CVE-2021-39242

Severity: high (CVSS 7.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 2.2% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2021-08-17 · updated 2026-06-17
Weakness: CWE-755

Description

An issue was discovered in HAProxy 2.2 before 2.2.16, 2.3 before 2.3.13, and 2.4 before 2.4.3. It can lead to a situation with an attacker-controlled HTTP Host header, because a mismatch between Host and authority is mishandled.

Affected products

Debian Linux: 11.0
Fedoraproject Fedora: 33, 34
HAProxy: from 2.2.0 before 2.2.16; from 2.3.0 before 2.3.13; from 2.4.0 before 2.4.3 — fixed in 2.2.16, 2.3.13, 2.4.3

How to fix

Upgrade to a fixed version: HAProxy 2.2.16, 2.3.13, 2.4.3. Check the vendor advisory for mitigations.

References

https://git.haproxy.org/?p=haproxy.git%3Ba=commit%3Bh=b5d2b9e154d78e4075db163826c5e0f6d31b2ab1
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/4ALECUZDIMT5FYGP6V6PVSI4BKVZTZWN/
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/RPNY4WZIQUAUOCLIMUPC37AQWNXTWIQM/
https://www.debian.org/security/2021/dsa-4960
https://www.mail-archive.com/haproxy%40formilux.org/msg41041.html
https://git.haproxy.org/?p=haproxy.git%3Ba=commit%3Bh=b5d2b9e154d78e4075db163826c5e0f6d31b2ab1
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/4ALECUZDIMT5FYGP6V6PVSI4BKVZTZWN/
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/RPNY4WZIQUAUOCLIMUPC37AQWNXTWIQM/
https://www.debian.org/security/2021/dsa-4960
https://www.mail-archive.com/haproxy%40formilux.org/msg41041.html

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