# CVE-2026-39977: flatpak-builder has a path traversal leading to arbitrary file read on host when installing licence files

> Attackers can read arbitrary host files during Flatpak builds via crafted manifests, exposing sensitive data to build outputs.

- **Severity:** High 7.1 (`CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** A user has to click or open something
- **Fix available:** Yes
- **Published:** 9 Apr 2026
- **Updated:** 9 Apr 2026
- **Also known as:** DEBIAN-CVE-2026-39977, GHSA-6gm9-3g7m-3965, UBUNTU-CVE-2026-39977, openSUSE-SU-2026:10590-1

## What is CVE-2026-39977?

CVE-2026-39977 is a high-severity path traversal vulnerability in flatpak-builder, published on 9 Apr 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.

### Official description

flatpak-builder is a tool to build flatpaks from source. From 1.4.5 to before 1.4.8, the license-files manifest key takes an array of paths to user defined licence files relative to the source directory of the module. The paths from that array are resolved using g_file_resolve_relative_path() and validated to stay inside the source directory using two checks - g_file_get_relative_path() which does not resolve symlinks and g_file_query_file_type() with G_FILE_QUERY_INFO_NOFOLLOW_SYMLINKS which only applies to the final path component. The copy operation runs on host. This can be exploited by using a crafted manifest and/or source to read arbitrary files from the host and capture them into the build output. This vulnerability is fixed in 1.4.8.

Weakness: CWE-22 (Improper Limitation of a Pathname to a Restricted Directory)

## Which versions are affected by CVE-2026-39977?

It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `flatpak-builder` | openSUSE:Tumbleweed | All versions | 1.4.8-1.1 |

Products: Flatpak Flatpak-Builder.

## How do I fix CVE-2026-39977?

Upgrade to a fixed version.

## Is CVE-2026-39977 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-39977 exists.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-39977 and fix it if it is affected.

What it is: CVE-2026-39977 is a high-severity path traversal vulnerability in flatpak-builder, published on 9 Apr 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-39977 exists.

Affected product: flatpak-builder. 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-2026-39977/ (Markdown: https://insights.sbomify.com/CVE-2026-39977/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-39977
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-39977
- OSV: https://osv.dev/vulnerability/CVE-2026-39977

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-39977/
Summaries are generated and may contain mistakes; the official description above is the record.