apport: Arbitrary file creation or overwrite via path traversal in base64-encoded problem report keys

In apport before 2.37.0, a path traversal vulnerability allows an attacker who can supply a crafted crash report to cause unintended file creation or overwrite outside the intended extraction directory when the report is processed with apport-unpack.

The key-name validation added for CVE-2026-77113 does not cover base64-encoded keys skipped by ProblemReport.load(binary=False), and ProblemReport.extract_keys() writes them to a file path built from the key without validation. This allows an attacker to create or overwrite arbitrary files writable by the invoking user, such as .bashrc or .profile, gaining persistence or code execution in that user's context.