CVE-2026-49383: JetBrains IntelliJ IDEA XXE Vulnerability

CVE-2026-49383 Overview

CVE-2026-49383 is an XML External Entity (XXE) vulnerability affecting JetBrains IntelliJ IDEA versions prior to 2026.1. The flaw resides in the UI Designer form parser, which improperly processes XML input containing external entity references [CWE-611]. An attacker exploits this issue by tricking a developer into opening a crafted .form file within the integrated development environment. Successful exploitation results in limited local information disclosure. JetBrains addressed the issue in IntelliJ IDEA 2026.1.

Critical Impact

Opening a malicious UI Designer form file in a vulnerable IntelliJ IDEA installation can disclose limited local file contents through XML external entity processing.

Affected Products

• JetBrains IntelliJ IDEA versions prior to 2026.1
• Projects containing untrusted UI Designer .form files
• Developer workstations running vulnerable IntelliJ IDEA builds

Discovery Timeline

• 2026-05-29 - CVE-2026-49383 published to NVD
• 2026-06-01 - Last updated in NVD database

Technical Details for CVE-2026-49383

Vulnerability Analysis

The vulnerability stems from insecure XML parsing within the IntelliJ IDEA UI Designer component. The UI Designer uses .form files, which are XML documents describing Swing user interface layouts. The parser processing these files does not disable external entity resolution, leaving it susceptible to XML External Entity (XXE) injection [CWE-611].

An attacker crafts a .form file containing a malicious external entity declaration. When a developer opens the file, the parser dereferences the entity and reads local file contents accessible to the IDE process. The retrieved data can be exfiltrated through error messages, file references embedded in the UI, or out-of-band channels depending on parser configuration.

The attack vector is local and requires user interaction, limiting practical exploitation to targeted scenarios such as malicious pull requests, shared project archives, or trojanized repository clones.

Root Cause

The root cause is an XML parser configured without secure defaults. The UI Designer form parser fails to set features such as disallow-doctype-decl or to disable external general and parameter entities, allowing entity resolution against the local filesystem.

Attack Vector

Exploitation requires local access and user interaction. An attacker delivers a malicious .form file through a repository, archive, or shared project. When the developer opens the file in the UI Designer, the parser processes the embedded external entity and reads contents from files readable by the IDE process. No elevated privileges are required by the attacker.

The vulnerability does not yield code execution or write access. Impact is restricted to confidentiality of files accessible to the IntelliJ IDEA process on the local system.

Detection Methods for CVE-2026-49383

Indicators of Compromise

• .form files containing <!DOCTYPE> declarations or <!ENTITY> references with SYSTEM identifiers
• Unexpected outbound network connections from the IntelliJ IDEA process to attacker-controlled hosts
• IntelliJ IDEA process reads of sensitive files such as /etc/passwd, SSH keys, or credential stores following a project open event

Detection Strategies

• Scan source repositories for .form files containing external entity declarations or SYSTEM keywords prior to opening projects
• Monitor IntelliJ IDEA process file access patterns for reads of sensitive files outside the project workspace
• Alert on outbound HTTP, FTP, or DNS traffic originating from the IDE process to unexpected destinations

Monitoring Recommendations

• Inventory installed IntelliJ IDEA versions across developer endpoints and flag instances below 2026.1
• Track project open events and correlate with subsequent file access and network activity from the IDE process
• Apply endpoint detection rules that capture XXE-style file reads triggered by IDE child processes

How to Mitigate CVE-2026-49383

Immediate Actions Required

• Upgrade JetBrains IntelliJ IDEA to version 2026.1 or later on all developer workstations
• Audit existing projects for untrusted .form files and remove or sanitize entries containing DOCTYPE or external entity declarations
• Restrict opening of untrusted projects, archives, and third-party pull requests until patching is complete

Patch Information

JetBrains released a fix in IntelliJ IDEA 2026.1 that disables external entity resolution in the UI Designer form parser. Refer to the JetBrains Security Issues Fixed page for the official advisory and version details.

Workarounds

• Avoid opening UI Designer .form files from untrusted sources until the IDE is upgraded
• Run IntelliJ IDEA under a least-privileged user account to limit files accessible through XXE
• Use network egress controls to block IDE process connections to non-approved destinations, reducing data exfiltration paths

# Verify installed IntelliJ IDEA version on Linux/macOS
idea --version

# Search a project for potentially malicious .form files
grep -rEl "<!DOCTYPE|<!ENTITY|SYSTEM\s+\"" --include="*.form" /path/to/project