How severe is CVE-2026-55203?

CVE-2026-55203 has a CVSS 3.x base score of 7.5, rated high severity. It is exploitable over network with high attack complexity, requires no privileges and no user interaction. Impact on confidentiality is low, integrity high, and availability none.

Is CVE-2026-55203 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (24th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2026-55203?

CVE-2026-55203 affects Haproxy. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2026-55203?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.

Does CVE-2026-55203 have an EU (EUVD) identifier?

Yes. CVE-2026-55203 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-37905.

When was CVE-2026-55203 published?

CVE-2026-55203 was published on 2026-06-18 and last updated on 2026-06-26.

CVE-2026-55203

CVE-2026-55203 is a high-severity vulnerability in Haproxy with a CVSS 3.x base score of 7.5. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-190.

Key facts

Severity: High (CVSS 3.x base score 7.5)
CVSS v4: 9.0
EPSS exploit prediction: 0% (24th percentile)
Actively exploited: Not listed in CISA KEV
EU (EUVD) id: EUVD-2026-37905
Weakness: CWE-190
Affected product: Haproxy
Published: 2026-06-18
Last modified: 2026-06-26

Description

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.

Frequently asked questions

What is CVE-2026-55203?: 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.
How severe is CVE-2026-55203?: CVE-2026-55203 has a CVSS 3.x base score of 7.5, rated high severity. It is exploitable over network with high attack complexity, requires no privileges and no user interaction. Impact on confidentiality is low, integrity high, and availability none.
Is CVE-2026-55203 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (24th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2026-55203?: CVE-2026-55203 affects Haproxy. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2026-55203?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.
Does CVE-2026-55203 have an EU (EUVD) identifier?: Yes. CVE-2026-55203 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-37905.
When was CVE-2026-55203 published?: CVE-2026-55203 was published on 2026-06-18 and last updated on 2026-06-26.

References

Affected products (1)

cpe:2.3:a:haproxy:haproxy:*:*:*:*:*:*:*:*

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