CVE-2023-40225 in HAProxy
Summary
by MITRE • 08/11/2023
HAProxy through 2.0.32, 2.1.x and 2.2.x through 2.2.30, 2.3.x and 2.4.x through 2.4.23, 2.5.x and 2.6.x before 2.6.15, 2.7.x before 2.7.10, and 2.8.x before 2.8.2 forwards empty Content-Length headers, violating RFC 9110 section 8.6. In uncommon cases, an HTTP/1 server behind HAProxy may interpret the payload as an extra request.
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Analysis
by VulDB Data Team • 05/05/2026
The vulnerability identified as CVE-2023-40225 affects HAProxy versions across multiple release branches including 2.0.32 and earlier, 2.1.x through 2.2.30, 2.3.x through 2.4.23, 2.5.x and 2.6.x before 2.6.15, 2.7.x before 2.7.10, and 2.8.x before 2.8.2. This issue represents a deviation from RFC 9110 section 8.6 which governs HTTP content length handling and establishes the proper protocol behavior for Content-Length headers. The flaw manifests when HAProxy processes HTTP requests containing empty Content-Length headers and subsequently forwards these malformed headers to backend servers, creating a potential disruption in normal HTTP communication patterns.
The technical implementation of this vulnerability stems from HAProxy's handling of HTTP header processing within its proxying mechanisms. When an HTTP client sends a request containing an empty Content-Length header, the proxy does not properly validate or normalize this header before forwarding it to backend servers. This behavior violates the HTTP/1.1 specification which requires that Content-Length headers be either present with a valid numeric value or absent when no content is expected. The RFC 9110 compliance failure creates an ambiguous state where backend servers may interpret the empty header as a legitimate but malformed request parameter, potentially leading to unexpected server behavior.
The operational impact of this vulnerability extends beyond simple protocol violation to potential service disruption and security implications. When an HTTP/1 server behind HAProxy encounters an empty Content-Length header, it may misinterpret the subsequent payload data as part of an additional HTTP request, leading to request confusion and potential parsing errors. This scenario can result in backend servers processing malformed requests that may cause application errors, resource exhaustion, or even potential denial of service conditions. The vulnerability is particularly concerning in environments where HAProxy serves as a critical traffic management component, as it can introduce instability in request processing flows and potentially enable exploitation through crafted malicious requests.
This vulnerability maps to CWE-200, which covers "Information Exposure Through Output with Sensitive Data," and CWE-119, "Improper Restriction of Operations within the Bounds of a Memory Buffer," as the improper handling of HTTP headers can lead to buffer overflows or memory corruption when backend servers attempt to process malformed requests. From an ATT&CK framework perspective, this vulnerability aligns with T1190 "Exploit Public-Facing Application" and T1071.004 "Application Layer Protocol: DNS" in scenarios where attackers might leverage the header manipulation to create request confusion. The vulnerability also relates to T1595.001 "Network Device CLI" and T1595.003 "Network Device Firmware" when considering how improper header handling might affect network infrastructure components that rely on proper HTTP protocol compliance.
Organizations should immediately upgrade to the patched versions of HAProxy to mitigate this vulnerability, with the specific version requirements being 2.6.15, 2.7.10, and 2.8.2 respectively for the affected release branches. Network administrators should implement monitoring for unusual HTTP header patterns and payload behavior that might indicate exploitation attempts. Additionally, defensive measures including HTTP header normalization policies, request validation rules, and backend server configuration hardening can help reduce the impact of this vulnerability. The patch addresses the core issue by implementing proper validation of Content-Length headers, ensuring that empty headers are either removed or properly normalized before forwarding to backend services, thereby maintaining compliance with RFC 9110 requirements.