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

highCVSS 7.5CVE-2026-55203

CSIRTS triage

vendor: HAProxy product: HAProxyOther

What: An integer overflow vulnerability exists in the FCGI demux record length field.
Who is affected: Deployments using HAProxy are affected.
Urgency: Remediation is urgent due to high severity and potential for exploitation.
Action: Update to the patched version for CVE-2026-55203.

AI-assisted analysis generated from the source advisory — verify against the original.

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Details

Source: Microsoft Security Response Center (INTL · vendor-psirt · site)
Severity: high — CVSS 7.5
Published: 2026-06-09
Exploitation: Not in CISA KEV at last sync

Original advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-55203

Exploitation outlook

EPSS (FIRST.org) estimates each CVE’s probability of exploitation in the next 30 days — here is the CSIRTS.com read on those numbers.

Low exploitation riskCVE-2026-552030.32% 30-day exploitation probability — currently an unlikely target, but scores change as exploit code circulates. Riskier than 24% of all scored CVEs.

Referenced CVEs

CVE	CSIRTS overview	External
CVE-2026-55203	coverage & exploitation status	NVD · CVE.org

Same CVEs, other sources

How other CERTs, PSIRTs and databases cover the vulnerabilities in this advisory.

high[UPDATE] [high] HAProxy: Multiple vulnerabilitiescert-bund

Recent advisories for HAProxy - Integer

A cluster of recent advisories against the same product widens the attack surface — attackers routinely chain freshly published CVEs on one product, so review these together.

high[UPDATE] [high] HAProxy: Multiple vulnerabilitiescert-bund · 2026-07-09
unknownVulnerability in HAProxy (June 29, 2026)cert-fr-avis · 2026-06-29
highGHSA-h2qv-fj59-j46j: Netty HAProxy: Unbalanced Reference Count in Nested PP2_TYPE_SSL TLV Parsing Leads to Mem…ghsa · 2026-06-11
highCVE-2026-55204: HAProxy - NULL Pointer Dereference in hpack_dht_insert Functionmsrc · 2026-06-09