π ~1 min read
Table of contents
Symptom & Impact
Administrators see apt update stop with NO_PUBKEY warnings and repository metadata cannot be trusted, blocking package operations and patch workflows.
Environment & Reproduction
The issue appears on Ubuntu 16.04 hosts using third-party or stale mirrors after key rotation.
Root Cause Analysis
Repository signing keys are missing, expired, or not stored in trusted keyrings expected by apt on this release.
Quick Triage
Confirm exact key IDs and identify affected repository definitions before modifying trust configuration.
Step-by-Step Diagnosis
Collect key IDs, validate mirror URLs, and verify whether repositories still publish supported metadata.
Solution – Primary Fix
Import valid keys from official sources, remove deprecated repositories, then refresh package lists cleanly.
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Solution – Alternative Approaches
Use archive snapshots, migrate to supported mirrors, or disable unneeded repos pending governance review.
Verification & Acceptance Criteria
apt update completes without GPG errors and all required repositories return signed Release files.
Rollback Plan
Revert key and sources list changes from backup and restore previous trusted keyring state.
Prevention & Hardening
Track repository key expirations, restrict third-party repos, and audit apt trust sources regularly.
Related Errors & Cross-Refs
EXPKEYSIG, BADSIG, repository no longer has a Release file, and hash sum mismatch incidents.
Related tutorial: View the step-by-step tutorial for Ubuntu 16.04 LTS.
View all Ubuntu 16.04 LTS tutorials on the Tutorials Hub β
Browse all common problems & solutions on the Tutorials Hub.
References & Further Reading
Ubuntu package management security guidance and repository signing key lifecycle documentation.
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