CVE-2026-75327: DocSys File Upload Vulnerability

CVE-2026-75327 Overview

CVE-2026-75327 is an arbitrary file upload vulnerability in DocSys-master version 2.02.85. The flaw resides in the uploadMarkdownPic interface implemented in src/com/DocSystem/controller/DocController.java. The endpoint accepts uploaded content without enforcing file type or extension restrictions. An attacker can abuse this interface to upload files of arbitrary types to the server, including executable web content.

Critical Impact
Successful exploitation allows an attacker to upload arbitrary files through the DocSys uploadMarkdownPic endpoint, which can lead to remote code execution and full compromise of the hosting application.

Affected Products

DocSys-master V2.02.85
Component: src/com/DocSystem/controller/DocController.java
Endpoint: uploadMarkdownPic interface

Discovery Timeline

2026-08-26 - CVE-2026-75327 published to NVD
2026-08-26 - Last updated in NVD database

Technical Details for CVE-2026-75327

Vulnerability Analysis

The vulnerability is an arbitrary file upload flaw [CWE-434] in the DocSys document management application. The uploadMarkdownPic interface is designed to accept image uploads associated with Markdown content. However, the implementation in DocController.java does not enforce content-type validation, extension allow-listing, or MIME sniffing on incoming files.

Because the endpoint trusts client-supplied filenames and content, an attacker can supply a payload with an executable extension such as .jsp, .war, or another server-interpreted format. If the destination directory is served by the application container, the uploaded file may be requested and executed directly. This transforms a file upload primitive into remote code execution on the DocSys host.

The issue is reachable by any user able to invoke the upload interface. Details of authentication requirements are documented in the referenced advisory.

Root Cause

The root cause is missing server-side validation in the uploadMarkdownPic handler. The controller writes uploaded content to disk without verifying that the file extension matches an allow-list of image formats. It also does not rename or sanitize the stored filename, allowing attacker-controlled extensions to persist on disk.

Attack Vector

An attacker sends a crafted multipart HTTP POST request to the uploadMarkdownPic endpoint. The request contains a file with a malicious extension and web shell contents. Once the file is stored under a web-accessible path, the attacker requests the uploaded resource to trigger execution within the servlet container. See the GitHub CVE Documentation for the reproduction steps.

No verified proof-of-concept code is included in the NVD record. The vulnerability mechanism is documented in prose in the referenced advisory.

Detection Methods for CVE-2026-75327

Indicators of Compromise

Files with executable extensions such as .jsp, .jspx, .war, or .class present in DocSys upload directories used by uploadMarkdownPic.
HTTP POST requests to the uploadMarkdownPic endpoint with Content-Type: multipart/form-data and filename parameters ending in non-image extensions.
Subsequent HTTP GET requests directly to uploaded filenames in the Markdown image upload path.

Detection Strategies

Inspect web server and application access logs for uploadMarkdownPic requests followed by direct retrieval of the returned resource path.
Alert on any file written to Markdown image directories whose extension is not in the image allow-list (.png, .jpg, .jpeg, .gif, .webp).
Correlate process creation events on the DocSys host with child processes spawned by the Java servlet container immediately after upload activity.

Monitoring Recommendations

Enable verbose HTTP request logging on the DocSys application for upload endpoints and retain logs for forensic review.
Deploy file integrity monitoring on all directories writable by the DocSys service account.
Monitor outbound network connections from the DocSys host for reverse shell or command-and-control patterns.

How to Mitigate CVE-2026-75327

Immediate Actions Required

Restrict network access to the DocSys uploadMarkdownPic endpoint using a web application firewall or reverse proxy allow-list.
Audit the Markdown image upload directory and remove any files that do not match approved image formats.
Rotate credentials and review privileged accounts on any DocSys host where suspicious uploads are identified.

Patch Information

No vendor patch is referenced in the NVD entry for CVE-2026-75327. Consult the DocSys project repository and the GitHub CVE Documentation for updates and remediation guidance from the maintainers.

Workarounds

Enforce a server-side extension allow-list for the uploadMarkdownPic handler and reject any file that does not match approved image MIME types.
Store uploaded files with server-generated random filenames and strip user-supplied extensions before writing to disk.
Configure the servlet container to disable script execution in the Markdown image upload directory.
Deploy WAF rules that block multipart uploads containing executable extensions to the uploadMarkdownPic path.

bash

# Example nginx configuration to block script execution in the uploads directory
location ~ ^/docsys/uploads/markdown/.*\.(jsp|jspx|war|class|sh|php)$ {
    deny all;
    return 403;
}