CVE-2023-25950: HAProxy HTTP Smuggling DoS Vulnerability

CVE-2023-25950 Overview

CVE-2023-25950 is an HTTP request/response smuggling vulnerability affecting HAProxy version 2.7.0, and versions 2.6.1 to 2.6.7. This vulnerability allows a remote attacker to alter a legitimate user's request, potentially leading to the disclosure of sensitive information or causing a denial-of-service (DoS) condition. As HAProxy is widely deployed as a load balancer and reverse proxy in enterprise environments, this vulnerability poses significant risk to organizations relying on it for traffic management and security.

Critical Impact

Remote attackers can manipulate HTTP requests to bypass security controls, access sensitive data, or disrupt service availability through request smuggling techniques.

Affected Products

• HAProxy version 2.7.0
• HAProxy versions 2.6.1 through 2.6.7

Discovery Timeline

• 2023-04-11 - CVE-2023-25950 published to NVD
• 2025-02-11 - Last updated in NVD database

Technical Details for CVE-2023-25950

Vulnerability Analysis

This vulnerability is classified as CWE-444 (Inconsistent Interpretation of HTTP Requests), commonly known as HTTP Request Smuggling. HTTP request smuggling occurs when front-end and back-end servers interpret the boundaries of HTTP requests differently, allowing attackers to inject malicious content into legitimate requests.

In the context of HAProxy, the vulnerable versions improperly handle certain HTTP request structures, creating a desynchronization between how HAProxy processes requests and how backend servers interpret them. This inconsistency can be exploited to smuggle additional HTTP requests through the proxy, bypassing security controls and potentially accessing protected resources.

The attack can be executed remotely over the network without requiring authentication, making it accessible to any attacker who can send HTTP traffic to the affected HAProxy instance. Successful exploitation can lead to confidentiality breaches through unauthorized data access, integrity violations through request manipulation, and availability impacts through denial-of-service conditions.

Root Cause

The root cause of this vulnerability lies in HAProxy's HTTP request parsing logic, where inconsistent interpretation of request boundaries allows for request desynchronization. This typically involves ambiguous handling of Content-Length and Transfer-Encoding headers, or improper processing of malformed HTTP requests that should be rejected but are instead forwarded to backend servers with different interpretations.

Attack Vector

The attack vector for CVE-2023-25950 is network-based, allowing remote attackers to exploit the vulnerability by sending specially crafted HTTP requests to the HAProxy instance. The attacker constructs HTTP requests that exploit parsing inconsistencies between HAProxy and backend servers. By carefully manipulating request headers, the attacker can cause HAProxy to interpret request boundaries differently than the backend server.

This allows the attacker to "smuggle" a secondary request within what HAProxy perceives as a single request. The smuggled request is then processed by the backend server as a separate, legitimate request. This technique can be used to bypass access controls, steal user credentials, poison web caches, or hijack other users' sessions.

Detection Methods for CVE-2023-25950

Indicators of Compromise

• Unusual HTTP traffic patterns with mismatched Content-Length and Transfer-Encoding headers
• Unexpected request sequences in backend server logs that don't correspond to HAProxy access logs
• Detection of multiple HTTP requests within a single TCP connection where only one was expected
• Web cache poisoning incidents or unexpected content being served to users

Detection Strategies

• Implement deep packet inspection to identify HTTP request smuggling patterns, particularly requests with conflicting Content-Length and Transfer-Encoding headers
• Deploy web application firewalls (WAF) with HTTP desync detection capabilities
• Monitor for discrepancies between HAProxy access logs and backend server logs that may indicate request manipulation
• Utilize network intrusion detection systems (IDS) with signatures for HTTP smuggling techniques

Monitoring Recommendations

• Enable detailed HTTP request logging on both HAProxy and backend servers to facilitate correlation analysis
• Implement alerting for requests containing both Content-Length and Transfer-Encoding headers
• Monitor for sudden increases in 4xx or 5xx error responses that may indicate exploitation attempts
• Track connection reuse patterns and flag unusual request sequences within persistent connections

How to Mitigate CVE-2023-25950

Immediate Actions Required

• Upgrade HAProxy to version 2.7.1 or later if running version 2.7.0
• Upgrade HAProxy to version 2.6.8 or later if running versions 2.6.1 through 2.6.7
• Review HAProxy configurations to ensure strict HTTP mode is enabled where possible
• Implement additional security layers such as WAF rules to detect and block request smuggling attempts

Patch Information

HAProxy has released patches addressing this vulnerability. The fix is available in the HAProxy Git repository. Organizations should update to the latest stable release of their respective HAProxy branch. Additional details can be found in the JVN Vulnerability Report and on the HAProxy Official Website.

Workarounds

• Configure HAProxy to reject requests containing both Content-Length and Transfer-Encoding headers using ACL rules
• Enable HTTP protocol normalization to standardize incoming requests before forwarding to backend servers
• Implement strict request validation using HAProxy's built-in capabilities to reject malformed or ambiguous requests
• Consider implementing a secondary security layer or reverse proxy with robust HTTP parsing to filter traffic before it reaches the vulnerable HAProxy instance

# Example HAProxy configuration to mitigate request smuggling
# Add these rules to the frontend or backend configuration

# Reject requests with both Content-Length and Transfer-Encoding headers
http-request deny if { req.hdr(content-length) -m found } { req.hdr(transfer-encoding) -m found }

# Enable strict HTTP mode
option http-use-htx

# Normalize incoming requests
option httplog
option http-server-close