CVE-2021-28976: CVE-2021-28976: Remote Code Execution vulnerability in GetSimpleCMS before 3.3.16 in admin/upload.php via …

What it is

Remote Code Execution vulnerability in GetSimpleCMS before 3.3.16 in admin/upload.php via phar filess.

Exploitation evidence

First exploit evidence seen: 2025-04-11 (earliest date across the sources below).

Exploited in the wild

No source on this site lists in-the-wild exploitation yet (CISA KEV, VulnCheck KEV).

Public exploit code

Exploit-DB

• EDB-52168: GetSimpleCMS 3.3.16 - Remote Code Execution (RCE) (2025-04-11)

Sources: Exploit-DB (OffSec).

How to fix it

Fixed versions

• get-simple getsimplecms: upgrade to 3.3.15 or later

Derived from the NVD CPE configuration (versionEndExcluding / versionEndIncluding). Always confirm with the vendor advisory.

Vendor advisories and patches

No vendor-tagged reference yet; see the full reference list.

MITRE ATT&CK mitigations for this chain

• M1048 Application Isolation and Sandboxing (for T1190, T1203) : Application isolation will limit what other processes and system features the exploited target can access.
• M1037 Filter Network Traffic (for T1190) : Restrict outbound network traffic from public-facing servers to prevent unauthorized connections from initiating communications with attacker-controlled infrastructure. While this may not prevent the initial exploitation, it limits the attacker's ability to verify and control the compromised server post-exploit, reduci
• M1030 Network Segmentation (for T1190) : Segment externally facing servers and services from the rest of the network with a DMZ or on separate hosting infrastructure.
• M1016 Vulnerability Scanning (for T1190) : Regularly scan externally facing systems for vulnerabilities and establish procedures to rapidly patch systems when critical vulnerabilities are discovered through scanning and through public disclosure.
• M1026 Privileged Account Management (for T1190) : Use least privilege for service accounts will limit what permissions the exploited process gets on the rest of the system.
• M1050 Exploit Protection (for T1190, T1203) : Web Application Firewalls may be used to limit exposure of applications to prevent exploit traffic from reaching the application.
• M1035 Limit Access to Resource Over Network (for T1190) : Ensure that all publicly exposed services are actually intended to be so, and restrict access to any that should only be available internally.
• M1051 Update Software (for T1190, T1203) : Update software regularly by employing patch management for externally exposed applications.
• M1042 Disable or Remove Feature or Program (for T1505.003) : Consider disabling functions from web technologies such as PHP’s `evaI()` that may be abused for web shells.
• M1018 User Account Management (for T1505.003) : Enforce the principle of least privilege by limiting privileges of user accounts so only authorized accounts can modify the web directory.

Detection Generic guidance

• Initial Access: Alert on unusual requests to the affected service (odd paths, oversized payloads, exploit strings) in WAF, reverse-proxy and application logs.
• Execution: Watch for the service process spawning shells or script interpreters (cmd.exe, powershell, sh, bash, python) via EDR process-tree rules.
• Persistence: Look for new files in web roots, new scheduled tasks/services, and new accounts created after the exposure window.

Response checklist Generic guidance

1. Inventory: find every instance of the affected product and version (include internet-facing and forgotten ones).
2. Patch or mitigate per the vendor advisory; if you cannot patch, remove internet exposure or apply the vendor workaround.
3. Assume compromise if the system was exposed before patching: hunt for the indicators in the advisory and review logs back to the disclosure date.
4. Rotate credentials and secrets stored on or used by the affected system if compromise cannot be ruled out.