CVE-2026-33150 in libfuse
Summary
by MITRE • 03/20/2026
libfuse is the reference implementation of the Linux FUSE. From version 3.18.0 to before version 3.18.2, a use-after-free vulnerability in the io_uring subsystem of libfuse allows a local attacker to crash FUSE filesystem processes and potentially execute arbitrary code. When io_uring thread creation fails due to resource exhaustion (e.g., cgroup pids.max), fuse_uring_start() frees the ring pool structure but stores the dangling pointer in the session state, leading to a use-after-free when the session shuts down. The trigger is reliable in containerized environments where cgroup pids.max limits naturally constrain thread creation. This issue has been patched in version 3.18.2.
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Analysis
by VulDB Data Team • 03/30/2026
The vulnerability identified as CVE-2026-33150 affects libfuse, which serves as the standard reference implementation for the Linux FUSE (Filesystem in Userspace) framework that enables non-privileged users to create custom filesystems without modifying kernel code. This particular flaw exists in versions 3.18.0 through 3.18.1 of libfuse and represents a critical use-after-free condition within the io_uring subsystem that can be exploited by local attackers to cause system instability and potentially achieve arbitrary code execution. The issue stems from improper memory management during the io_uring thread initialization process where the system fails to properly handle resource exhaustion scenarios that are common in modern containerized environments.
The technical root cause of this vulnerability lies in the fuse_uring_start() function which manages io_uring thread creation and resource allocation. When the system encounters resource constraints such as cgroup pids.max limits that naturally restrict thread creation in containerized deployments, the function attempts to free memory allocated for the ring pool structure but fails to properly nullify the pointer reference. This creates a dangling pointer situation where subsequent operations during session shutdown attempt to access memory that has already been freed, resulting in a use-after-free condition. The vulnerability is particularly dangerous because the trigger mechanism is highly reliable in containerized environments where resource limits are commonly enforced, making it an attractive target for exploitation.
The operational impact of this vulnerability extends beyond simple system crashes to potentially enable privilege escalation and arbitrary code execution within the affected environment. Local attackers who can control or influence the conditions that lead to io_uring thread creation failure can reliably trigger the use-after-free scenario, causing FUSE filesystem processes to crash and potentially allowing for code execution with the privileges of the affected process. This is particularly concerning in containerized environments where FUSE filesystems are commonly used for storage and file system operations, as the vulnerability can be exploited to compromise container integrity and potentially escalate privileges to the host system. The vulnerability affects not only the stability of individual processes but also the overall security posture of systems that rely on FUSE for file system operations.
Mitigation strategies for CVE-2026-33150 primarily focus on upgrading to libfuse version 3.18.2 or later where the issue has been resolved through proper memory management practices that prevent the dangling pointer condition. Organizations should prioritize patching affected systems, particularly those running containerized workloads where resource constraints can reliably trigger the vulnerability. Additionally, system administrators should monitor for resource exhaustion conditions and implement proper cgroup configurations to avoid hitting pids.max limits that could lead to the vulnerable code path. The fix addresses the underlying memory management issue by ensuring that freed memory pointers are properly cleared and that session state management accounts for potential resource allocation failures. This vulnerability aligns with CWE-416, which describes use-after-free conditions, and represents a significant concern for security practitioners managing containerized environments where resource constraints are common. The ATT&CK framework categorizes this as a privilege escalation technique through resource exhaustion and memory corruption vulnerabilities, emphasizing the need for robust memory management practices in system libraries that handle concurrent operations and resource allocation.