CVE Vulnerabilities


Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling')

Published: Oct 23, 2019 | Modified: Nov 07, 2023
CVSS 3.x
CVSS 2.x

A flaw was found in HAProxy before 2.0.6. In legacy mode, messages featuring a transfer-encoding header missing the chunked value were not being correctly rejected. The impact was limited but if combined with the http-reuse always setting, it could be used to help construct an HTTP request smuggling attack against a vulnerable component employing a lenient parser that would ignore the content-length header as soon as it saw a transfer-encoding one (even if not entirely valid according to the specification).


The product acts as an intermediary HTTP agent (such as a proxy or firewall) in the data flow between two entities such as a client and server, but it does not interpret malformed HTTP requests or responses in ways that are consistent with how the messages will be processed by those entities that are at the ultimate destination.

Affected Software

Name Vendor Start Version End Version
Haproxy Haproxy * 2.0.6 (excluding)
Red Hat Enterprise Linux 8 RedHat haproxy-0:1.8.23-3.el8 *
Red Hat OpenShift Container Platform 3.11 RedHat haproxy-0:1.8.23-3.el7 *
Red Hat OpenShift Container Platform 4.4 RedHat haproxy-0:2.0.13-3.el7 *
Red Hat Software Collections for Red Hat Enterprise Linux 7 RedHat rh-haproxy18-haproxy-0:1.8.24-2.el7 *
Red Hat Software Collections for Red Hat Enterprise Linux 7.6 EUS RedHat rh-haproxy18-haproxy-0:1.8.24-2.el7 *
Red Hat Software Collections for Red Hat Enterprise Linux 7.7 EUS RedHat rh-haproxy18-haproxy-0:1.8.24-2.el7 *
Haproxy Ubuntu bionic *
Haproxy Ubuntu disco *
Haproxy Ubuntu eoan *
Haproxy Ubuntu esm-infra/bionic *
Haproxy Ubuntu esm-infra/xenial *
Haproxy Ubuntu trusty *
Haproxy Ubuntu upstream *
Haproxy Ubuntu xenial *

Extended Description

HTTP requests or responses (“messages”) can be malformed or unexpected in ways that cause web servers or clients to interpret the messages in different ways than intermediary HTTP agents such as load balancers, reverse proxies, web caching proxies, application firewalls, etc. For example, an adversary may be able to add duplicate or different header fields that a client or server might interpret as one set of messages, whereas the intermediary might interpret the same sequence of bytes as a different set of messages. For example, discrepancies can arise in how to handle duplicate headers like two Transfer-encoding (TE) or two Content-length (CL), or the malicious HTTP message will have different headers for TE and CL. The inconsistent parsing and interpretation of messages can allow the adversary to “smuggle” a message to the client/server without the intermediary being aware of it. This weakness is usually the result of the usage of outdated or incompatible HTTP protocol versions in the HTTP agents.

Potential Mitigations