Description

A malicious Flatpak extension can probe the host filesystem to determine what files and directories exist at arbitrary paths, and host directory listings can be disclosed to sandboxed applications using the extension. Additionally, unvalidated extension metadata can cause extension content to be mounted at unintended locations inside the sandbox.

Executive Summary

Technical Details

Mitigations

References

Additional Metadata

{
  "score": "3.1",
  "pubDate": "2026-09-27T22:17:06.430Z",
  "pubdate": "2026-09-27T22:17:06.430Z",
  "executiveSummary": "This vulnerability involves improper validation and isolation of Flatpak extensions, leading to unauthorized host filesystem access and sandbox containment breaches.\nThe vulnerability type is classified as an authorization and validation flaw within the Flatpak extension management mechanism.\nImpact includes the potential for host filesystem reconnaissance, disclosure of sensitive directory listings to sandboxed applications, and arbitrary mounting of extension content.\nAffected systems are those utilizing the Flatpak extension architecture, which relies on the assumption that extensions are trusted and correctly scoped.\nRisk implications are high as the vulnerability undermines the core security promise of Flatpak, which is to isolate applications from the host environment.\nAn attacker capable of delivering a malicious Flatpak extension can compromise the security boundaries of the sandbox without requiring elevated privileges on the host system.\nExploitation requires the victim to install or utilize a malicious extension, at which point the attacker gains visibility into the host filesystem and can manipulate the application's internal environment.",
  "technicalDetails": "The root cause of this vulnerability lies in insufficient validation of extension metadata and the reliance on flawed mechanisms for mounting extension content into the Flatpak sandbox environment.\nFlatpak extensions are intended to provide supplemental functionality; however, the lack of rigorous input sanitization regarding metadata allows an attacker to manipulate mount points.\nSpecifically, when an extension is processed, the system fails to restrict the paths specified in the metadata, enabling the mount of content at unintended locations within the application's sandbox. This facilitates a path traversal-like condition, where the extension can map directories from the host filesystem directly into the sandbox namespace.\nAttack flow involves the following steps: First, the attacker crafts a malicious Flatpak extension containing specially crafted metadata fields designed to bypass existing path constraints. Second, the attacker distributes this extension to target users. Third, upon installation or runtime loading, the Flatpak subsystem processes the malicious metadata without verifying the integrity or intended scope of the mount point. Fourth, the malicious extension maps host-level directory structures into the sandbox. Fifth, the sandboxed application, now containing the malicious extension, gains the ability to traverse these unauthorized mount points.\nThe vulnerability allows the sandboxed application to determine the existence of arbitrary files and directories on the host filesystem via standard file system APIs. By probing for specific paths, an attacker can map the host's directory structure, identify installed software, or locate sensitive data files residing outside the expected container boundaries.\nFurthermore, the ability to control mounting locations means that extension content can be placed where it might override critical application configuration or library files, potentially leading to local code execution or privilege escalation within the context of the sandboxed application.\nThe vulnerability does not require prior authentication or elevated privileges, provided the user can be enticed to install the malicious extension. There is no requirement for network exposure as the exploitation occurs locally upon the execution of the Flatpak runtime components during the extension loading process."
}