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CVE-2026-61511 – Unauthenticated Remote Code Execution via Eval Injection – vBulletin 5.x through…

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Summary

CVE-2026-61511 is a critical eval injection vulnerability affecting vBulletin 5.x through 5.7.5 and 6.x through 6.2.1, rated CVSS 9.8 (Critical) under v3.1. The flaw resides in the vB5_Template_Runtime::runMaths() method and allows unauthenticated remote attackers to inject and execute arbitrary PHP code through a publicly accessible AJAX endpoint. No credentials, session token, or user interaction of any kind is required to exploit this vulnerability.

Technical details

  • Root cause: The vB5_Template_Runtime::runMaths() method in vb5/template/runtime.php applies an insufficiently restrictive regex filter to input before passing it to PHP’s eval() function, allowing crafted content to escape the intended character whitelist.
  • Trigger conditions: An attacker sends a specially crafted HTTP request to the unauthenticated ajax/render template route, supplying malicious input via the pagenav[pagenumber] parameter. By encoding the payload using phpfuck-style encoding — which constructs arbitrary PHP expressions from only characters permitted by the regex filter — the attacker bypasses the filter entirely.
  • Attack vector: Remotely exploitable over the network (AV:N) with no authentication (PR:N), no user interaction (UI:N), and low attack complexity (AC:L). The vulnerable endpoint is publicly accessible on any standard vBulletin deployment.
  • Impact: Successful exploitation results in arbitrary PHP code execution on the server, granting an attacker full control over confidentiality, integrity, and availability of the affected host.

Affected software

  • vBulletin 5.x: all versions through 5.7.5
  • vBulletin 6.x: all versions through 6.2.1

Severity

CVSS v3.1 Base Score: 9.8 (Critical)
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

Mitigation and recommended actions

  • Immediate (6.x users): Apply the official vBulletin security patch released for versions 6.1.6 through 6.2.1, or upgrade to vBulletin 6.2.2 or later.
  • Immediate (5.x users): vBulletin 5.x does not receive active security updates. Upgrade to a patched 6.x release as soon as operationally possible.
  • Interim network mitigation: If immediate patching is not feasible, restrict external access to the ajax/render AJAX endpoint at the network perimeter or web server layer (e.g., via WAF rule or reverse-proxy ACL) to reduce attack surface while a patch is prepared.

IONIX Status

The IONIX research team is tracking ongoing exploitation attempts and recommends immediate patching. Potentially affected assets are outlined in this post.

References

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1

Map your entire attack surface (continously)

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2

Monitor for new CVEs

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3

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4

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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.

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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

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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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