CVE-2026-123838 in IntelliJ IDEA
Summary
by MITRE • 11/06/2026
In JetBrains IntelliJ IDEA before 2026.3 code execution was possible in Safe Mode via UI Designer form file
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Analysis
by VulDB Data Team • 11/06/2026
The vulnerability identified in versions of JetBrains IntelliJ IDEA prior to version 2026.3 represents a critical security flaw within the integrated development environment's user interface designer component. This issue specifically affects the handling of form files when the application is operating in Safe Mode, a state typically designed to disable plugins and potentially restrict certain dynamic behaviors to ensure stability or prevent malicious code execution from third-party extensions. The core technical flaw lies in the insufficient validation and sanitization of data parsed from UI Designer form files during this restricted operational mode. While Safe Mode is intended to provide a sandboxed environment that limits potential attack surfaces, the parser responsible for interpreting these XML-based configuration files fails to adequately restrict object instantiation or method invocation capabilities. This oversight allows an attacker who can influence the content of a loaded form file to bypass the expected restrictions imposed by the safe execution context.
From a technical perspective, this vulnerability is classified under CWE-94 as Improper Control of Generation of Code (Code Injection). The root cause stems from the application's reliance on dynamic deserialization or reflection mechanisms when loading UI components defined in these files. When IntelliJ IDEA processes a form file, it instantiates various Java objects corresponding to visual elements like buttons, text fields, and panels. In standard operation, this process is controlled by strict whitelists of allowed classes. However, within Safe Mode, the security controls intended to limit execution paths are either bypassed or incorrectly applied due to logic errors in the parsing routine. Consequently, an attacker can craft a malicious form file containing specially constructed object definitions that trigger unintended code execution upon loading. This effectively neutralizes one of the primary safety features of the IDE, allowing arbitrary Java bytecode to be executed within the context of the running application process.
The operational impact of this vulnerability is significant for developers and organizations relying on IntelliJ IDEA as their primary development tool. An attacker who can trick a user into opening a maliciously crafted form file could achieve remote code execution with the privileges of the current user account. This level of access allows for comprehensive compromise of the local system, including theft of sensitive source code, credentials stored in configuration files, and other proprietary intellectual property. Furthermore, because IntelliJ IDEA is often used to manage complex projects involving multiple dependencies, successful exploitation could serve as an initial foothold for lateral movement within a corporate network if connected resources are accessible from the compromised workstation. The ability to execute code in Safe Mode is particularly dangerous because users may perceive this mode as secure and therefore lower their guard regarding file sources they open or share with colleagues.
This vulnerability aligns with MITRE ATT&CK technique T1059, Command and Scripting Interpreter, specifically through Java-based execution vectors such as Jython or Groovy if applicable to the specific implementation details of the UI designer's scripting capabilities. It also relates to T1204, User Execution, as it requires a user action to open the malicious file, highlighting the importance of social engineering in this attack vector. The flaw underscores the risks associated with complex software ecosystems that utilize dynamic loading and reflection for functionality like GUI design tools. These features are powerful but inherently difficult to secure completely against crafted inputs that exploit edge cases in parsing logic.
Mitigation strategies primarily involve upgrading to IntelliJ IDEA version 2026.3 or later, where JetBrains has addressed the validation gaps in the UI Designer's form file parser. Until an upgrade is feasible, users should exercise extreme caution when opening project files or design forms from untrusted sources, even if they intend to use Safe Mode for stability reasons. Security teams should also consider implementing application control policies that restrict the execution of Java processes unless explicitly authorized by whitelisted signatures. Additionally, network segmentation and endpoint detection and response solutions can help monitor for anomalous behavior indicative of code injection attempts within development environments. Regular patching cycles are essential to maintain the integrity of these sophisticated tools against evolving threats targeting their complex internal architectures.