Description

A flaw was found in Flatpak. A Time-of-check to time-of-use (TOCTOU) race condition exists in the `org.freedesktop.Flatpak.SystemHelper` component. This vulnerability occurs because a privileged `chmod` operation executes before the OSTree repository validation within the `Deploy()` function. An attacker can exploit this timing window to redirect symlinks to arbitrary files, potentially leading to unauthorized file manipulation or information disclosure.

Executive Summary

Technical Details

Mitigations

References

Additional Metadata

{
  "score": "5.8",
  "pubDate": "2026-09-04T22:17:18.190Z",
  "pubdate": "2026-09-04T22:17:18.190Z",
  "executiveSummary": "A critical Time-of-Check to Time-of-Use (TOCTOU) race condition vulnerability has been identified within the org.freedesktop.Flatpak.SystemHelper component. The flaw arises from an insecure sequence of operations during the deployment process, where file permissions are modified via chmod prior to verifying the integrity of the underlying OSTree repository.\nThis vulnerability allows an authenticated local attacker to manipulate the filesystem by exploiting the timing window between the privileged permission change and the subsequent validation check. By redirecting symlinks during this race window, an attacker can coerce the SystemHelper process to interact with arbitrary files on the host system. The impact of this exploit includes potential unauthorized file manipulation, privilege escalation, or sensitive information disclosure, depending on the target file accessed. Exploitation requires local access to the system and the ability to influence filesystem structures within the target environment. Given that the vulnerable component operates with elevated privileges, this flaw represents a significant risk to the integrity and confidentiality of the host operating system.",
  "technicalDetails": "The vulnerability resides within the Deploy() function of the org.freedesktop.Flatpak.SystemHelper service, which is responsible for managing application deployment and maintenance tasks with elevated system privileges. The core of the issue is a design flaw in the handling of file operations that fails to guarantee atomicity or consistent state validation during the deployment sequence.\nThe attack vector centers on the race condition created by the ordering of operations: the service initiates a privileged chmod operation on deployment-related files or directories before performing the necessary OSTree repository validation. Because the validation occurs after the permissions have already been altered, the state of the filesystem can be modified by a malicious actor during the interval between the chmod call and the validation routine.\nAn attacker can exploit this window by replacing legitimate files or directory components with symlinks pointing to sensitive locations on the host filesystem. When the Deploy() function reaches the validation phase, it traverses the path that now points to an arbitrary location. Because the system service is executing with high privileges, the process follows the symlink, effectively performing actions on unintended target files. This can be used to bypass security controls or perform unauthorized operations such as modifying critical configuration files or reading data that should be restricted.\nThe technical flow is as follows: 1) The attacker initiates a deployment process or monitors for one. 2) The SystemHelper executes chmod, modifying permissions on a controlled directory. 3) During the narrow race window, the attacker swaps a target directory or file with a symlink pointing to a sensitive resource (e.g., /etc/shadow or other root-owned files). 4) The SystemHelper proceeds to the validation step, unknowingly operating on the symlink destination rather than the intended deployment path. 5) The process completes, potentially applying the permission changes to the attacker-supplied target or performing integrity checks on unauthorized data. This bypasses the security intended by the repository validation checks, as the validation only confirms the integrity of data located at the path AFTER the attacker has already manipulated the filesystem structure."
}