Caddy: Unicode case-folding length expansion causes incorrect split_path index in FastCGI transport

Exploitation

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

CVSS 4.0

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

EPSS

• CVE-2026-27590: 0.61% estimated exploitation activity in the next 30 days; percentile 49.48 (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 FastCGI path splitting logic computes the split index on a lowercased copy of the request path and then uses that byte index to slice the original path. This is unsafe for Unicode because strings.ToLower() can change UTF-8 byte length for some characters. As a result, Caddy can derive an incorrect SCRIPTNAME/SCRIPTFILENAME and PATHINFO, potentially causing a request that contains .php to execute a different on-disk file than intended (path confusion). In setups where an attacker can control file contents (e.g., upload features), this can lead to unintended PHP execution of non-.php files (potential RCE depending on deployment). Details The issue is in github.com/caddyserver/caddy/modules/caddyhttp/fastcgi.Trasnport.splitPos() (and the subsequent slicing in buildEnv()): The returned index is computed in the byte space of lowerPath, but buildEnv() applies it to the original path: - docURI = path[:splitPos] - pathInfo = path[splitPos:] - scriptName =…

Published by the source

24 Feb 2026, 20:39 UTC

Last modified by the source

27 Feb 2026, 19:55 UTC

Source last checked

26 Oct 2026, 15:14 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-5r3v-vc8m-m96g

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# Patch Next advisory brief: CVE-2026-27590

Advisory: Caddy: Unicode case-folding length expansion causes incorrect split_path index in FastCGI transport
IDs: CVE-2026-27590, GHSA-5r3v-vc8m-m96g
Record: https://patchnext.com/advisories/cve-2026-27590 (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 8.9)
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-5r3v-vc8m-m96g (GitHub-reviewed advisory GHSA-5r3v-vc8m-m96g lists affected ranges: Go github.com/caddyserver/caddy/v2 >=0 <2.11.1)

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