CVE-2026-55204 — haproxy
Fix: haproxy/haproxy@9a6d1feCVE-2026-55204 is a NULL Pointer Dereference vulnerability. A fix is available — see the affected versions and patch details below.
HAProxy - NULL Pointer Dereference in hpack_dht_insert Function
Exploitation Status
No confirmed exploitation observed yet
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-55204.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
Description
HAProxy through 3.4.0, fixed in commit 9a6d1fe, contains a null pointer dereference vulnerability in hpack_dht_insert() within src/hpack-tbl.c that fails to validate the return value of hpack_dht_defrag() when the memory pool is exhausted. An attacker can trigger HPACK dynamic table insertions under memory pressure to dereference a NULL pointer and crash HAProxy worker processes, causing denial of service.
Affected Products
haproxyhaproxyDetection & mitigation playbook
VulnerabilityDetect
Identify every host running the affected component and compare the installed build against the fixed version below — for source-built or distro-packaged software the version string, not a lockfile, is the source of truth (`dpkg -l`, `rpm -q`, or the binary's own `--version`).
Fix
Upgrade the affected component to the fixed release for CVE-2026-55204, or apply your distribution's backported patch — distro builds are often patched at an older version number, so check your vendor's advisory rather than the upstream version alone.
Workarounds
Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.
Fixing This On Your OS
If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.
This Important flaw in HAProxy can lead to a denial of service. An unauthenticated remote attacker could exploit a null pointer dereference by triggering HPACK dynamic table insertions under memory pressure, causing HAProxy worker processes to crash. This impacts the availability of services relying on HAProxy as a…
Mitigation for this issue is either not available or the currently available options do not meet the Red Hat Product Security criteria comprising ease of use and deployment, applicability to widespread installation base, or stability.Source: Red Hat security advisory for CVE-2026-55204 (CC BY 4.0)
| Product | Fixed in | Advisory |
|---|---|---|
| Red Hat Enterprise Linux 10 | haproxy-0:3.0.5-6.el10_2.2 | RHSA-2026:55679 |
| Red Hat Enterprise Linux 10.0 Extended Update Support | haproxy-0:3.0.5-4.el10_0.3 | RHSA-2026:55800 |
| Red Hat Enterprise Linux 8 | haproxy-0:1.8.27-5.el8_10.2 | RHSA-2026:55859 |
| Red Hat Enterprise Linux 8.4 Advanced Mission Critical Update Support | haproxy-0:1.8.27-2.el8_4.2 | RHSA-2026:55861 |
| Red Hat Enterprise Linux 8.6 Advanced Mission Critical Update Support | haproxy-0:1.8.27-4.el8_6.2 | RHSA-2026:55862 |
| Red Hat Enterprise Linux 8.8 Telecommunications Update Service | haproxy-0:1.8.27-5.el8_8.2 | RHSA-2026:55860 |
| Red Hat Enterprise Linux 9 | haproxy-0:2.8.14-3.el9_8.2 | RHSA-2026:55772 |
| Red Hat Enterprise Linux 9.2 Update Services for SAP Solutions | haproxy-0:2.4.17-6.el9_2.5 | RHSA-2026:55802 |
Frequently Asked Questions
Is CVE-2026-55204 in your dependencies?
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