Atomicorp WAF Research Note: CVE-2026-48126

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

CVE Details

• Already protected by existing Atomicorp WAF rules
• Protection status: Understanding Atomicorp WAF Protection Status
• CVE: CVE-2026-48126
• Name: Algernon: Host header path traversal in –domain mode reads files and runs Lua from parent dir
• Severity: High
• CVSS: 8.2 (v3.1) (High)
• CVSS version: 3.1
• CVSS source: GitHub_M CNA
• CVSS vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:L/A:N
  • Attack vector: Network
  • Attack complexity: Low
  • Privileges required: None
  • User interaction: None
  • Scope: Unchanged
  • Confidentiality impact: High
  • Integrity impact: Low
  • Availability impact: None
• CWE: CWE-22 — Improper Limitation of a Pathname to a Restricted Directory (‘Path Traversal’), CWE-23 — Relative Path Traversal, CWE-644 — Improper Neutralization of HTTP Headers for Scripting Syntax
• Related CAPEC attack patterns (via MITRE CWE relationships): CAPEC-126 (opens in a new tab) , CAPEC-139 (opens in a new tab) , CAPEC-64 (opens in a new tab) , CAPEC-76 (opens in a new tab) , CAPEC-78 (opens in a new tab) , CAPEC-79 (opens in a new tab)
• Vendor: xyproto
• Vulnerable product: algernon
• CVE published: 2026-05-26
• CVE last modified: 2026-07-24
• EPSS: 0.0034; 26.5th percentile (FIRST (opens in a new tab) )
• Latest Update: 2026-09-06
• Selected Rule IDs observed: 340007 , 344360 , 347009 , 390709
• Matching rules: 4; rules with blocking actions: 4
• Would-block Rule IDs: 340007 , 344360 , 347009 , 390709
• Request surfaces inspected: JSON request data, Request URI, Request argument names, Request arguments, Request cookies, Request filename, SOAP request data, XML request data

Vulnerability 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.

Recommended Actions

1. Maintain the observed Atomicorp protections. Keep your Atomicorp commercial rules current and confirm that the selected observed rules (340007 , 344360 , 347009 , 390709 ) have not been locally disabled. Atomicorp commercial rules are enabled by default. If you use Atomic WAF, review the applicable policy and rule settings.

2. Apply the vendor’s remediation. WAF protection is defense in depth and does not replace upgrading or following the affected vendor’s guidance.

   • Affected versions: xyproto algernon version < 1.17.8.
   • No specific CNA/vendor solution or workaround is included in the current CVE Record (opens in a new tab) . Review the vendor’s security advisories and supported-release guidance before making changes.

3. Validate after changes. Confirm rule updates are current, review WAF events for the affected application, and test application behavior after vendor upgrades, mitigations, or local policy changes.

Research Notes

Atomicorp WAF protections are designed around attack techniques and exploit behaviors, allowing existing protections to apply across multiple vulnerabilities without requiring a unique rule for every CVE.

Public Vulnerability References

• NVD (opens in a new tab)
• CVE Record (opens in a new tab)
• CVE authority reference: https://github.com/xyproto/algernon/security/advisories/GHSA-jc3j-x6pg-4hmv (opens in a new tab)

Test or Research Source

• Source: Atomicorp AI Research Framework
• Tags: cve, derived, traversal, waf-probe

Research Note Context

This research note documents exploit-technique analysis associated with this vulnerability and references Atomicorp WAF protections observed during engineering review, testing, or validation activities.

Atomicorp security controls are generally designed to identify and mitigate attack techniques and exploit behaviors rather than individual CVE identifiers. As a result, protections developed for one vulnerability may also apply to related attack variants and future vulnerabilities using similar techniques.

Learn More

Atomicorp develops commercially supported security technologies for web applications, servers, endpoints, workloads, and legacy enterprise platforms.

Organizations interested in protecting systems against exploit techniques associated with vulnerabilities such as this one can learn more about:

• Atomic ModSecurity Rules: commercially maintained WAF rules for ModSecurity deployments, including protections for web attacks, bots, scanners, malware, exploitation attempts, and emerging vulnerability classes.
• Atomic WAF: an enterprise web application firewall with Atomicorp rules, centralized management, reporting, authentication controls, and operational tooling.
• Atomic ModSecurity Integrator: a special licensing option for MSPs, MSSPs, OEMs, VARs, reverse proxy providers, and other integrators who want to incorporate Atomicorp ModSecurity rule technology into their own products or services.
• Atomic OSSEC: an enterprise OSSEC-based intrusion detection, compliance, and security monitoring platform.
• Atomic Protector: workload protection for Linux, web, cloud, data center, and hybrid environments.

For additional information, product evaluations, or technical discussions, visit https://www.atomicorp.com/ or https://modsecurity.io/ (opens in a new tab) .