📖 ~1 min read
Table of contents
Symptom & Impact
apt update fails with NO_PUBKEY and routine security updates are blocked until trust is restored.
Environment & Reproduction
Debian 13 host with a third-party source in /etc/apt/sources.list.d and a missing or mismatched signed-by keyring.
Root Cause Analysis
The repository metadata is signed by a key that is not present in the local keyring path referenced by the source file.
Quick Triage
Identify the failing source entry and fingerprint from apt output before changing repository definitions.
Step-by-Step Diagnosis
Compare source signed-by paths with files in /usr/share/keyrings and validate permissions and key fingerprints.
Solution – Primary Fix
Import the vendor key with gpg –dearmor, update signed-by in the source file, and rerun apt update.
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Solution – Alternative Approaches
Temporarily disable only the failing third-party source and continue updates from official Debian repositories.
Verification & Acceptance Criteria
apt update completes cleanly and no signature trust warnings remain for enabled sources.
Rollback Plan
Restore previous source and keyring files from backup and re-run apt update to confirm baseline behavior.
Prevention & Hardening
Use per-repo keyrings, expiry monitoring, and configuration management validation for new repository onboarding.
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Related Errors & Cross-Refs
Related issues include EXPKEYSIG, apt-secure trust failures, and signature verification mismatch warnings.
Related tutorial: View the step-by-step tutorial for Debian 13.
View all Debian 13 tutorials on the Tutorials Hub →
Browse all common problems & solutions on the Tutorials Hub.
References & Further Reading
Debian apt-secure documentation and repository signing guidance for external package sources.
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