Is CVE-2026-55204 exploited in the wild?

No. Junglewise Threat Intelligence has no report of CVE-2026-55204 being exploited in the wild.

How severe is CVE-2026-55204?

High severity, with a CVSS score of 7.5, published 18 June 2026.

Which products and versions are affected?

HAProxy Up to and including 3.4.0.

Is there a fix for CVE-2026-55204?

No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.

How is CVE-2026-55204 attacked?

It is attacked over the network, in a hosted service.

Am I affected by CVE-2026-55204?

Check whether you run HAProxy at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

CVE-2026-55204: HAProxy NULL pointer dereference in hpack_dht_insert

CVE-2026-55204 · Severity: high · CVSS 7.5 · Published 2026-06-18

HAProxy is a widely used load balancer and proxy server that manages web traffic for high-availability environments. A flaw in how it handles compressed web headers can allow a remote attacker to crash the service by sending specific requests during periods of high memory usage. This results in a denial-of-service (DoS) condition, potentially taking websites or applications offline.

Technologies: HAProxy. Vendors: HAProxy.

Quick answers

Is CVE-2026-55204 exploited in the wild?: No. Junglewise Threat Intelligence has no report of CVE-2026-55204 being exploited in the wild.
How severe is CVE-2026-55204?: High severity, with a CVSS score of 7.5, published 18 June 2026.
Which products and versions are affected?: HAProxy Up to and including 3.4.0.
Is there a fix for CVE-2026-55204?: No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.
How is CVE-2026-55204 attacked?: It is attacked over the network, in a hosted service.
Am I affected by CVE-2026-55204?: Check whether you run HAProxy at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

Technical details

A NULL pointer dereference exists in the hpack_dht_insert() function within src/hpack-tbl.c. The vulnerability occurs because the code fails to validate the return value of hpack_dht_defrag() when the memory pool (pool_head_hpack_tbl) is exhausted. An attacker can exploit this by triggering HPACK dynamic table insertions under memory pressure, leading the application to dereference a NULL pointer plus an offset. This results in a SIGSEGV and a crash of the HAProxy worker process. The issue is fixed in commit 9a6d1fe and affects versions through 3.4.0.

Affected products

HAProxy HAProxy Up to and including 3.4.0

Timeline

2026-06-18: disclosed
2026-06-18: advisory

References

This page as Markdown. Free to reuse under CC BY 4.0 with a link back.