Algernon: Host header path traversal in --domain mode reads files and runs Lua from parent dir

Exploitation

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

CVSS

8.2 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:L/A:N

EPSS

• CVE-2026-48126: 0.34% estimated exploitation activity in the next 30 days; percentile 26.95 (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 When algernon is started with --domain (or --letsencrypt, which silently turns on --domain at engine/flags.go:372), the request handler resolves the served directory by joining the configured --dir with the value of the client-supplied Host header. The join is performed by filepath.Join with no validation, so a Host: .. header walks one level above the document root. Subsequent file resolution then exposes everything in that parent directory — arbitrary file read, full directory listing, and, if any .lua file is present, server-side Lua execution. Algernon 1.17.7 and earlier are affected. Details engine/handlers.go (function RegisterHandlers, around line 510): utils/web.go (GetDomain): utils/files.go (URL2filename) only sanitises the URL path — it never inspects dirname: engine/flags.go (auto-enable in CertMagic / Let's Encrypt mode): Putting it together: 1. The client sends Host: ... Go's HTTP server accepts the value because . is in the URI host whitelist and there are no…

Published by the source

23 Jun 2026, 21:49 UTC

Last modified by the source

23 Jun 2026, 21:49 UTC

Source last checked

26 Oct 2026, 15:24 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-jc3j-x6pg-4hmv

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# Patch Next advisory brief: CVE-2026-48126

Advisory: Algernon: Host header path traversal in --domain mode reads files and runs Lua from parent dir
IDs: CVE-2026-48126, GHSA-jc3j-x6pg-4hmv
Record: https://patchnext.com/advisories/cve-2026-48126 (revision 1, source checked 26 Oct 2026, 15:24 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 8.2)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: github.com/xyproto/algernon 1.17.8. A vendor fix does not mean a deployment is fixed.

## Affected components
- Go github.com/xyproto/algernon: affected <= 1.17.7; fixed in 1.17.8

## Suggested inspection (read-only)
- Go github.com/xyproto/algernon: 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/xyproto/algernon.
- 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-jc3j-x6pg-4hmv (GitHub-reviewed advisory GHSA-jc3j-x6pg-4hmv lists affected ranges: Go github.com/xyproto/algernon >=0 <=1.17.7)

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