CVE-2020-27386: FlexDotnetCMS unrestricted file upload enables remote code execution

Automated analysis

Generated by VULONE's analysis model from the NVD record, CISA KEV and EPSS data on 18 September 2026. Confidence: high.

high priorityThe flaw gives authenticated remote code execution with a high CVSS score and very high EPSS, though it requires valid credentials and is not in KEV.

What it is

FlexDotnetCMS before v1.5.9 lets an authenticated user upload a file with a safe extension (for example TXT) through the FileManager, then rename it to an executable extension (for example ASP) using the FileEditor or the FileManager rename function. The renamed file can then be executed by requesting it over HTTP. This turns a low-privileged authenticated account into arbitrary code execution on the web server.

Impact

An attacker with a valid account gains remote code execution under the web server's process identity, allowing full control of the application and its data. CVSS 3.1 scores this 8.8 (HIGH) with high confidentiality, integrity and availability impact.

Attack surface

Reached over the network through the CMS FileManager and FileEditor interfaces; the attacker must be authenticated (PR:L) and no user interaction is required (UI:N). The upload and rename steps are performed through normal CMS functionality, and the final payload is triggered by an HTTP GET to the uploaded file path.

Exploitation

Not listed in CISA KEV, but EPSS is very high (0.72872, 99.4th percentile) and public references include exploit write-ups and a Metasploit pull request, indicating mature public exploitation tooling. No ransomware group use is documented in the record.

What to do

• Upgrade FlexDotnetCMS to v1.5.9 or later, which the release notes identify as the fixed version.
• Restrict FileManager and FileEditor access to trusted administrative accounts and remove unused accounts.
• Enforce server-side upload validation that blocks executable extensions and prevents renaming uploaded files to executable types.
• Configure the web server to deny execution of script files in upload and content directories.
• Monitor and alert on file rename and upload events in the CMS, especially changes to executable extensions.

Detection

• Alert on HTTP requests to uploaded file paths with executable extensions such as .asp, .aspx or .ashx.
• Monitor CMS FileManager and FileEditor activity for uploads followed by renames to executable extensions.
• Watch web server logs for POST uploads and subsequent GET requests to the same file within a short window.
• Audit file system changes in upload directories for newly created executable files.

This assessment is produced automatically and is not human-reviewed. Verify against the vendor advisory before acting on it.