
This vulnerability is classified as CWE-93 (Improper Neutralization of CRLF Sequences), commonly known as HTTP Response Splitting or CRLF Injection. The flaw resides in the decodeenc() function of mod_proxy_cluster, which fails to properly sanitize CRLF sequences in user-controlled input before processing cluster configuration data.
The attack requires adjacent network access to the MCMP protocol port, which is typically used for communication between the Apache HTTP Server and backend application servers in a clustered environment. Since no authentication is required to interact with the MCMP protocol, any attacker with network proximity can attempt exploitation.
When CRLF sequences are injected, they can corrupt the response body returned by INFO endpoint queries. This corruption could potentially be leveraged to manipulate cluster state information, inject malicious headers, or interfere with load balancing decisions.
Root Cause
The root cause of this vulnerability lies in insufficient input validation within the decodeenc() function. The function processes encoded data from MCMP protocol messages but does not properly filter or escape CRLF sequences (\r\n). This allows attackers to inject arbitrary line terminators that are interpreted as part of the HTTP response structure, breaking the expected response format and enabling response manipulation.
Attack Vector
The attack vector requires adjacent network access (AV:A), meaning the attacker must be on the same network segment or have routing access to the MCMP protocol port. The attack complexity is low, requiring no special privileges or user interaction. An attacker can craft malicious MCMP requests containing CRLF-encoded payloads targeting the decodeenc() function, which processes these requests and returns corrupted INFO endpoint responses.
The exploitation flow involves sending specially crafted cluster configuration data containing encoded CRLF sequences to the MCMP service. When the decodeenc() function processes this input, the injected sequences corrupt the response structure, potentially allowing the attacker to inject arbitrary content into HTTP responses.
Detection Methods for CVE-2026-3234
Indicators of Compromise
- Unusual MCMP protocol traffic containing encoded CRLF sequences (%0d%0a or %0D%0A)
- Malformed INFO endpoint responses with unexpected line breaks or injected content
- Log entries showing requests to MCMP endpoints with suspicious encoded characters
Detection Strategies
- Monitor network traffic to MCMP protocol ports for requests containing URL-encoded CRLF sequences
- Implement deep packet inspection rules to detect %0d%0a patterns in MCMP communications
- Review Apache HTTP Server logs for anomalous cluster management requests
- Deploy intrusion detection signatures for CRLF injection patterns targeting mod_proxy_cluster
Monitoring Recommendations
- Enable verbose logging for mod_proxy_cluster to capture detailed request information
- Configure network monitoring to alert on unusual traffic patterns to MCMP ports
- Implement baseline monitoring for INFO endpoint response sizes and formats to detect corruption
- Set up alerts for repeated failed or malformed MCMP protocol requests
How to Mitigate CVE-2026-3234
Immediate Actions Required
- Restrict network access to MCMP protocol ports using firewall rules to trusted hosts only
- Implement network segmentation to isolate cluster management traffic from untrusted networks
- Review and audit current mod_proxy_cluster configurations for exposure to adjacent networks
- Consider disabling the INFO endpoint if not required for operations
Patch Information
Red Hat has acknowledged this vulnerability and is tracking it via Red Hat CVE-2026-3234 Advisory. Additional details are available in Red Hat Bug Report #2442889. Organizations should monitor vendor advisories for updated packages that address this vulnerability.
Workarounds
- Apply strict firewall rules to limit MCMP protocol port access to trusted management hosts only
- Deploy a Web Application Firewall (WAF) or proxy in front of MCMP endpoints to filter CRLF injection attempts
- Consider using VPN or encrypted tunnels for cluster management traffic to prevent adjacent network attacks
- Implement input validation at the network perimeter for MCMP protocol communications
# Example: Restrict MCMP port access using iptables
# Allow only trusted management hosts to access MCMP port (default 6666)
iptables -A INPUT -p tcp --dport 6666 -s 10.0.1.100 -j ACCEPT
iptables -A INPUT -p tcp --dport 6666 -s 10.0.1.101 -j ACCEPT
iptables -A INPUT -p tcp --dport 6666 -j DROP
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