CVE-2026-45721: xyproto Algernon remote code execution via handler.lua path traversal

CVE-2026-45721 · Severity: critical · CVSS 9 · Published 2026-05-26

Algernon is a standalone web server and application engine. A vulnerability in how the server searches for configuration files allows it to execute scripts found outside of the intended web directory. If an attacker or another process can place a specific script file in a parent directory on the server's host, they can achieve full remote code execution and gain complete control over the server and its data without needing to log in.

Technologies: github.com/xyproto/algernon (Go), Xyproto Algernon. Vendors: Go, Xyproto.

Quick answers

Is CVE-2026-45721 exploited in the wild?
No. Junglewise Threat Intelligence has no report of CVE-2026-45721 being exploited in the wild.
How severe is CVE-2026-45721?
Critical severity, with a CVSS score of 9, published 26 May 2026.
Which products and versions are affected?
github.com/xyproto/algernon (Go) <= 1.17.6, SEMVER:introduced=0,fixed=1.17.7; Xyproto Algernon < 1.17.7, <= 1.17.6.
Is there a fix for CVE-2026-45721?
Yes. github.com/xyproto/algernon (Go): upgrade to 1.17.7 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-45721 attacked?
It is attacked over the network, in a hosted service.
Am I affected by CVE-2026-45721?
Check whether you run github.com/xyproto/algernon (Go) or Xyproto Algernon at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

How to fix

  • github.com/xyproto/algernon (Go): upgrade to 1.17.7 or later
    go get github.com/xyproto/algernon@v1.17.7

Technical details

The vulnerability exists in the 'DirPage' function within 'engine/dirhandler.go'. When a requested URL resolves to a directory without an index file, the server performs an unbounded upward search (up to 100 iterations) for a file named 'handler.lua'. Because the loop does not stop at the configured server root, it can reach the filesystem root (/ or drive letter). If such a file is found, it is executed in a Lua interpreter with full access to the Algernon API, including 'os.execute' and 'io.popen'. This occurs before authentication checks are finalized, allowing an unauthenticated remote attacker to trigger the execution of a malicious script if they can place it in any parent directory of the web root.

Affected products

  • xyproto Algernon < 1.17.7

Identifiers

  • CVE-2026-45721
  • GHSA-xwcr-wm99-g9jc
  • GO-2026-5772

Timeline

  • 2026-05-12: advisory: GitHub Security Advisory published
  • 2026-05-26: disclosed: CVE-2026-45721 published to NVD
  • 2026-05-26: patched: Fixed in version 1.17.7

References

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