How severe is CVE-2026-27588?

CVE-2026-27588 has a CVSS 3.x base score of 9.1, rated critical severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability none.

Is CVE-2026-27588 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (29th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2026-27588?

CVE-2026-27588 affects Caddyserver Caddy. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2026-27588?

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

Does CVE-2026-27588 have an EU (EUVD) identifier?

Yes. CVE-2026-27588 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-8540.

When was CVE-2026-27588 published?

CVE-2026-27588 was published on 2026-02-24 and last updated on 2026-06-17.

CVE-2026-27588

CVE-2026-27588 is a critical-severity vulnerability in Caddyserver Caddy with a CVSS 3.x base score of 9.1. 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-178.

Key facts

Severity: Critical (CVSS 3.x base score 9.1)
CVSS v4: 7.7
EPSS exploit prediction: 0% (29th percentile)
Actively exploited: Not listed in CISA KEV
EU (EUVD) id: EUVD-2026-8540
Weakness: CWE-178
Affected product: Caddyserver Caddy
Published: 2026-02-24
Last modified: 2026-06-17

Description

Caddy is an extensible server platform that uses TLS by default. Prior to version 2.11.1, Caddy's HTTP `host` request matcher is documented as case-insensitive, but when configured with a large host list (>100 entries) it becomes case-sensitive due to an optimized matching path. An attacker can bypass host-based routing and any access controls attached to that route by changing the casing of the `Host` header. Version 2.11.1 contains a fix for the issue.

Frequently asked questions

What is CVE-2026-27588?: Caddy is an extensible server platform that uses TLS by default. Prior to version 2.11.1, Caddy's HTTP `host` request matcher is documented as case-insensitive, but when configured with a large host list (>100 entries) it becomes case-sensitive due to an optimized matching path. An attacker can bypass host-based routing and any access controls attached to that route by changing the casing of the `Host` header. Version 2.11.1 contains a fix for the issue.
How severe is CVE-2026-27588?: CVE-2026-27588 has a CVSS 3.x base score of 9.1, rated critical severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability none.
Is CVE-2026-27588 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (29th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2026-27588?: CVE-2026-27588 affects Caddyserver Caddy. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2026-27588?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its critical severity, prioritise patching exposed systems.
Does CVE-2026-27588 have an EU (EUVD) identifier?: Yes. CVE-2026-27588 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-8540.
When was CVE-2026-27588 published?: CVE-2026-27588 was published on 2026-02-24 and last updated on 2026-06-17.

References

Affected products (1)

cpe:2.3:a:caddyserver:caddy:*:*:*:*:*:*:*:*

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