CVE-2026-3749: Bytedesk Bytedesk RCE Vulnerability

CVE-2026-3749 Overview

A vulnerability has been identified in Bytedesk up to version 1.3.9 affecting the handleFileUpload function within the SVG File Handler component. This flaw allows for unrestricted file upload attacks, enabling authenticated attackers to bypass file validation controls and upload potentially malicious SVG files to the server. The vulnerability stems from improper access control (CWE-284) and unrestricted upload of files with dangerous types (CWE-434).

Critical Impact
Authenticated attackers can remotely exploit this vulnerability to upload arbitrary SVG files, potentially leading to stored cross-site scripting (XSS) attacks, server-side request forgery (SSRF), or other malicious activities through embedded scripts within SVG content.

Affected Products

Bytedesk versions up to and including 1.3.9
Component: source-code/src/main/java/com/bytedesk/core/upload/UploadRestService.java
SVG File Handler functionality

Discovery Timeline

March 8, 2026 - CVE-2026-3749 published to NVD
March 10, 2026 - Last updated in NVD database

Technical Details for CVE-2026-3749

Vulnerability Analysis

The vulnerability exists in the file upload handling mechanism of Bytedesk's core upload service. The handleFileUpload function in UploadRestService.java fails to properly validate and sanitize SVG file uploads, allowing attackers to upload files containing malicious content. SVG files are particularly dangerous because they can contain embedded JavaScript, external entity references, and other active content that executes when the file is rendered in a browser context.

The attack can be executed remotely by any authenticated user with access to the file upload functionality. This represents a significant risk as uploaded malicious SVG files could affect other users viewing the content, potentially leading to account compromise or data theft.

Root Cause

The root cause is insufficient input validation in the file upload mechanism. The handleFileUpload function does not adequately verify the contents of uploaded SVG files, relying instead on file extension or MIME type checks that can be easily bypassed. This allows attackers to upload SVG files containing embedded scripts, external entity declarations, or other malicious payloads that are not stripped or sanitized before storage.

Attack Vector

The attack is network-based and requires low-privilege authentication. An attacker with valid credentials can craft a malicious SVG file containing embedded JavaScript or other dangerous content and upload it through the vulnerable file handler. When other users view or interact with the uploaded SVG content, the malicious payload executes in their browser context.

The exploit has been made publicly available, increasing the likelihood of exploitation in the wild. Organizations running affected versions should prioritize remediation.

java

// Security patch excerpt - URI import addition for improved validation
// Source: https://github.com/Bytedesk/bytedesk/commit/975e39e4dd527596987559f56c5f9f973f64eff7

  */
 package com.bytedesk.ai.springai.providers.gitee;
 
+import java.net.URI;
 import java.util.List;
 import java.util.Map;

The patch introduces java.net.URI imports across multiple service files, enabling proper URI validation and sanitization to prevent malicious file uploads and related attack vectors.

Detection Methods for CVE-2026-3749

Indicators of Compromise

Unusual SVG file uploads containing embedded <script> tags or JavaScript event handlers
SVG files with external entity references (XXE patterns) in upload directories
Web server logs showing repeated file upload requests to the /upload endpoints
Client-side error reports indicating script execution from unexpected SVG sources

Detection Strategies

Implement web application firewall (WAF) rules to inspect SVG file contents for malicious patterns
Monitor file upload endpoints for suspicious activity patterns, including rapid uploads or unusual file sizes
Deploy content security policies (CSP) to restrict script execution from uploaded content
Use file integrity monitoring to detect unauthorized modifications to the upload handling code

Monitoring Recommendations

Enable detailed logging on the UploadRestService.java component and related upload endpoints
Configure alerts for SVG file uploads containing known malicious patterns such as <script>, onload, onerror, or external entity declarations
Monitor for anomalous user behavior around file upload functionality
Implement real-time scanning of uploaded files before they are stored or served

How to Mitigate CVE-2026-3749

Immediate Actions Required

Upgrade Bytedesk to version 1.4.5.1 or later immediately
Review existing uploaded SVG files for malicious content and remove any suspicious files
Implement temporary restrictions on SVG file uploads until the patch is applied
Audit user activity logs for potential exploitation attempts

Patch Information

The vulnerability has been addressed in Bytedesk version 1.4.5.1. The fix is contained in commit 975e39e4dd527596987559f56c5f9f973f64eff7, which introduces proper URI validation and input sanitization for file uploads. Organizations should apply this patch by upgrading to the fixed version.

Patch resources:

Workarounds

Disable SVG file upload functionality entirely until the patch can be applied
Implement server-side SVG sanitization using libraries that strip embedded scripts and external entity references
Configure content security policy headers to prevent script execution from uploaded content directories
Restrict file upload functionality to trusted users only through access control modifications

bash

# Configuration example - Disable SVG uploads via file extension filtering
# Add to application configuration or web server rules

# Nginx example - Block SVG uploads at web server level
location /api/upload {
    # Deny SVG file uploads temporarily
    if ($request_body ~* "\.svg") {
        return 403;
    }
}

# Application-level - Add to allowed file types configuration
# Remove 'svg' from ALLOWED_FILE_EXTENSIONS until patch is applied
ALLOWED_FILE_EXTENSIONS=jpg,jpeg,png,gif,pdf,doc,docx