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CVE-2026-86426 – Authentication Bypass Leading to RCE – LibreNMS ≤ 26.7.0

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Summary

CVE-2026-86426 is a critical authentication bypass vulnerability in the LibreNMS REST API, caused by a type-confusion flaw in API token validation. It affects all LibreNMS versions before 26.8.0 and allows unauthenticated remote attackers to gain the privileges of an existing API token holder — potentially escalating to remote code execution. The vulnerability carries a CVSS score of 9.2 (Critical).

Technical details

  • Root cause: LibreNMS’s ApiTokenGuard accepts API tokens supplied as JSON body parameters. Laravel preserves JSON data types, so a value like {"api_token": 9} is decoded as an integer rather than a string.
  • The application compares this integer against the token_hash text column in MySQL. MySQL coerces the text column to a number by its leading characters, so a hash like 9cd9a5e2... evaluates to 9 while a hash beginning with a letter (e.g., abcdef01...) evaluates to 0.
  • Sending {"api_token": [0]} bypasses empty() validation (since an array is not empty), and Laravel’s query builder then flattens the array to the integer 0 before executing the query.
  • Attack vector: unauthenticated, network-based requests to REST API endpoints with a crafted numeric api_token value; no rate limiting exists on the API route group, enabling rapid guessing across digits 0–9 to match a valid token.
  • Impact: an attacker who matches a token inherits that token’s full permissions, including access to device credentials and administrative functions. If the matched token belongs to an administrator, alert templates can be abused (via @php blocks) to achieve remote code execution.
  • Precondition: at least one enabled API token must exist on the target instance.

Affected software

  • LibreNMS versions prior to 26.8.0 (all versions up to and including 26.7.0)

Severity

  • CVSS v4.0 Base Score: 9.2 (Critical)
  • Vector: CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N

Mitigation and recommended actions

  • Immediate: Upgrade LibreNMS to version 26.8.0 or later, which fixes the type-confusion flaw in API token validation.
  • If immediate patching is not possible: restrict network access to the LibreNMS REST API (e.g., via firewall rules or VPN) to trusted hosts only, disable or rotate all existing API tokens, and monitor API access logs for requests using numeric or array-formatted api_token values.

How IONIX identifies potentially affected assets

IONIX matches the following signals against data already collected when it crawled the asset; identifying the technology sends no request beyond that crawl.

  • Raw response body: <title>LibreNMS</title>

References

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1

Map your entire attack surface (continously)

IONIX uses multi-factor discovery methods, including DNS analysis, certificate mapping, metadata inspection, and more, to automatically map every internet-facing asset across your environment. This includes cloud instances, third-party platforms, shadow IT, and even forgotten infrastructure that traditional tools miss.

2

Monitor for new CVEs

Dozens of threat intel feeds using agentic technology are continuously analyzed to detect the appearance of proof-of-concept code, exploit kits, and indicators of active targeting. IONIX goes further by applying AI to proactively evaluate whether emerging vulnerabilities are likely to be exploited, even before PoCs go public.

3

Identify Potential External Exposures

Not all CVEs matter. IONIX filters vulnerabilities by asking attacker-centric questions: Can it be reached from the internet? Does it require authentication? Is it being exploited in the wild? This dramatically reduces noise and focuses teams on threats that can actually be weaponized.

4

Create Safe, Scalable Exploit Validations

IONIX transforms real-world PoCs into safe, non-intrusive test payloads that can be run in production environments without disruption. These simulations are precisely targeted to the systems that are vulnerable, ensuring rapid validation without unnecessary load.

5

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By combining context about software stack, versioning, exposure status, and reachability, IONIX ensures that only the right payloads are executed against the right assets, maximizing efficiency and minimizing risk.

6

Drive Fast and Actionable Remediation

Results are routed through integrations with ticketing, SOAR, and SIEM tools. Issues are written in plain language, bundled into remediation clusters, and prioritized based on asset criticality, exploitability, and blast radius. This shortens mean time to remediation (MTTR) and empowers teams to act with confidence.

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