CVE-2026-74618 in Linux
Summary
by MITRE • 08/22/2026
In the Linux kernel, the following vulnerability has been resolved:
binfmt_misc: don't warn when the mount is completed from another user namespace
fsopen() records the caller's user namespace in fc->user_ns and hands back an ordinary file descriptor. Nothing ties the task that calls fsconfig(FSCONFIG_CMD_CREATE) to the task that created the context. The fd is inherited across fork() and exec() and it can be passed over a unix socket.
Completing a context from another user namespace is allowed on purpose. vfs_cmd_create() authorizes the create with mount_capable(), which for FS_USERNS_MOUNT checks ns_capable(fc->user_ns, CAP_SYS_ADMIN), and that succeeds for a task holding CAP_SYS_ADMIN in an ancestor of fc->user_ns. So an unprivileged task can reach the WARN_ON() in bm_fill_super(): create a user and a mount namespace in a child, call fsopen("binfmt_misc") there, send the fscontext fd to the parent and let the parent issue FSCONFIG_CMD_CREATE. Both namespaces come from a plain unshare(1) and no capability is needed anywhere:
WARNING: fs/binfmt_misc.c:938 at bm_fill_super+0xa2/0xc0 [binfmt_misc]
CPU: 15 UID: 1000 PID: 3243382 Comm: fswarn Call Trace: get_tree_keyed+0x7d/0xb0 bm_get_tree+0x34/0x90 [binfmt_misc]
vfs_get_tree+0x2a/0x100 vfs_cmd_create+0x60/0xf0 __do_sys_fsconfig+0x4b2/0x500
The child needs the mount namespace because fsopen() itself gates on may_mount(), which asks for CAP_SYS_ADMIN in the user namespace owning the caller's mount namespace. fsconfig() doesn't repeat that check.
It is a WARN_ON() and not a WARN_ON_ONCE(), so the condition can be raised in a loop to taint the kernel and flood the log, and it panics a kernel booted with panic_on_warn.
Keep refusing the mount and stop warning about it. Nothing in bm_fill_super() depends on the two namespaces matching, it derives everything from sb->s_user_ns.
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Analysis
by VulDB Data Team • 08/23/2026
The Linux kernel vulnerability identified in binfmt_misc involves an improper validation of user namespace context during filesystem mounting operations, specifically when utilizing the fsopen and fsconfig system calls. The core technical flaw resides in the bm_fill_super function within the binfmt_misc driver, which contains a WARN_ON macro that triggers if the mount is completed from a different user namespace than the one recorded by fsopen. This check was originally intended to ensure consistency between the context creation task and the execution task but fails to account for legitimate use cases where file descriptors are passed across process boundaries or namespaces via mechanisms such as fork, exec, or unix domain sockets. The vulnerability arises because vfs_cmd_create correctly authorizes the mount operation using mount_capable with a check against fc->user_ns, allowing tasks holding CAP_SYS_ADMIN in an ancestor namespace to successfully complete mounts initiated by unprivileged processes in descendant namespaces. However, the subsequent WARN_ON condition incorrectly flags this authorized and functional behavior as anomalous, leading to false positive warnings that disrupt system stability and logging integrity.
From an operational impact perspective, this flaw allows for denial of service through log flooding and potential kernel panic conditions. Since the warning is implemented using WARN_ON rather than WARN_ON_ONCE, it can be triggered repeatedly in a loop by an unprivileged user who creates separate user and mount namespaces, opens a binfmt_misc filesystem context, passes the file descriptor to another process or namespace, and then initiates the creation command. This results in continuous kernel warnings that taint the kernel state and fill system logs with noise. In environments where panic_on_warn is enabled for security hardening purposes, this vulnerability can be exploited to force a complete kernel panic and subsequent system reboot without requiring any elevated privileges beyond those available to standard unprivileged users within their own user namespace boundaries. This represents a significant availability risk as it enables local denial of service attacks through resource exhaustion or forced reboots via simple syscall sequences that are otherwise valid under the Linux security model.
The technical root cause is rooted in the decoupling of context creation and execution phases allowed by modern filesystem interface designs, combined with overly strict validation logic within specific drivers like binfmt_misc. The fsopen function records the caller's user namespace but does not enforce a binding between that initial task and the task later calling fsconfig to create the mount. While vfs_cmd_create performs adequate capability checks against fc->user_ns, bm_fill_super redundantly verifies namespace equality without justification from functional requirements. This discrepancy highlights a gap in security validation where defensive coding practices inadvertently introduce stability issues by rejecting valid operational patterns supported by the kernel's architecture. The fix involves removing the unnecessary WARN_ON check since all necessary authorization is already handled at higher levels and no functionality within bm_fill_super depends on the two namespaces being identical, relying instead solely on sb->s_user_ns for security decisions.
This vulnerability aligns with CWE-20 Improper Input Validation as it reflects a failure to correctly validate input parameters regarding namespace context during critical system operations. It also relates to CWE-754 Improper Check for Unusual or Exceptional Conditions, where the code incorrectly treats a valid state change as an error condition leading to adverse effects. In terms of ATT&CK mapping, this issue facilitates Local Privilege Escalation via Denial of Service techniques under T1053 Scheduled Task/Job and specifically leverages local resource exhaustion methods similar to those described in T1499 Endpoint Denial of Service. Mitigation strategies primarily involve applying the upstream kernel patch that removes the erroneous warning check, thereby aligning driver behavior with actual security requirements rather than artificial constraints. System administrators should ensure their kernels are updated to versions containing this fix and monitor for any residual log anomalies indicating incomplete patches. Additionally, organizations running systems with panic_on_warn enabled must prioritize these updates to prevent accidental service disruptions caused by benign administrative actions or automated scripts that legitimately utilize cross-namespace filesystem operations.