Ubuntu 24.04 LTS – apt update fails with NO_PUBKEY for third-party repository – Fix & Prevention

📖 ~1 min read

Table of contents

1. Symptom & Impact
2. Environment & Reproduction
3. Root Cause Analysis
4. Quick Triage
5. Step-by-Step Diagnosis
6. Solution – Primary Fix
7. Solution – Alternative Approaches
8. Verification & Acceptance Criteria
9. Rollback Plan
10. Prevention & Hardening
11. Related Errors & Cross-Refs
12. References & Further Reading

Symptom & Impact

apt update fails with NO_PUBKEY and security updates are blocked.

Environment & Reproduction

Ubuntu 24.04 LTS host with a third-party source in /etc/apt/sources.list.d and missing signed-by keyring.

Root Cause Analysis

The repository metadata is signed with a key that is not installed in /usr/share/keyrings or referenced correctly.

Quick Triage

Check source files and run apt update to identify the failing repo and key fingerprint.

Step-by-Step Diagnosis

Review apt output, then compare the source entry signed-by path with existing keyring files under /usr/share/keyrings.

Solution – Primary Fix

Import the vendor GPG key with gpg –dearmor into /usr/share/keyrings, set signed-by in the repo file, and rerun apt update.

Solution – Alternative Approaches

Temporarily disable the faulty third-party source and continue updates from official Ubuntu archives only.

Verification & Acceptance Criteria

apt update completes without NO_PUBKEY and apt policy shows healthy repository metadata.

Rollback Plan

Remove the added source list and keyring file, then run apt update to return to baseline state.

Prevention & Hardening

Use per-repo signed-by keyrings and track key expiry dates in configuration management.

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Related Errors & Cross-Refs

EXPKEYSIG, InRelease signature verification failure, and apt-secure trust warnings are related.

Related tutorial: View the step-by-step tutorial for Ubuntu 24.04 LTS.

View all Ubuntu 24.04 LTS tutorials on the Tutorials Hub →

Browse all common problems & solutions on the Tutorials Hub.

References & Further Reading

Ubuntu apt-secure documentation and repository signing best practices.