CVE-2026-96807 in Flatpak
Summary
by MITRE • 09/23/2026
In Flatpak before 1.18.1, a malicious sandboxed app can replace ~/.var/app/$appid/.ld.so with a symlink, causing regenerate_ld_cache to write files at an arbitrary location. The filenames and content are not attacker controlled, making this hard to exploit.
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Analysis
by VulDB Data Team • 09/23/2026
The vulnerability identified in Flatpak versions prior to 1.18.1 represents a significant security flaw within the application sandboxing mechanism, specifically affecting how dynamic linker caches are managed for individual applications. Flatpak is designed to provide a secure environment by isolating applications from the host system and other applications through namespaces and file system restrictions. However, this specific issue arises during the maintenance phase of an application's runtime environment, particularly when the ld.so.cache regeneration process is triggered. The core technical flaw lies in the handling of symbolic links within the per-application configuration directory located at ~/.var/app/$appid/. By allowing a malicious sandboxed app to create or replace the .ld.so file with a symlink pointing outside its designated sandbox boundary, an attacker can manipulate the path resolution logic used by the regenerate_ld_cache utility. This operation is typically performed when new shared libraries are installed or updated for a specific application, ensuring that the dynamic linker knows where to find required dependencies efficiently.
From a technical perspective, this vulnerability falls under the category of Path Traversal and Symbolic Link Following issues, which aligns with CWE-59: Improper Link Resolution Before File Access. The attacker's ability to control the target of the symbolic link allows them to redirect file write operations initiated by the system utility regenerate_ld_cache. Although the content written is not directly controlled by the attacker in terms of payload data, and the filenames are generated deterministically based on library names, the destination path can be manipulated. This means that if an application with elevated privileges or specific capabilities runs this cache regeneration process while a malicious app has planted such a symlink, files could be created at arbitrary locations within the user's home directory or potentially other accessible paths depending on the execution context of the daemon handling these operations.
The operational impact of this vulnerability is nuanced due to the constraints mentioned in the original description. Because the attacker cannot control the content of the written files nor their exact filenames, direct code execution via traditional buffer overflow techniques is not feasible through this vector alone. Instead, the primary risk involves file system manipulation and potential privilege escalation if the regeneration process runs with higher privileges than the sandboxed application itself. For instance, creating unexpected configuration files or overwriting existing non-critical data could disrupt normal operation of other applications or lead to information disclosure by revealing directory structures that were previously inaccessible. In scenarios where Flatpak daemons run as root or have broader system access, this symlink attack vector becomes more dangerous, potentially allowing the creation of files in sensitive directories like /etc/ or within another user's profile if permissions are misconfigured.
This behavior also relates to ATT&CK technique T1548: Abuse Elevation Control Mechanism, specifically regarding abuse of setuid/setgid programs or privileged processes that perform file operations without adequate validation of input paths. The lack of strict canonicalization and symlink safety checks in the cache regeneration utility constitutes a failure in secure coding practices related to path handling. While the difficulty of exploitation limits the immediate threat level for most standard users, it remains a critical flaw for environments where Flatpak is used with elevated trust models or when combined with other vulnerabilities that allow an attacker to trigger the ld.so.cache update process remotely or via social engineering tactics that compel a user to install malicious applications.
Mitigation strategies primarily involve upgrading to Flatpak version 1.18.1 or later, which addresses this issue by implementing stricter validation of symbolic links during cache regeneration operations. Developers should ensure that any file system interactions involving dynamic linker caches verify the real path of target files and reject operations where symlinks point outside the allowed sandbox boundaries. Additionally, applying principle of least privilege to the processes responsible for managing application runtimes can reduce the impact if such a vulnerability were present in other components. System administrators should monitor for unusual symlink creations within ~/.var/app/ directories as part of their security posture, although given the difficulty of exploitation, this is more of a defensive measure than an immediate remediation requirement for most deployments until updates are applied.