CVE-2026-48126

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

State: Published
Severity: 8.2 High
Published: May 26, 2026
Updated: May 26, 2026
Reserved: May 20, 2026

Description

Algernon is a small self-contained pure-Go web server. Prior to 1.17.8, 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. This vulnerability is fixed in 1.17.8.

Scores

CVSS	Score	Vector	Source
3.1	8.2 High	CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:L/A:N	GitHub_M

Weaknesses

CWE-22 Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')
CWE-23 Relative Path Traversal
CWE-644 Improper Neutralization of HTTP Headers for Scripting Syntax

Affected Products

Product	Versions
algernonxyproto	< 1.17.8Affected

References

https://github.com/xyproto/algernon/security/advisories/GHSA-jc3j-x6pg-4hmv

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