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CVE-2026-45117 – Remote Code Execution – MyBB 1.8.13 through 1.8.39 (Installer)

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Summary

CVE-2026-45117 is a code injection (RCE) vulnerability in the MyBB forum software’s web-based installer, affecting versions 1.8.13 through 1.8.39. Improper sanitization of user-supplied database configuration values allows an attacker to inject arbitrary PHP code into the generated configuration file, leading to remote code execution. The flaw carries a CVSS v3.1 base score of 9.8 (Critical) and requires no authentication or user interaction.

Technical details

  • Root cause: the MyBB installer writes user-supplied database configuration values (e.g., database name/host/credentials) directly into a generated PHP configuration file.
  • MyBB 1.8.13 added addcslashes() sanitization to prevent installer RCE, but the character set passed to addcslashes() did not include the backslash (“) character.
  • This omission allows an attacker to craft input containing a backslash that escapes the PHP string context in the generated config file, injecting arbitrary PHP code.
  • Attack vector is network-based, requires no privileges and no user interaction, and is exploitable when the installer (install/) is reachable and can be run or re-run.
  • Successful exploitation results in arbitrary PHP code execution on the host, giving full compromise of confidentiality, integrity, and availability.

Affected software

  • MyBB versions 1.8.13 through 1.8.39 (inclusive)
  • Fixed in MyBB 1.8.40

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: Upgrade MyBB to version 1.8.40 or later, which corrects the addcslashes() character set used during installer sanitization.
  • If patching is not immediately possible: Remove or restrict access to the install/ directory after installation/upgrade is complete, since the installer should not remain accessible on a production forum.
  • As a general precaution, back up forum files and the database before applying the upgrade, and verify the installer directory is deleted or blocked from external access following any install or upgrade.

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: <a title="Powered By MyBB
  • Rendered page (JavaScript object present at runtime): MyBB

References

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How IONIX’s External Exposure Management Platform Detects and Validates
Zero-Days to Shrink MTTR

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

Execute Exploit Validations

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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  • Identification of potentially exposed assets to this CVE
  • Confirmation of verified exploitable assets
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