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Vulnerability record · CVE-2019-18277 · published 23 October 2019

CVE-2019-18277: Haproxy http request smuggling vulnerability

Haproxy · Haproxy

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).

7.5 CVSS 3.1 High EPSS 10% · top 4.5% CWE-444 · HTTP request smuggling
7.5CVSS 3.1 base score, v2 4.3
10%EPSS exploitation probability, 30 days
NoNot in CISA KEV
1Affected product versions listed by NVD
14References, 2 tagged exploit
17 Jun 2026Last modified by NVD

Description

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).

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

Affected products

1 vulnerable configurations from NVD's CPE data, grouped by vendor and product.

References

LinkTags
http://lists.opensuse.org/opensuse-security-announce/2019-12/msg00016.html
http://lists.opensuse.org/opensuse-security-announce/2019-12/msg00019.html
https://git.haproxy.org/?p=haproxy-2.0.git%3Ba=commit%3Bh=196a7df44d8129d1adc795da020b722614d6a581
https://lists.debian.org/debian-lts-announce/2022/05/msg00045.html
https://nathandavison.com/blog/haproxy-http-request-smuggling ExploitThird Party Advisory
https://usn.ubuntu.com/4174-1/
https://www.mail-archive.com/haproxy%40formilux.org/msg34926.html
http://lists.opensuse.org/opensuse-security-announce/2019-12/msg00016.html
http://lists.opensuse.org/opensuse-security-announce/2019-12/msg00019.html
https://git.haproxy.org/?p=haproxy-2.0.git%3Ba=commit%3Bh=196a7df44d8129d1adc795da020b722614d6a581
https://lists.debian.org/debian-lts-announce/2022/05/msg00045.html
https://nathandavison.com/blog/haproxy-http-request-smuggling ExploitThird Party Advisory
https://usn.ubuntu.com/4174-1/
https://www.mail-archive.com/haproxy%40formilux.org/msg34926.html

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Related vulnerabilities

Same products first, then exploited flaws of the same weakness class.

9.8CVE-2019-19330Haproxy injection vulnerabilityThe HTTP/2 implementation in HAProxy before 2.0.10 mishandles headers, as demonstrated by carriage return (CR, ASCII 0xd), line feed (LF, ASCII 0xa),…EPSS 4.0%9.1CVE-2023-25725Haproxy http request smuggling vulnerabilityHAProxy before 2.7.3 may allow a bypass of access control because HTTP/1 headers are inadvertently lost in some situations, aka "request smuggling." …EPSS 5.4%9.0CVE-2026-55203Haproxy integer overflow vulnerabilityHAProxy through 3.4.0, fixed in commit 5985276, contains an integer overflow vulnerability in the fcgi_conn structure's drl field that allows buffer …EPSS 0.58%8.8CVE-2020-11100HAProxy HPACK decoder heap out-of-bounds writeThe HPACK decoder in HAProxy 1.8 through 2.x before 2.1.4 contains an out-of-bounds write in hpack_dht_insert in hpack-tbl.c. A crafted HTTP/2 reques…EPSS 61%analysed8.7CVE-2026-55204Haproxy null pointer dereference vulnerabilityHAProxy through 3.4.0, fixed in commit 9a6d1fe, contains a null pointer dereference vulnerability in hpack_dht_insert() within src/hpack-tbl.c that f…EPSS 0.48%8.2CVE-2023-45539Haproxy vulnerabilityHAProxy before 2.8.2 accepts # as part of the URI component, which might allow remote attackers to obtain sensitive information or have unspecified o…EPSS 1.5%7.5CVE-2025-11230Haproxy aloha appliance vulnerabilityInefficient algorithm complexity in mjson in HAProxy allows remote attackers to cause a denial of service via specially crafted JSON requests.EPSS 0.69%7.5CVE-2024-45506Haproxy vulnerabilityHAProxy 2.9.x before 2.9.10, 3.0.x before 3.0.4, and 3.1.x through 3.1-dev6 allows a remote denial of service for HTTP/2 zero-copy forwarding (h2_sen…EPSS 1.2%

Source: NIST National Vulnerability Database (record CVE-2019-18277), CISA KEV, FIRST EPSS (scores of 2026-09-26). This page is refreshed as NVD updates the record.