Caddy: MatchHost becomes case-sensitive for large host lists (>100), enabling host-based route/auth bypass

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS 4.0

7.7 · CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N/E:P · score source

EPSS

• CVE-2026-27588: 0.43% estimated exploitation activity in the next 30 days; percentile 36.80 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary 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. Details In Caddy v2.10.2, the MatchHost matcher states it matches the Host value case-insensitively: - modules/caddyhttp/matchers.go: type MatchHost matches requests by the Host value (case-insensitive). However, in MatchHost.MatchWithError, when the host list is considered "large" (len(m) 100): - MatchHost.large() returns true for len(m) 100 (modules/caddyhttp/matchers.go, around the large() helper). - The matcher takes a "fast path" using binary search over the sorted host list, and checks for an exact match using a case-sensitive string comparison (m[pos] == reqHost). - After the fast path fails, the fallback loop short-circuits for large lists by breaking as…

Published by the source

24 Feb 2026, 20:34 UTC

Last modified by the source

27 Feb 2026, 19:53 UTC

Source last checked

26 Oct 2026, 15:14 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-x76f-jf84-rqj8

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# Patch Next advisory brief: CVE-2026-27588

Advisory: Caddy: MatchHost becomes case-sensitive for large host lists (>100), enabling host-based route/auth bypass
IDs: CVE-2026-27588, GHSA-x76f-jf84-rqj8
Record: https://patchnext.com/advisories/cve-2026-27588 (revision 1, source checked 26 Oct 2026, 15:14 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 7.7)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: github.com/caddyserver/caddy/v2 2.11.1. A vendor fix does not mean a deployment is fixed.

## Affected components
- Go github.com/caddyserver/caddy/v2: affected < 2.11.1; fixed in 2.11.1

## Suggested inspection (read-only)
- Go github.com/caddyserver/caddy/v2: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on github.com/caddyserver/caddy/v2.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-x76f-jf84-rqj8 (GitHub-reviewed advisory GHSA-x76f-jf84-rqj8 lists affected ranges: Go github.com/caddyserver/caddy/v2 >=0 <2.11.1)

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