# 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.

- **Severity:** High 8.2 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:L/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 26 May 2026
- **Updated:** 26 May 2026
- **Also known as:** GHSA-jc3j-x6pg-4hmv, GO-2026-5460

## 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.

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

## Which versions are affected by CVE-2026-48126?

3 open source packages ship an affected version, including github.com/xyproto/algernon.


| Package | Ecosystem | 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.

## Is CVE-2026-48126 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-48126 exists.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-48126 and fix it if it is affected.

What it is: 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.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-48126 exists.

Affected packages and fixed versions:
- github.com/xyproto/algernon (Go): affected all versions; fixed in 1.17.8
- github.com/xyproto/algernon (Go): affected all versions; fixed in 1.17.8
- github.com/xyproto/algernon (Go): affected all versions; fixed in 1.17.8

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-48126/ (Markdown: https://insights.sbomify.com/CVE-2026-48126/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-48126
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-48126
- OSV: https://osv.dev/vulnerability/CVE-2026-48126

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-48126/
Summaries are generated and may contain mistakes; the official description above is the record.