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CVE-2026-63952
memfd: deny writeable mappings when implying SEAL_WRITE
Published 2026-07-19 (3 months ago) by the Linux kernel CNA. Subsystem mm. Debian urgency: unimportant.
What to do, per Debian release
Debian's own status for CVE-2026-63952 in every release this site tracks, and what each one means in practice. Every status below is the one the Debian Security Team publishes for this CVE. source
| Release | Status | Version | What this means |
|---|---|---|---|
| Debian 12bookworm | not affected | 6.1.190-1 | Nothing to do. bookworm never shipped the vulnerable code. |
| Debian 13trixie | fixed | 6.12.94-1 | Update the kernel and reboot. Fixed in 6.12.94-1. A running kernel keeps the old code until the machine restarts. |
| Debian 14forky | fixed | 7.0.12-1 | Update the kernel and reboot. Fixed in 7.0.12-1. A running kernel keeps the old code until the machine restarts. |
| Debian unstablesid | fixed | 7.0.12-1 | Update the kernel and reboot. Fixed in 7.0.12-1. A running kernel keeps the old code until the machine restarts. |
The vulnerability
What the bug is
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.
Where it lives
The fix touches these files. If the subsystem is one you do not use, the practical exposure is lower, though the package is still the vulnerable one.
mm/memfd.c
- What this is
- TMPFS (SHMEM FILESYSTEM)MAINTAINERS section matched on mm/memfd.c · linux-mm@kvack.org · Maintained
Check it on the machine
grep -w CONFIG_MEMFD_CREATE /boot/config-$(uname -r)Reads the booted kernel's own configuration, on the machine and nowhere else. =y is built in, =m is a module, and an is not set line means the code is not there.
Show whether Debian builds this, per release
Debian builds this code. Every release below compiles the affected file into the kernels it ships.
- bookworm, trixie, forky, sidy
- y
- built into the kernel image on every flavour - only a patch removes it
- Build switch
- CONFIG_MEMFD_CREATE
From the configuration Debian publishes for every kernel flavour it builds. It covers only the architectures Debian ships a linux-config package for, and nothing about a kernel you built yourself. Debian's own status above stays authoritative about the source package.MAINTAINERS kbuild Makefiles Debian linux-config packages
Signals
- Published
- 2026-07-19 (3 months ago)source
- CVSS
- 8.4 high
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:Nsource recompute it - Attack vector
- local (AV:L)source
- EPSS
- 0.16%, higher than 5.3% of all CVEssource
- CISA KEV
- not listedthe KEV catalogue
- Debian urgency
- unimportantsource
- Subsystem
- mm
Upstream fixes
The versions the CNA records as introducing and fixing this, and the commit for each. source
| Introduced | Fixed in | Commit |
|---|---|---|
| 6.3 | 6.6.143 | b3f4f82d1315 |
| 6.3 | 6.12.93 | 3be2a24f7f72 |
| 6.3 | 6.18.35 | 0995d1f79aed |
| 6.3 | 7.0.12 | 555702282d45 |
| 6.3 | 7.1 | 3b041514cb6e |
Why the CNA scored it this way
Quoted from the kernel CNA's own scoring file, word for word. A highlighted metric carries the most severe value CVSS v3.1 defines for it, which is what pushes the score up. source
- AV:LAttack vector, local
- Exploitation requires local syscalls (memfd_create, mmap, fcntl) with no network-facing code path; the vulnerable logic is reached only through the local fcntl(F_ADD_SEALS) interface on a memfd file descriptor.
- AC:LAttack complexity, low
- The attack is a reliable, deterministic syscall sequence (create executable memfd, establish a writable MAP_SHARED mapping, then apply F_SEAL_EXEC) with no race or attacker-uncontrollable conditions.
- PR:LPrivileges required, low
- Any unprivileged local user can invoke memfd_create, mmap, and fcntl without capabilities; these interfaces are available inside unprivileged user namespaces with no real-root requirement.
- UI:NUser interaction, none
- Exploitation does not require a human victim action; in the highest-impact scenario a sandboxed process programmatically passes the sealed-appearing memfd to a trusting peer via IPC (e.g., SCM_RIGHTS) without user interaction.
- S:CScope, changed
- The bug breaks the memfd seal security contract designed for mutually untrusted parties, allowing a malicious local peer to deceive a separate process that relies on F_SEAL_WRITE immutability, crossing the inter-process trust boundary (sandbox/W^X enforcement).
- C:HConfidentiality impact, high
- Bypassing write-seal/W^X guarantees enables arbitrary modification of executable memfd contents presented as immutable, which can be leveraged for arbitrary code execution in a trusting victim process and thereby full read of that process's memory.
- I:HIntegrity impact, high
- The vulnerability allows arbitrary modification of memory that is advertised as write-sealed (F_SEAL_WRITE), defeating the kernel's immutability guarantee and enabling W^X bypass and code injection into trusted execution contexts.
- A:NAvailability impact, none
- The flaw is a logic error in seal enforcement that does not cause kernel crashes, panics, hangs, or denial of service; exploitation maintains availability while subverting integrity guarantees.