Atomicorp WAF Research Note: CVE-2026-66898

Path traversal via unvalidated instance name in backup tarball restore enables root file write / RCE

CVE Details

• Already protected by existing Atomicorp WAF rules
• Protection status: Understanding Atomicorp WAF Protection Status
• CVE: CVE-2026-66898
• Name: Path traversal via unvalidated instance name in backup tarball restore enables root file write / RCE
• Severity: Critical
• CVSS: 9.9 (v3.1) (Critical)
• CVSS version: 3.1
• CVSS source: canonical CNA
• CVSS vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H
  • Attack vector: Network
  • Attack complexity: Low
  • Privileges required: Low
  • User interaction: None
  • Scope: Changed
  • Confidentiality impact: High
  • Integrity impact: High
  • Availability impact: High
• CWE: CWE-22 — Improper Limitation of a Pathname to a Restricted Directory (‘Path Traversal’)
• Related CAPEC attack patterns (via MITRE CWE relationships): CAPEC-126 (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: canonical
• Vulnerable product: lxd
• CVE published: 2026-08-12
• CVE last modified: 2026-09-11
• Affected versions (NVD configurations):
  • lxd: >= 4.0.0, < 4.0.12
  • lxd: >= 5.0.0, < 5.0.4
  • lxd: >= 5.1, < 5.21.2
  • lxd: 6.0
• EPSS: 0.0034; 27.5th percentile (FIRST (opens in a new tab) )
• Latest Update: 2026-09-06
• Selected Rule IDs observed: 390719
• Matching rules: 1; rules with blocking actions: 1
• Would-block Rule IDs: 390719
• Request surfaces inspected: Request headers

Vulnerability Description

A path traversal vulnerability in LXD allows an attacker to manipulate file system paths during backup import and restore operations. When importing or restoring a backup archive, LXD fails to validate instance and storage volume names contained within the archive metadata. An attacker can exploit this flaw by supplying a crafted backup archive with malicious instance or volume names containing path traversal sequences, potentially allowing file access or overwriting outside the designated restore directory.

Recommended Actions

1. Maintain the observed Atomicorp protections. Keep your Atomicorp commercial rules current and confirm that the selected observed rules (390719 ) 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: Canonical LXD versions earlier than 4.0.12.
   • Affected versions: Canonical LXD versions earlier than 5.0.4.
   • Affected versions: Canonical LXD versions earlier than 5.21.2.
   • Affected versions: Canonical LXD versions earlier than 6.1.
   • Recommended upgrade: Upgrade Canonical LXD to 4.0.12 or later.
   • Recommended upgrade: Upgrade Canonical LXD to 5.0.4 or later.
   • Recommended upgrade: Upgrade Canonical LXD to 5.21.2 or later.
   • Recommended upgrade: Upgrade Canonical LXD to 6.1 or later.
   • CNA guidance source: canonical (opens in a new tab)
   • Vendor recommendation: Upgrade to LXD version 4.0.12 or later, 5.0.4 or later, or 5.12.2 or later, or 6.0 or later.

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)
• Vendor advisory: https://github.com/canonical/lxd/security/advisories/GHSA-m857-c7gc-c984 (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) .