CVE-2026-41203: CI4MS Theme::upload is vulnerable to Zip Slip leading to 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 0.7%, percentile 53: FIRST’s model estimates a 0.7% chance of exploitation activity in the next 30 days, higher than 53% of all scored CVEs (moderate). EPSS is a probability, not a verdict.
• Needs an account (low (any account)). Exposure depends on who can log in.
• Only applies if your dependency tree includes ci4-cms-erp/ci4ms (Composer (PHP)) < 0.31.5.0, fixed in 0.31.5.0. Check your lockfile, including transitive dependencies.

What it is

CI4MS is a CodeIgniter 4-based CMS skeleton that delivers a production-ready, modular architecture with RBAC authorization and theme support. Prior to version 0.31.5.0, ci4ms Theme::upload extracts user uploaded ZIP archives without validating entry names, allowing an authenticated backend user with the theme create permission to write files to arbitrary filesystem locations (Zip Slip) and achieve remote code execution by dropping a PHP file under the public web root. This issue has been patched in version 0.31.5.0.

From the GitHub advisory

Summary ci4ms Theme::upload extracts user uploaded ZIP archives without validating entry names, allowing an authenticated backend user with the theme create permission to write files to arbitrary filesystem locations (Zip Slip) and achieve remote code execution by dropping a PHP file under the public web root.

Details modules/Theme/Controllers/Theme.php:13-56 implements the theme upload action. ZipArchive::extractTo() is called directly with no iteration over entry names to verify they resolve inside the destination:

A ZIP containing entries like ../../public/shell.php is extracted outside writable/tmp/ into directories served by PHP. The author validates entries correctly in modules/Methods/Controllers/Methods.php:165-175 with a realpath + regex loop; the same check is missing here.

Routing: modules/Theme/Config/Routes.php binds POST backend/themes/themesUpload to Theme::upload with role=create. Although ThemeConfig itself does not list the route in csrfExcept, the upload handler is still reachable cross-site by any admin browser that has create on the Theme module, and any admin with that role can trigger it directly.

A companion Zip Slip bug in Backup::restore is tracked separately as GHSA-xp9f-pvvc-57p4.

Weakness type: CWE-22.

Product: ci4-cms-erp/ci4ms (Composer (PHP)) · Assigned by: GitHub_M · Published: 2026-05-07

Affected and fixed versions

Severity scores

• Attack vector: network (remotely reachable)
• Attack complexity: low
• Attack requirements: none
• Privileges required: low (any account)
• User interaction: none

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): 0.72% probability of exploitation activity in the next 30 days; percentile 53.

Timeline

• 2026-04-22 — GitHub-reviewed advisory GHSA-xv3r-vr59-95rg published (critical)
• 2026-05-07 — CVE record last updated
• 2026-05-07 — CVE record published by GitHub_M

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-xv3r-vr59-95rg github.com
2. CVE.org record CVE-2026-41203 cve.org
3. NVD entry CVE-2026-41203 nvd.nist.gov

Other references from the CVE record and advisory

1. github.com github.com
2. github.com github.com

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