Ubuntu 24.04 LTS – apt update shows NO_PUBKEY for repository signatures

📖 ~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

Repository metadata cannot be trusted, so updates and installs fail.

Environment & Reproduction

Occurs after adding third-party repositories without proper keyring configuration.

sudo apt-get update
ls -l /etc/apt/keyrings

Root Cause Analysis

The signing key is missing, outdated, or not linked through signed-by in source entries.

Quick Triage

Locate the failing repository and key fingerprint from apt output.

sudo grep -R '^deb ' /etc/apt/sources.list /etc/apt/sources.list.d 2>/dev/null

Step-by-Step Diagnosis

Inspect repository files and keyring references.

grep -R 'signed-by' /etc/apt/sources.list /etc/apt/sources.list.d 2>/dev/null
ls -l /etc/apt/keyrings

Solution – Primary Fix

Install the correct vendor key to /etc/apt/keyrings and update source configuration.

curl -fsSL  | sudo gpg --dearmor -o /etc/apt/keyrings/vendor.gpg
sudo apt-get update

Solution – Alternative Approaches

Disable untrusted repositories until valid signed metadata is available.

sudo sed -i 's/^deb/#deb/g' /etc/apt/sources.list.d/.list

Verification & Acceptance Criteria

apt update runs without NO_PUBKEY or signature verification errors.

sudo apt-get update

Rollback Plan

Remove incorrect keyring file and restore previous source list backup.

sudo rm -f /etc/apt/keyrings/vendor.gpg

Prevention & Hardening

Require signed-by keyrings for all external repos and document fingerprints.

sudo apt-config dump | grep -i trusted

Related Errors & Cross-Refs

Related to InRelease signature mismatch and expired signing keys.

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 secure apt repository and keyring guidance.