InfrastructureLet us Encrypt DST Root CA X3 expiration
On 30 September 2021 the DST Root CA X3 certificate used by Let us Encrypt expired, causing TLS failures on older devices and systems that did not trust the newer ISRG Root X1.
Fact-checked and reviewed — Kodi C.
On , the DST Root CA X3 certificate that Let us Encrypt used for cross-signing expired. While Let us Encrypt had transitioned to their own ISRG Root X1 certificate (trusted by most modern systems), older devices and operating systems without ISRG Root X1 in their trust stores experienced TLS validation failures. The expiration affected legacy Android devices (pre-7.1.1), older macOS/iOS versions, and systems with outdated CA bundles.
Technical background
Let us Encrypt began operations in 2015 using certificates cross-signed by IdenTrust's DST Root CA X3, which had broad trust store inclusion. This allowed Let us Encrypt certificates to be trusted by clients before ISRG Root X1 achieved widespread distribution. By 2021, ISRG Root X1 was included in most modern operating systems and browsers.
To maintain compatibility with older Android devices, Let us Encrypt obtained a special cross-sign from IdenTrust allowing certificates to chain to DST Root CA X3 through ISRG Root X1, exploiting Android's trust validation behavior. This extends compatibility until 2024 for many older Android devices.
Impact and mitigation
Systems affected included Android 7.0 and earlier (approximately 33% of active Android devices at the time), older embedded systems, IoT devices with static trust stores, and servers with outdated CA certificate bundles. Affected clients received certificate validation errors when connecting to sites using Let us Encrypt certificates.
Server operators could mitigate issues by configuring their web servers to send the appropriate certificate chain—either including or excluding the expired cross-sign depending on client population. OpenSSL 1.0.2 users experienced issues due to strict chain validation, requiring upgrades or configuration changes.
Lessons for infrastructure teams
The DST Root CA X3 expiration highlighted the importance of maintaining current CA certificate bundles across all systems, including servers, clients, and embedded devices. If you are affected, inventory systems with static trust stores and establish processes for trust store updates.
Plan for future root certificate transitions by monitoring certificate authority announcements and testing client compatibility. Consider TLS termination architectures that simplify certificate management and enable rapid chain configuration changes. The incident showed how certificate authority trust relationships can create unexpected dependencies affecting service availability.
Infrastructure Planning and Design
Infrastructure planning should incorporate the technical requirements and operational considerations associated with this development. Capacity planning, performance requirements, availability targets, and disaster recovery considerations should be addressed during the design phase to ensure infrastructure supports organizational objectives.
Change management processes should account for the specific technical dependencies and potential impacts of infrastructure modifications. Testing procedures should validate that changes do not introduce operational disruptions or security vulnerabilities before deployment to production environments.
Assessing infrastructure
Infrastructure teams should conduct full assessments to identify affected systems and focus on remediation based on exposure and criticality. Patch management processes should account for the specific technical requirements and potential compatibility considerations associated with this update. Testing procedures should validate that patches do not introduce operational disruptions before deployment to production environments.
Monitoring should continue post-remediation to verify successful setup and detect any exploitation attempts targeting systems that remain vulnerable during the patching window.
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Coverage intelligence
- Published
- Coverage pillar
- Infrastructure
- Source credibility
- 91/100 — high confidence
- Topics
- Let us Encrypt · certificate expiration · TLS · trust stores
- Sources cited
- 3 sources (letsencrypt.org, cvedetails.com, iso.org)
- Reading time
- 5 min
Source material
- DST Root CA X3 Expiration (September 2021) — Let us Encrypt
- CVE Details - Vulnerability Database — CVE Details
- ISO/IEC 27017:2015 — Cloud Service Security Controls — International Organization for Standardization
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