# CVE-2019-15794: Reference counting error in overlayfs/shiftfs error path when used in conjuction with aufs

> A reference counting error in the Linux kernel's overlayfs/shiftfs error path when used with aufs can lead to use-after-free, potentially allowing privilege escalation or denial of service.

- **Severity:** High 7.1 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 23 Apr 2020
- **Updated:** 16 Sep 2024
- **Also known as:** DEBIAN-CVE-2019-15794, GHSA-33f2-r445-jvq6, ROOT-OS-DEBIAN-11-CVE-2019-15794, ROOT-OS-UBUNTU-2204-CVE-2019-15794, ROOT-OS-UBUNTU-2404-CVE-2019-15794, UBUNTU-CVE-2019-15794

## What is CVE-2019-15794?

CVE-2019-15794 is a high-severity use after free vulnerability in Canonical Ubuntu Linux, published on 23 Apr 2020. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

Overlayfs in the Linux kernel and shiftfs, a non-upstream patch to the Linux kernel included in the Ubuntu 5.0 and 5.3 kernel series, both replace vma->vm_file in their mmap handlers. On error the original value is not restored, and the reference is put for the file to which vm_file points. On upstream kernels this is not an issue, as no callers dereference vm_file following after call_mmap() returns an error. However, the aufs patchs change mmap_region() to replace the fput() using a local variable with vma_fput(), which will fput() vm_file, leading to a refcount underflow.

Weakness: CWE-672 (Operation on a Resource after Expiration or Release)

## Which versions are affected by CVE-2019-15794?

It affects 2 products.


Products: Canonical Ubuntu Linux, Linux kernel.

## How do I fix CVE-2019-15794?

Apply the vendor's patch.

## Is CVE-2019-15794 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2019-15794 exists.

Public exploit code:

- Ubuntu 19.10 - ubuntu-aufs-modified mmap_region() Breaks Refcounting in overlayfs/shiftfs Error Path (exploitdb)

## Other vulnerabilities in Canonical Ubuntu Linux

- [CVE-2020-11651](https://insights.sbomify.com/CVE-2020-11651/): SaltStack Salt Authentication Bypass (critical 9.8, exploited in the wild)
- [CVE-2020-7247](https://insights.sbomify.com/CVE-2020-7247/): OpenSMTPD Remote Code Execution (critical 9.8, exploited in the wild)
- [CVE-2019-16928](https://insights.sbomify.com/CVE-2019-16928/): Exim Out-of-bounds Write (critical 9.8, exploited in the wild)
- [CVE-2018-6789](https://insights.sbomify.com/CVE-2018-6789/): Exim Buffer Overflow (critical 9.8, exploited in the wild)
- [CVE-2015-2590](https://insights.sbomify.com/CVE-2015-2590/): Oracle Java SE and Java SE Embedded Remote Code Execution (critical 9.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2019-15794 and fix it if it is affected.

What it is: CVE-2019-15794 is a high-severity use after free vulnerability in Canonical Ubuntu Linux, published on 23 Apr 2020. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2019-15794 exists.

Affected product: Canonical Ubuntu Linux. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2019-15794/ (Markdown: https://insights.sbomify.com/CVE-2019-15794/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2019-15794
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2019-15794
- OSV: https://osv.dev/vulnerability/CVE-2019-15794

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From sbomify Insights: https://insights.sbomify.com/CVE-2019-15794/
Summaries are generated and may contain mistakes; the official description above is the record.