CVE-2026-56672 in ComfyUI
Summary
by MITRE • 07/31/2026
ComfyUI is a node-based diffusion model GUI, API, and backend. Prior to 0.28.0, GET /userdata/{file} served user-controlled HTML and SVG files with extension-derived content types, allowing stored cross-site scripting in the ComfyUI origin and access to browser-stored API tokens, settings, workflows, and authenticated-equivalent API calls. The handler used web.FileResponse(path), so an uploaded .html/.svg was served as text/html/image/svg+xml. POST /userdata stores arbitrary request bodies (confined to the user's userdata directory). When a victim navigated to the file URL, the embedded script executed same-origin. The /view endpoint already forced dangerous MIME types to download; that protection had never been applied to /userdata. This issue is fixed in version 0.28.0.
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Analysis
by VulDB Data Team • 07/31/2026
ComfyUI represents a sophisticated node-based interface for diffusion model workflows that has gained significant traction in machine learning and generative AI communities. The vulnerability under examination affects versions prior to 028.0 and resides within the user data handling mechanism of this application. This flaw manifests as a critical stored cross-site scripting vulnerability that leverages the application's permissive file serving behavior, creating a pathway for attackers to execute malicious code within the context of the ComfyUI origin domain. The vulnerability stems from improper content type handling when serving user-uploaded files through the GET /userdata/{file} endpoint.
The technical exploitation mechanism relies on the application's failure to properly validate and sanitize file extensions before determining MIME types for response headers. When users upload HTML or SVG files, the system employs a naive approach by deriving content types directly from file extensions rather than performing proper content inspection. This allows attackers to upload malicious files with .html or .svg extensions that are then served with dangerous content types such as text/html or image/svg+xml respectively. The web.FileResponse(path) implementation in the application's backend does not enforce proper MIME type restrictions, creating an environment where user-controlled content can be interpreted by browsers as executable scripts rather than static assets.
The operational impact of this vulnerability extends beyond simple XSS execution to encompass complete session hijacking and privilege escalation within the ComfyUI environment. When victims navigate to URLs serving malicious files, browser-stored API tokens, user settings, workflow configurations, and authenticated-equivalent API calls become accessible to attackers through same-origin script execution. This creates a sophisticated attack vector where adversaries can establish persistent access to machine learning workflows, potentially compromising sensitive training data, model parameters, and operational configurations. The vulnerability effectively transforms the application's user data storage functionality into an attack surface for privilege escalation and data exfiltration.
The security implications of this flaw align with CWE-79 (Cross-site Scripting) and CWE-20 (Improper Input Validation) classifications, representing a classic case of insufficient input sanitization combined with improper output encoding. From an ATT&CK framework perspective, this vulnerability maps to T1566 (Phishing) through malicious file delivery mechanisms and T1078 (Valid Accounts) as it enables attackers to leverage legitimate user sessions for unauthorized access. The fix implemented in version 0.28.0 specifically addresses this by applying the same protective measures that were already in place for the /view endpoint, ensuring that dangerous MIME types are forced to download rather than execute within the browser context. This remediation aligns with security best practices for content type handling and demonstrates the importance of consistent security controls across all application endpoints.
The vulnerability highlights fundamental security design issues within ComfyUI's file handling architecture, where the distinction between safe and dangerous file types was not properly enforced. Unlike the /view endpoint that already implemented proper MIME type restrictions, the /userdata handler lacked equivalent protections, creating an inconsistent security posture within the same application. This oversight represents a classic example of security by obscurity failing in production environments, where attackers can exploit the least-secure path to achieve maximum impact. The fix requires comprehensive testing of all file upload and serving mechanisms to ensure no similar vulnerabilities exist in other endpoints that might expose user data or session information through improper content type handling.
Organizations utilizing ComfyUI should immediately implement version 0.28.0 or later to remediate this vulnerability, while also considering broader security measures such as implementing Content Security Policy headers, conducting regular security audits of file upload mechanisms, and establishing proper input validation for all user-controlled data. The vulnerability serves as a reminder that even seemingly benign features like user data storage can become critical attack vectors when proper security controls are not applied consistently across all application components.