CVE-2026-48126: Xyproto Algernon

High severity, CVSS 8.2. EPSS: 0.5% chance of exploitation in the next 30 days.

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.

Affected products

• Xyproto Algernon: before 1.17.8 (fixed in 1.17.8)

Published 2026-05-26. Last modified 2026-07-24.