CVE-2026-100844 in MONAI
Summary
by MITRE • 09/27/2026
MONAI before 1.6.0 is vulnerable to OS command injection in the nnUNetV2Runner component (monai.apps.nnunet.nnunetv2_runner). User-controlled values taken from the YAML configuration file (notably dataset_name_or_id) and from CLI/kwargs arguments are concatenated into a command string without quoting or validation and then passed to subprocess with shell=True, so shell metacharacters (e.g., ';' on Linux, '&' on Windows) are interpreted. If a victim loads and processes a crafted configuration file — for example by instantiating nnUNetV2Runner with the malicious YAML and invoking a training/validation job such as train_single_model() — arbitrary commands are executed with the privileges of the user running the job.
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Analysis
by VulDB Data Team • 09/27/2026
The vulnerability identified in MONAI versions prior to 1.6.0 represents a critical OS command injection flaw within the nnUNetV2Runner component, specifically located at monai.apps.nnunet.nnunetv2_runner. This security defect arises from improper neutralization of special elements used in operating system commands, allowing an attacker to inject and execute arbitrary shell commands through maliciously crafted inputs. The core technical issue lies in how the application constructs command strings for subprocess execution. User-controlled values, which are sourced directly from YAML configuration files—particularly the dataset_name_or_id parameter—and from Command Line Interface arguments or keyword arguments passed during initialization, are concatenated into a single string without any form of quoting, escaping, or validation checks. This lack of sanitization means that shell metacharacters such as semicolons on Linux systems and ampersands on Windows systems are not treated as literal characters but are instead interpreted by the underlying operating system shell as command separators or logical operators.
When a victim instantiates the nnUNetV2Runner class with a malicious YAML configuration file and subsequently invokes methods that trigger model training or validation, such as train_single_model(), the application passes this unsanitized string to subprocess functions configured with shell=True. The use of shell=True instructs Python to execute the command via the system's default shell rather than directly invoking the executable. Consequently, any special characters embedded within the user-controlled input are processed by the shell before execution. An attacker can exploit this behavior by embedding commands like ; rm -rf / or & whoami into the dataset_name_or_id field of a YAML file. Upon processing, these injected sequences cause the system to execute additional arbitrary commands alongside the intended medical imaging analysis tasks, effectively bypassing any application-level security controls that might otherwise restrict execution scope.
The operational impact of this vulnerability is severe due to the context in which MONAI typically operates. Medical image analysis pipelines often run with elevated privileges or access to sensitive datasets containing patient health information. If an attacker can induce a system administrator or data scientist to load and process a crafted configuration file, they achieve remote code execution with the same permissions as the user running the job. This could lead to complete compromise of the host system, including theft of confidential medical records, installation of malware, pivoting to other systems on the network, or destruction of critical infrastructure. The attack vector is particularly dangerous because it does not require direct interaction with a web service; instead, it relies on social engineering or supply chain attacks where a malicious YAML file is distributed and subsequently loaded by legitimate users within their trusted environments.
This vulnerability aligns closely with CWE-78 Improper Neutralization of Special Elements used in an OS Command, commonly known as OS Command Injection. It also maps to the MITRE ATT&CK framework technique T1059 Command and Scripting Interpreter, specifically under sub-techniques related to shell commands such as sh or cmd.exe. The exploitation path involves crafting a malicious input payload that leverages shell metacharacters to break out of the intended command context and execute arbitrary code. To mitigate this risk, developers must immediately upgrade MONAI to version 1.6.0 or later where these issues have been addressed. For systems still running older versions, it is imperative to avoid using shell=True in subprocess calls whenever possible, opting instead for direct execution with a list of arguments which prevents the shell from interpreting metacharacters. Additionally, strict input validation and whitelisting should be implemented for all user-controlled parameters that are passed into system commands, ensuring that only expected alphanumeric characters or specific safe patterns are accepted.