CVE-2026-45721 Algernon: handler.lua discovery walks parent directories above the server root
Attacker achieves remote code execution by placing a handler.lua file in a parent directory, which the server loads without authentication.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-45721?
CVE-2026-45721 is a critical 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 1.17.7, when Algernon is asked for any URL path that resolves to a directory without an index file, DirPage walks upward through parent directories — past the configured server root — looking for a file named handler.lua to execute as the request handler. The loop terminates only after 100 ancestor steps or when filepath.Dir returns ., so on any absolute server-root path the search reaches the filesystem root (/ on Unix, drive letter on Windows). The first handler.lua it finds is loaded into the Lua interpreter with the full Algernon API exposed — including run3(), httpclient, os.execute, io.popen, PQ, MSSQL, raw filesystem access, and the userstate database. Any process that can write handler.lua anywhere in a parent directory of the server root obtains pre-authenticated remote code execution on the next HTTP request. This is reachable without authentication — the lookup happens before the permission check returns a hit (the perm system only gates URL prefixes, not the handler-resolution step), and any URL pointing at a directory without an index triggers the walk. On a fresh stock Algernon install the request GET / is enough. This vulnerability is fixed in 1.17.7.
Which versions are affected by CVE-2026-45721?
3 open source packages ship an affected version, including github.com/xyproto/algernon.
| Package | Affected | Fixed in |
|---|---|---|
github.com/xyproto/algernon Go | All versions | 1.17.7 |
github.com/xyproto/algernon Go | All versions | 1.17.7 |
github.com/xyproto/algernon Go | All versions | 1.17.7 |
How do I fix CVE-2026-45721?
Upgrade github.com/xyproto/algernon to 1.17.7 or later.
Upgrade, and resolve every user-supplied path against an allowed base folder before opening it. More about Path traversal.
Is CVE-2026-45721 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-45721 exists.
Sources
Also known as GHSA-xwcr-wm99-g9jc, GO-2026-5772.