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NVD / CVE Program· CNA VulnCheck

CVE-2026-55203: HAProxy - Integer Overflow in FCGI Demux Record Length Field

Noroxi assessment

Priority: Monitor (action score 36/100)

  • The sources we track (CISA KEV, Exploit-DB, Metasploit, Nuclei, GitHub) show no public exploit code or exploitation in the wild.
  • Probability of exploitation in the next 30 days: 0.33% (FIRST EPSS).
  • Anyone who can reach it over the internet can trigger it. No account or password is required. No user action is required. No special conditions are required; it is repeatable.
  • Fixed version: azl3 haproxy 2.9.11-6 on Azure Linux 3.0 2.9.11-7; azl3 haproxy 2.9.11-7 on Azure Linux 3.0 2.9.11-7.

Low priority for now. Upgrade to azl3 haproxy 2.9.11-6 on Azure Linux 3.0 2.9.11-7; azl3 haproxy 2.9.11-7 on Azure Linux 3.0 2.9.11-7 or later.

Generated automatically from the record's data: CVSS (NVD and CNA), CISA KEV and SSVC, FIRST EPSS, exploit sources, fixed versions. How we calculate it

Primary sources: NVD · CVE.org · FIRST EPSS

HAProxy through 3.4.0, fixed in commit 5985276, contains an integer overflow vulnerability in the fcgi_conn structure's drl field that allows buffer misparse as new FCGI record headers. When contentLength is 65535 and paddingLength is 1 or more, the drl field wraps to 0, causing incorrect record consumption and allowing malicious FastCGI backends to desynchronize the FCGI framing parser, potentially causing request routing errors, response smuggling, or memory safety issues.

CriticalCVSS 9.0 · v4.0—No exploit Fix available
Published
Jun 18, 2026
Updated
Oct 26, 2026
EPSS
0.3% · 26th percentile

Report tools

JSON

Action score

36

Monitor

Low priority for now.

CVSS
36 / 40 · 9.0 / 10
CISA KEV
0 / 30 · Not listed
EPSS
0 / 30 · 0.3%

CISA SSVC decision

Exploitation
none
Automatable
no
Technical impact
partial

Vulnrichment: CISA's decision-tree inputs.

CNA vs NVD score

NVD
—
CNA · VulnCheck
9.0
NVD has not scored this yet; the score shown is the CNA’s.

CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:H/VA:N/SC:L/SI:H/SA:N

The score given by the assigning authority versus NVD’s independent score. A gap means the severity is contested.

Get told when this record changes

  • Not in KEV: no record of exploitation in the wild
  • No public exploit code on record
  • A fix has been released

If any of these changes, we'll tell you in the daily email digest.

Follow CVE-2026-55203

When you'll hear from us

  • when it enters CISA KEV
  • when exploit code appears or is weaponized
  • when the record is updated (including a fixed version)
  • when EPSS rises 20 points in a week

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Noroxi analysis

What does it mean in your environment?

The assessment above is generated automatically from the record's data; it doesn't know your environment. Our team verifies on your systems whether this vulnerability is actually exploitable and which mitigation is enough.

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Affected systems

Is your version affected?

Pick the product and type the version you run. It's compared with this record's version ranges in your browser; the version isn't sent anywhere.

VendorProduct
haproxyhaproxy

Affected versions

NVD version ranges (for catalog products). Add the product to your stack with a version and matching uses these.

  • haproxy haproxybefore 2.4.36
  • haproxy haproxy2.5 and later · before 2.6.30
  • haproxy haproxy2.7 and later · before 2.8.25
  • haproxy haproxy2.9 and later · before 3.0.24
  • haproxy haproxy3.1 and later · before 3.2.20
  • haproxy haproxy3.3 and later · before 3.4.1

Versions reported by the vendor

Affected version ranges reported by the assigning authority (VulnCheck). Independent of NVD's CPE analysis and usually ahead of it.

  • haproxy haproxy

    • 5985276735777634d8c85f1d73bb7764aab0d6ddnot affected
    • up to and including 3.4.0affected · semver

Package-level exposure

OSV and GitHub Advisory data: ecosystem, package and range. SBOM matching uses this table.

EcosystemPackageAffected rangeFix
AlmaLinux:10haproxybefore 3.0.5-6.el10_2.23.0.5-6.el10_2.2
AlmaLinux:9haproxybefore 2.8.14-3.el9_8.22.8.14-3.el9_8.2
Debian:12haproxyfrom 0—
Debian:14haproxybefore 3.2.20-13.2.20-1
openSUSE:Leap 16.0haproxybefore 3.2.21+git0.dbe43be37-160000.1.13.2.21+git0.dbe43be37-160000.1.1
openSUSE:Tumbleweedhaproxybefore 3.4.0+git31.fc300e9f2-1.13.4.0+git31.fc300e9f2-1.1
Red Hat:enterprise_linux_eus:10.0haproxybefore 0:3.0.5-4.el10_0.30:3.0.5-4.el10_0.3
Red Hat:enterprise_linux_eus:10.0haproxy-debuginfobefore 0:3.0.5-4.el10_0.30:3.0.5-4.el10_0.3
Red Hat:enterprise_linux_eus:10.0haproxy-debugsourcebefore 0:3.0.5-4.el10_0.30:3.0.5-4.el10_0.3
Red Hat:enterprise_linux:10.2haproxybefore 0:3.0.5-6.el10_2.20:3.0.5-6.el10_2.2
Red Hat:enterprise_linux:10.2haproxy-debuginfobefore 0:3.0.5-6.el10_2.20:3.0.5-6.el10_2.2
Red Hat:enterprise_linux:10.2haproxy-debugsourcebefore 0:3.0.5-6.el10_2.20:3.0.5-6.el10_2.2
Red Hat:enterprise_linux:9::appstreamhaproxybefore 0:2.8.14-3.el9_8.20:2.8.14-3.el9_8.2
Red Hat:enterprise_linux:9::appstreamhaproxy-debuginfobefore 0:2.8.14-3.el9_8.20:2.8.14-3.el9_8.2
Red Hat:enterprise_linux:9::appstreamhaproxy-debugsourcebefore 0:2.8.14-3.el9_8.20:2.8.14-3.el9_8.2
Red Hat:hummingbird:1haproxybefore 0:3.0.25-0.1.hum10:3.0.25-0.1.hum1
Red Hat:rhel_e4s:9.2::appstreamhaproxybefore 0:2.4.17-6.el9_2.50:2.4.17-6.el9_2.5
Red Hat:rhel_e4s:9.2::appstreamhaproxy-debuginfobefore 0:2.4.17-6.el9_2.50:2.4.17-6.el9_2.5
Red Hat:rhel_e4s:9.2::appstreamhaproxy-debugsourcebefore 0:2.4.17-6.el9_2.50:2.4.17-6.el9_2.5
Red Hat:rhel_e4s:9.4::appstreamhaproxybefore 0:2.4.22-3.el9_4.30:2.4.22-3.el9_4.3
Red Hat:rhel_e4s:9.4::appstreamhaproxy-debuginfobefore 0:2.4.22-3.el9_4.30:2.4.22-3.el9_4.3
Red Hat:rhel_e4s:9.4::appstreamhaproxy-debugsourcebefore 0:2.4.22-3.el9_4.30:2.4.22-3.el9_4.3
Red Hat:rhel_eus:9.6::appstreamhaproxybefore 0:2.4.22-4.el9_6.30:2.4.22-4.el9_6.3
Red Hat:rhel_eus:9.6::appstreamhaproxy-debuginfobefore 0:2.4.22-4.el9_6.30:2.4.22-4.el9_6.3

+8

Same product

haproxy: all records

Other highest-scoring records for the same primary product.

  • CVE-2020-11100In hpack_dht_insert in hpack-tbl.c in the HPACK decoder in HAProxy 1.8 through 2.x before 2.1.4, a remote attacker can write arbitrary bytes
    53Plan
  • CVE-2019-14241HAProxy through 2.0.2 allows attackers to cause a denial of service (ha_panic) via vectors related to htx_manage_client_side_cookies in prot
    51Plan
  • CVE-2021-40346An integer overflow exists in HAProxy 2.0 through 2.5 in htx_add_header that can be exploited to perform an HTTP request smuggling attack, a
    48Plan

Published before and after this one in the same product

  • ← EarlierCVE-2025-32464Apr 8, 2025 · HAProxy 2.2 through 3.1.6, in certain uncommon configurations, has a sample_conv_regsub heap-based buffer overflow because of mishandling of
    27Monitor
  • ← EarlierCVE-2026-33555Apr 13, 2026 · An issue was discovered in HAProxy before 3.3.6.
    23Monitor
  • Later →CVE-2026-55204Jun 18, 2026 · HAProxy - NULL Pointer Dereference in hpack_dht_insert Function
    34Monitor

Remediation

Which version to upgrade to

Fix versions compiled from the vendor, package registries and Microsoft. Verify the vendor's note before upgrading.

Product / packageFixed versionSource
azl3 haproxy 2.9.11-6 on Azure Linux 3.02.9.11-7 · CBL-Mariner ReleasesMicrosoft (MSRC)
azl3 haproxy 2.9.11-7 on Azure Linux 3.02.9.11-7 · CBL-Mariner ReleasesMicrosoft (MSRC)
almalinux:haproxy3.0.5-6.el10_2.2 · AlmaLinux:10Package registry (OSV)
debian:haproxy3.2.20-1 · Debian:14Package registry (OSV)
opensuse:haproxy3.4.0+git31.fc300e9f2-1.1 · openSUSE:TumbleweedPackage registry (OSV)
red hat:haproxy0:3.0.25-0.1.hum1 · Red Hat:hummingbird:1Package registry (OSV)
red hat:haproxy-debuginfo0:3.0.5-6.el10_2.2 · Red Hat:enterprise_linux:10.2Package registry (OSV)
red hat:haproxy-debugsource0:3.0.5-6.el10_2.2 · Red Hat:enterprise_linux:10.2Package registry (OSV)
rocky linux:haproxy0:3.0.5-6.el10_2.2 · Rocky Linux:10Package registry (OSV)
suse:haproxy2.8.11+git0.01c1056a4-4.1 · SUSE:Linux Micro 6.0Package registry (OSV)

Exploit status

No known public exploit

No public exploit has been observed yet. That doesn't mean you're safe, only that the bar is a little higher.

Research context

For pentesters and researchers: attack profile, score disagreement, timeline, patch commits, credits, variant and chain candidates, bug bounty scope. All derived from existing data; no exploit code.

Timeline

From publication to today: proof of concept, Metasploit module, CISA KEV and fix record. Dates are as reported by the sources.

No dated events beyond publication.

No history yet (it accumulates with the daily EPSS job).

Patch and commit links

Commit, PR and diff links among the references. A starting point for patch-diffing and variant hunting; fixes, not exploits.

Credits

All researchers

Finders, reporters and analysts named in the CNA record. Click a name for that researcher’s other records.

Variant candidates

Same product, same weakness class, within 18 months. If the patch missed the root cause, the sibling bug is here.

No nightly-computed relations.

Chain candidates

An authentication bypass and a privilege-requiring bug in the same product, published close together: combined they may become an unauthenticated path.

—

Bug bounty scope

No known public program.

Source: bounty-targets-data (public HackerOne, Bugcrowd, Intigriti, YesWeHack listings).

Technical details

Attack conditions

  • Anyone who can reach it over the internet can trigger it.
  • No account or password is required.
  • No user action is required.
  • No special conditions are required; it is repeatable.

If successful

Confidentiality
Integrity
Availability
Attack vector
Network
Attack complexity
Low
Privileges required
None
User interaction
None
Scope
X

Weakness class (CWE)

CWE-190 · Integer Overflow or Wraparound

CVSS vector

CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:H/VA:N/SC:L/SI:H/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

nvd-secondary

Attack context

MITRE CAPEC attack patterns and ATT&CK techniques for this weakness class (CWE). A starting point for detection rules and threat hunting.

ATT&CK techniques

—

Change log

  1. Fix✗ → ✓

For records you follow, these changes also arrive as notifications. →

References

Show all references (1 more)

Vendor advisories and official records. Exploit/PoC links are deliberately left out.

All records