CVE-2026-21861: baserCMS has OS Command Injection Leading to Remote Code Execution (RCE)

Should you act?

Critical, preconditions apply

Severe if exploited, but exploitation needs conditions that may not apply to you. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.

• Not in CISA KEV, but CISA’s SSVC assessment records a public proof-of-concept, so attack code exists.
• EPSS 2.3%, percentile 83: FIRST’s model estimates a 2.3% chance of exploitation activity in the next 30 days, higher than 83% of all scored CVEs (elevated). EPSS is a probability, not a verdict.
• Needs an account (high (admin)). Exposure depends on who can log in.
• Only applies if your dependency tree includes baserproject/basercms (Composer (PHP)) <= 5.2.2, fixed in 5.2.3. Check your lockfile, including transitive dependencies.

What it is

baserCMS is a website development framework. Prior to version 5.2.3, baserCMS contains an OS command injection vulnerability in the core update functionality. An authenticated administrator can execute arbitrary OS commands on the server due to improper handling of user-controlled input that is directly passed to exec() without sufficient validation or escaping. This issue has been patched in version 5.2.3.

From the GitHub advisory

Summary

In the core update functionality of baserCMS, some parameters sent from the admin panel are passed to the exec() function without proper validation or escaping. This issue allows an authenticated CMS administrator to execute arbitrary OS commands on the server (Remote Code Execution, RCE).

This vulnerability is not a UI-level issue such as screen manipulation or lack of CSRF protection, but rather stems from a design that directly executes input values received on the server side as OS commands. Therefore, even if buttons are hidden in the UI, or even if CakePHP's CSRF/FormProtection (SecurityComponent) ensures that only legitimate POST requests are accepted, an attack is possible as long as a request containing a valid token is processed within an administrator session.

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Vulnerability Information

Weakness type: CWE-78.

Product: baserproject/basercms (Composer (PHP)) · Assigned by: GitHub_M · Published: 2026-03-31

Affected and fixed versions

Severity scores

• Attack vector: network (remotely reachable)
• Attack complexity: low
• Privileges required: high (admin)
• User interaction: none
• Scope: changed (can affect other components)

CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.

Exploitation and urgency

• CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
• CISA SSVC assessment: exploitation poc, automatable no, technical impact total.
• FIRST EPSS (model of 2026-10-21): 2.28% probability of exploitation activity in the next 30 days; percentile 83.

Timeline

• 2026-03-31 — CVE record last updated
• 2026-03-31 — CVE record published by GitHub_M
• 2026-03-31 — GitHub-reviewed advisory GHSA-qxmc-6f24-g86g published (critical)

Sources and references

Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.

Vulnerability records

1. GitHub Advisory GHSA-qxmc-6f24-g86g github.com
2. CVE.org record CVE-2026-21861 cve.org
3. NVD entry CVE-2026-21861 nvd.nist.gov

Other references from the CVE record and advisory

1. github.com github.com
2. basercms.net basercms.net
3. github.com github.com

Critical Composer (PHP) advisories · Today’s Security Digest