CVE-2026-63952 in Linux
Summary
by MITRE • 07/19/2026
In the Linux kernel, the following vulnerability has been resolved:
memfd: deny writeable mappings when implying SEAL_WRITE
When SEAL_EXEC is added, SEAL_WRITE is implied to make W^X. But the implied seal is set after the check that makes sure the memfd can not have any writable mappings. This means one can use SEAL_EXEC to apply SEAL_WRITE while having writeable mappings.
This breaks the contract that SEAL_WRITE provides and can be used by an attacker to pass a memfd that appears to be write sealed but can still be modified arbitrarily.
Fix this by adding the implied seals before the call for mapping_deny_writable() is done.
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Analysis
by VulDB Data Team • 07/19/2026
The vulnerability resides in the Linux kernel's memory file descriptor implementation, specifically within the memfd subsystem where the sealing mechanism fails to properly enforce write restrictions. This issue occurs when the SEAL_EXEC flag is applied to a memory file descriptor, as it implicitly implies the SEAL_WRITE flag to maintain W^X (Write XOR Execute) security properties. The flaw manifests in the order of operations during the sealing process, where the implied SEAL_WRITE seal is applied after the validation check that prevents writable mappings from being established. This temporal discrepancy creates a window where attackers can exploit the system by first creating a memfd with SEAL_EXEC while maintaining writable mappings, thereby circumventing the intended security controls.
The technical implementation flaw stems from improper ordering of operations within the kernel's memory management subsystem, specifically in how it handles the sealing flags for memory file descriptors. When SEAL_EXEC is set, the kernel should immediately enforce all implied seals including SEAL_WRITE to prevent any writable mappings from being created. However, the current implementation delays this enforcement until after the mapping_deny_writable() check has already been completed, allowing malicious actors to establish writable mappings before the proper sealing restrictions are applied. This violation of the expected security contract undermines the fundamental purpose of memory file descriptor seals and creates a persistent attack vector.
The operational impact of this vulnerability extends beyond simple privilege escalation as it fundamentally compromises the integrity of the memory file descriptor security model. Attackers can exploit this weakness to create seemingly secure memory file descriptors that actually remain modifiable, potentially allowing them to inject malicious code into processes or manipulate data structures in ways that should have been prevented by the sealing mechanism. This vulnerability affects any system running Linux kernels where memfd operations are utilized, particularly those applications relying on memory file descriptors for secure temporary storage or inter-process communication. The implications are significant for containerized environments and systems implementing strict memory access controls.
The fix implemented addresses this vulnerability by reordering the seal application process to ensure that implied seals are applied before any mapping validation occurs. This modification aligns with established security principles and follows the principle of least privilege by ensuring that all security restrictions are properly enforced from the outset. The solution directly corresponds to the common weakness pattern identified in CWE-284, which deals with improper access control mechanisms, and addresses ATT&CK technique T1059.007 for process injection through memory manipulation. By enforcing the implied SEAL_WRITE flag before checking mapping restrictions, the kernel ensures that all security properties are properly maintained throughout the entire initialization sequence of memory file descriptors, thereby preventing attackers from exploiting the temporal window that previously existed between seal application and mapping validation.