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TI-2026-35564 Aliases CVE-2026-48126GHSA-jc3j-x6pg-4hmv

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

Priority HIGH 75CWE-22Published · Updated

Key facts

Attack vector
Network
Privileges required
None
User interaction
None
Attack complexity
Low
Impact type
—
Confidentiality · integrity · availability
High · Low · None
Fix status
Fixed

Attack profile from CVSS 3.1 · CNA. Fix status from CNA and GitHub.

Summary

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.

Why it matters now

Exploitation Proof of concept CISA SSVC · not in CISA KEV
EPSS 0.00335 27th percentileScored 2026-11-07
Fix Fixed algernon (xyproto): 1.17.8github.com/xyproto/algernon (Go): 1.17.8

SSVC decision points

CISA ADP · SSVC 2.0.3

Exploitation
Poc
Automatable
Yes
Technical impact
Partial

The decision (Track, Attend, Act) also depends on your mission and well-being impact; see CISA's SSVC guide.

Affected products

VendorProductAffected versionsFixed inStatusSource
xyprotoalgernonbefore 1.17.81.17.8Patch availableCNA
—github.com/xyproto/algernon (Go)before 1.17.81.17.8Patch availableGHSA-jc3j-x6pg-4hmv
  • algernon Vendorxyproto Affected before 1.17.8 Fixed in 1.17.8 Patch available CNA
  • github.com/xyproto/algernon (Go) Vendor— Affected before 1.17.8 Fixed in 1.17.8 Patch available GHSA-jc3j-x6pg-4hmv

Remediation

  • Fix · CNA algernon (xyproto) Fixed in 1.17.8
  • Fix · GHSA-jc3j-x6pg-4hmv github.com/xyproto/algernon (Go) Fixed in 1.17.8 Advisory →

Timeline

  1. 2026-10-02 Threatist record created from CVE-2026-48126
  2. 2026-10-03 Alias added: GHSA-jc3j-x6pg-4hmv

References

Advisory 1

Other 2

  • TI-2026-37902 Algernon: Server-side script source disclosure on Windows via NTFS filename CVSS 4.0HIGH 8.7
  • TI-2026-33460 Algernon: handler.lua discovery walks parent directories above the server root CVSS 3.1CRITICAL 9.0
  • TI-2026-33466 Algernon: Single-file mode unconditionally enables debug mode CVSS 3.1HIGH 7.5
  • TI-2026-34142 Algernon: Auto-refresh SSE event server sets Access-Control-Allow-Origin: * CVSS 3.1MEDIUM 4.3
  • TI-2026-34141 Algernon: Auto-refresh SSE event server binds to all interfaces by default on Linux/macOS CVSS 3.1MEDIUM 4.3
  • TI-2026-32026 Algernon: Path traversal file write via savein() CVSS 4.0HIGH 8.7
See all 7 →

Sources

This product uses the NVD API but is not endorsed or certified by the NVD. Sources and licences