How severe is CVE-2026-39977?

CVE-2026-39977 has a CVSS 3.x base score of 6.3, rated medium severity. It is exploitable over local access with low attack complexity, requires no privileges and user interaction. Impact on confidentiality is high, integrity none, and availability none.

Is CVE-2026-39977 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (21st percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2026-39977?

CVE-2026-39977 affects Flatpak Flatpak-builder. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2026-39977?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround.

Does CVE-2026-39977 have an EU (EUVD) identifier?

Yes. CVE-2026-39977 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-21045.

When was CVE-2026-39977 published?

CVE-2026-39977 was published on 2026-04-09 and last updated on 2026-06-17.

CVE-2026-39977

CVE-2026-39977 is a medium-severity vulnerability in Flatpak Flatpak-builder with a CVSS 3.x base score of 6.3. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-22.

Key facts

Severity: Medium (CVSS 3.x base score 6.3)
CVSS v4: 7.1
EPSS exploit prediction: 0% (21st percentile)
Actively exploited: Not listed in CISA KEV
EU (EUVD) id: EUVD-2026-21045
Weakness: CWE-22
Affected product: Flatpak Flatpak-builder
Published: 2026-04-09
Last modified: 2026-06-17

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.

Frequently asked questions

What is CVE-2026-39977?: 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.
How severe is CVE-2026-39977?: CVE-2026-39977 has a CVSS 3.x base score of 6.3, rated medium severity. It is exploitable over local access with low attack complexity, requires no privileges and user interaction. Impact on confidentiality is high, integrity none, and availability none.
Is CVE-2026-39977 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (21st percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2026-39977?: CVE-2026-39977 affects Flatpak Flatpak-builder. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2026-39977?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround.
Does CVE-2026-39977 have an EU (EUVD) identifier?: Yes. CVE-2026-39977 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-21045.
When was CVE-2026-39977 published?: CVE-2026-39977 was published on 2026-04-09 and last updated on 2026-06-17.

References

https://github.com/flatpak/flatpak-builder/security/advisories/GHSA-6gm9-3g7m-3965

Affected products (1)

cpe:2.3:a:flatpak:flatpak-builder:*:*:*:*:*:*:*:*

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