Security Briefing — Let's Encrypt DST Root CA X3 Expiration

Executive briefing: The DST Root CA X3 certificate used to cross-sign Let's Encrypt chains expired on 30 September 2021. Organizations relying on outdated trust stores encountered TLS failures on Android <9, embedded devices, and legacy enterprise appliances.

Key updates

• Trust store remediation. Devices without the ISRG Root X1 certificate failed to validate Let's Encrypt leaf certificates.
• Compatibility guidance. Let's Encrypt published mitigations including chain switching and certificate pinning updates.
• Monitoring requirements. CDN, IoT, and API operators needed proactive telemetry to catch TLS handshake spikes and client drop-offs.

Implementation guidance

• Audit TLS termination points, agents, and embedded systems to ensure ISRG Root X1 is trusted and firmware updates are available.
• Coordinate certificate rotation plans for constrained devices that cannot update trust stores, considering alternate CAs.
• Document certificate expiration response runbooks and validate monitoring for future root transitions.

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