CVE-2026-39977: flatpak-builder is a tool to build flatpaks from source

Routine

MEDIUM 6.3UPGRADE

CVE-2026-39977 is a medium-severity flaw (CVSS 6.3) affecting flatpak-builder. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

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()

Why this priority

Public exploit reference

A fixed release or distro patch exists.

Risk facts

CVSS score	6.3 (Medium)
Attack vector	Local
Attack complexity	Low
Privileges needed	None
User interaction	Required
EPSS exploit chance	0.3% (top 79%)
CISA exploited list	No
Weakness	CWE-22
Published	2026-04-09
Last modified	2026-10-26

Publisher (CNA): GitHub, Inc. · Vendor: flatpak

Last updated 2026-10-26 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-04-09 20:16 UTCPublished CVE record published
2026-10-26 00:10 UTCUpdated Record last modified

flatpak flatpak-builder UPGRADE

Affected: >= 1.4.5 and < 1.4.8

Fixed in: 1.4.8

Safe to downgrade to: a release before 1.4.5

1. Check installed version

apt-cache policy flatpak-builder || rpm -q flatpak-builder || flatpak-builder --version

2. UPGRADE to 1.4.8 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade flatpak-builder

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade flatpak-builder

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

3. If no update is available yet, DOWNGRADE to a release before 1.4.5 (Debian / Ubuntu)

apt-cache madison flatpak-builder sudo apt install flatpak-builder=<VERSION_BEFORE_1.4.5> sudo apt-mark hold flatpak-builder

3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list flatpak-builder sudo dnf downgrade flatpak-builder-<VERSION_BEFORE_1.4.5> sudo dnf versionlock add flatpak-builder

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-39977?

flatpak-builder: upgrade to 1.4.8. Exact commands for each OS are on this page.

Is CVE-2026-39977 being exploited?

CVE-2026-39977 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-39977 affect?

CVE-2026-39977 affects flatpak-builder. Severity is medium (CVSS 6.3). 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 ar

How urgent is CVE-2026-39977?

Priority 32/100. Public exploit reference

References

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