CVE-2026-48126 Algernon: Host header path traversal in --domain mode reads files and runs Lua from parent dir
Attackers can read arbitrary files and execute Lua code by manipulating the Host header in domain mode, requiring an upgrade to fix.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-48126?
CVE-2026-48126 is a high-severity path traversal vulnerability in xyproto algernon, published on 26 May 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official 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.
Which versions are affected by CVE-2026-48126?
3 open source packages ship an affected version, including github.com/xyproto/algernon.
| Package | Affected | Fixed in |
|---|---|---|
github.com/xyproto/algernon Go | All versions | 1.17.8 |
github.com/xyproto/algernon Go | All versions | 1.17.8 |
github.com/xyproto/algernon Go | All versions | 1.17.8 |
How do I fix CVE-2026-48126?
Upgrade github.com/xyproto/algernon to 1.17.8 or later.
Upgrade, and resolve every user-supplied path against an allowed base folder before opening it. More about Path traversal.
Is CVE-2026-48126 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-48126 exists.
Sources
Also known as GHSA-jc3j-x6pg-4hmv, GO-2026-5460.