Routing Exchange ActiveSync CBA to Regional Endpoints: TLS 1.3 Security Impacts

Microsoft is routing Exchange ActiveSync Certificate‑Based Authentication (CBA) traffic to new, dedicated CBA endpoints by cloud region — a seemingly small change with important operational and security consequences for any organization that terminates, inspects, or filters ActiveSync traffic at a gateway.

Background​

Exchange ActiveSync (EAS) is the protocol mobile and lightweight mail clients use to synchronize mail, calendar, contacts, and device policies with Exchange and Exchange Online. Certificate‑Based Authentication (CBA) allows organizations to authenticate devices and users using client certificates instead of (or alongside) username/password credentials, and it’s commonly used in high‑security environments, regulated industries, and government clouds.
Microsoft has announced a change to how Exchange Online handles CBA for EAS: CBA traffic will now be routed to dedicated endpoints based on tenant location. The stated rationale is to support TLS 1.3 and to strengthen security and reliability. The change has already started rolling out to the worldwide multi‑tenant cloud and is scheduled to begin rolling out to other clouds in November. For most clients the change is transparent, but gateways that inspect or filter ActiveSync traffic — Secure Email Gateways (SEGs), reverse proxies, or TLS‑intercepting appliances — may need configuration updates.

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What’s changing — the new CBA endpoints​

Microsoft will route Exchange ActiveSync CBA traffic to the following dedicated endpoints depending on tenant location:

• Multi‑tenant: outlook‑cba.office365.com
• DoD: outlook‑dod‑cba.office365.us
• GCC‑High: outlook‑cba.office365.us
• Gallatin (China): outlook‑cba.partner.outlook.cn

The change is intended to centralize and isolate CBA traffic from the general EAS endpoints and to enable TLS 1.3 support for CBA flows. Microsoft indicates the rollout is underway for the multi‑tenant cloud and will begin for other clouds in November.

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Why this matters: security and protocol implications​

Moving CBA to dedicated endpoints and enabling TLS 1.3 for those flows has several technical and security consequences:

• Stronger TLS security: TLS 1.3 removes legacy cipher suites, accelerates the handshake, and reduces attack surface compared with older TLS versions. This improves confidentiality and resilience against certain classes of attacks.
• Isolated handling of client certificates: Dedicated endpoints allow Microsoft to apply CBA‑specific TLS and client certificate handling without impacting the wider Exchange ActiveSync ecosystem.
• Potential change to connection targets: Instead of connecting to a general Exchange ActiveSync endpoint, client devices (or intermediaries) will be directed to the CBA hostnames above. That means any network device that authoritatively filters or proxies ActiveSync traffic must be able to reach or allow these hostnames.
• Interaction with TLS inspection: TLS 1.3 makes deep packet inspection more difficult for intermediaries that perform on‑the‑fly TLS interception, because key‑exchange and handshake semantics differ and some TLS‑level information commonly used by appliances may be encrypted in ways the appliance cannot transparently handle.
• Operational dependence on DNS and SNI: Client connections and redirects will rely on DNS and SNI (Server Name Indication). Gateways that use hostname‑based policies must ensure they handle or forward SNI correctly.

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Which environments are most likely to be affected​

Most end users and native clients will see no interruption: modern EAS clients will follow the implicit redirection to the new CBA endpoints without manual user intervention. However, these scenarios may require changes:

• Organizations using a Secure Email Gateway (SEG) or reverse proxy to inspect, filter, or offload ActiveSync traffic.
• Appliances that perform TLS interception (man‑in‑the‑middle) and terminate TLS for inspection.
• Firewalls or perimeter devices that allow traffic only to specific FQDNs or IP ranges for Exchange services.
• Legacy or custom network appliances that perform hostname filtering but do not support SNI forwarding, or which are configured with a static list of allowed hostnames.
• Environments using strict outbound allow lists (proxy or firewall policies that only allow pre‑approved FQDNs and ports).

If any of the above matches your environment, action is likely required to avoid end‑user disruption.

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Immediate operational checklist (what to do now)​

• Inventory ActiveSync pathing: Identify whether EAS traffic from mobile devices is routed through an SEG, reverse proxy, or other gateway that performs filtering or TLS inspection.
• Update allowlists: Ensure the new CBA hostnames are permitted in your firewall, proxy, or gateway allowlist:
• outlook‑cba.office365.com
• outlook‑dod‑cba.office365.us
• outlook‑cba.office365.us
• outlook‑cba.partner.outlook.cn
• Verify SNI and DNS handling: Confirm your gateway preserves or properly forwards SNI and DNS resolution for outbound TLS connections so Microsoft’s redirections and host‑based routing work as intended.
• Coordinate with SEG vendor: Open a support/incident ticket or consult vendor documentation to confirm the appliance supports the new hostnames and TLS 1.3 behavior.
• Plan testing: Prepare pilot groups and a testing schedule to exercise CBA connections through the gateway after allowing the new endpoints.
• Bypass TLS inspection where necessary: If the SEG cannot properly inspect or forward TLS 1.3 client certificate flows, configure an inspection bypass for the CBA hostnames or connections to Microsoft token/CBA endpoints.
• Communicate internally: Notify helpdesk and mobile support teams about the change and provide guidance for when to escalate user problems.
• Monitor telemetry: After the change, monitor authentication errors, sync failures, bounce rates, and gateway logs to detect issues early.

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Technical deep dive — what causes potential failures​

Understanding the failure modes helps craft correct mitigations. Common technical causes include:

TLS 1.3 and interception incompatibility​

TLS 1.3 changed handshake internals and removed certain negotiated parameters previously available to middleboxes. Appliances that rely on older TLS behaviors for inspection may not support TLS 1.3 cleanly. This commonly manifests as:

• Failed client certificate presentation or verification.
• Connection resets or TLS alerts from the gateway.
• Clients falling back to non‑CBA flows and failing authentication.

Mitigation: Update SEG firmware to a TLS 1.3–capable release, disable TLS interception for these endpoints, or configure an explicit bypass for the CBA hostnames.

Hostname allowlists and hardcoded FQDNs​

Firewalls or proxies that work from a static list of allowed FQDNs or IPs will block traffic to the new CBA hosts unless updated. Because Microsoft uses large, frequently changing IP pools, allowlisting IPs is fragile; allowlist by hostname where possible.
Mitigation: Allow hostnames above at the proxy or permit the outbound TLS flow to the Microsoft CBA hostnames.

SNI, redirects, and split TLS flows​

EAS clients may be redirected to a different hostname for CBA; such redirects depend on accurate SNI forwarding. Gateways that terminate TLS and do not forward SNI or rewrite it may prevent the redirect from working.
Mitigation: Configure the gateway to preserve and forward SNI for outbound connections, or avoid terminating TLS for these flows.

Certificate‑pinning and client certificate flows​

Some clients or internal proxies may expect a certain certificate chain. If a gateway intercepts TLS and presents a different certificate, client certificate verification for CBA may fail.
Mitigation: Bypass interception for CBA endpoints, or ensure your inspection appliance supports client certificate passthrough and can present or forward client certificates correctly.

DNS resolution and split‑brain or internal DNS overrides​

If internal DNS resolves the Exchange endpoint to an internal IP for on‑premises services (split DNS), CBA flows to the Microsoft CBA endpoints could be misrouted.
Mitigation: Ensure DNS resolution for the new hostnames either resolves externally or is configured correctly in split‑DNS environments.

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Recommended configuration examples​

Below are pragmatic configuration patterns to reduce risk. Tailor these examples to your environment and vendor documentation.

• Firewall / Proxy allowlist:
• Permit outbound TCP 443 to the new CBA FQDNs.
• If your device only supports IP allowlisting, work with your vendor or Microsoft to obtain appropriate IP ranges (note: IP ranges may change; hostname allowlisting is preferred).
• Proxy PAC or WPAD:
• Add an explicit rule to bypass proxy for the CBA FQDNs so TLS negotiation is end‑to‑end.
• SEG/TLS appliance:
• Create an exception to avoid TLS interception for connections where SNI matches the CBA hostnames.
• If interception must remain, validate that the appliance supports client certificate passthrough and TLS 1.3 before enabling interception for those flows.
• Conditional access / Intune policies:
• No direct changes should be needed to Azure AD conditional access or Intune; however, monitor conditional access logs for unexpected device noncompliance or certificate errors after the rollout.

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Testing and troubleshooting playbook​

A structured test and troubleshooting plan will shorten the time to resolution for any issues that arise.

Pre‑deployment testing​

• Create a test tenant or use a small pilot group in the organization.
• Use mobile test devices representative of your fleet: iOS Mail, Outlook for iOS/Android, Android native exchange client, Windows Mail.
• Confirm successful initial sync with normal users and with users configured for CBA.

Debugging failed CBA flows​

• Reproduce the failure and capture logs from the SEG/gateway.
• Check gateway logs for TLS handshake errors, TLS alerts, or blocks referencing the CBA hostnames.
• Validate the client certificate is being sent by the device (device logs or mobile device management debug logs).
• Use packet captures at the gateway and client to examine TLS handshake behavior and SNI values.
• Check DNS resolution from the gateway for the CBA FQDNs to ensure correct address resolution.
• Review authentication/exchange logs in your MDM/Exchange admin console for CBA errors (certificate trust failures, 4xx/5xx responses).
• If the gateway is terminating TLS, temporarily bypass it to see if direct connectivity to Microsoft resolves the issue.

Key error signatures​

• TLS alert 40x or 0x14: Likely TLS termination/filtering issue.
• HTTP 401/403 with client certificate references: Client certificate was rejected or not presented.
• Repeated TLS handshake failures with TLS 1.3 cipher mismatch: Appliance or client fails to negotiate TLS 1.3.

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Risk assessment — benefits and potential downsides​

Benefits​

• Improved security: TLS 1.3 reduces protocol vulnerabilities and enforces modern cipher suites.
• Better performance: TLS 1.3’s abbreviated handshake can reduce latency for initial connections.
• Operational isolation: Dedicated endpoints allow Microsoft to manage CBA behavior independently of general EAS services.

Potential downsides​

• Operational disruption: Gateways and appliances that aren’t updated may block or break CBA flows, causing sync failures.
• Increased support burden: Helpdesk and security teams may see a spike in calls related to mobile sync failures during rollout.
• Compatibility gaps: Older SEGs, middleboxes, or custom inspection solutions may be incompatible with TLS 1.3 or with client certificate forwarding semantics.
• Management complexity for regulated clouds: DoD, GCC‑High, and China clouds have unique networking and compliance constraints; testing and vendor coordination can take more time.

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Communication and vendor coordination​

Large organizations with centralized mail routing or inspection must coordinate across multiple stakeholders:

• Security / Network teams: Update firewall and proxy rules, verify TLS inspection capabilities, and configure bypasses.
• SEG vendors: Confirm support for TLS 1.3 and guidance for handling CBA hostnames. Obtain recommended firmware versions and configuration steps.
• Identity / MDM teams: Monitor conditional access and device compliance. Verify that client certificates remain trusted and valid.
• Helpdesk and support staff: Prepare troubleshooting scripts and escalation procedures for mobile sync issues.
• End users: Prepare a brief, clear communication for end users in affected groups identifying possible short‑term service impacts and known workarounds.

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Migration roadmap and recommended timeline​

• Week 0: Inventory and planning
• Identify SEGs, proxies, and firewall policies that affect ActiveSync.
• Assemble a cross‑functional team including network, security, MDM, and helpdesk.
• Week 1: Vendor validation and lab testing
• Contact SEG vendor; confirm TLS 1.3/CBA support and any required configuration.
• Build a lab test with representative devices and gateway configuration.
• Week 2: Pilot rollout
• Deploy changes for a limited pilot group (e.g., 1–5% of users) and monitor.
• Validate logs and metrics for authentication success and sync performance.
• Week 3–4: Progressive rollout
• Gradually expand to additional user groups and business units.
• Maintain regular communications and provide an escalation path.
• Ongoing: Post‑rollout monitoring and hardening
• Retain monitoring for at least one month after full rollout.
• Review policies, finalize documentation, and implement permanent firewall/proxy rules.

Note: The schedule above is a suggested framework. Timelines should be adapted to organizational risk tolerance, change windows, and vendor guidance.

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Practical examples of policy entries (illustrative)​

• Proxy bypass rule (PAC-style):
• if (shExpMatch(host, "outlook-cba.office365.com") || shExpMatch(host, "outlook-dod-cba.office365.us") || shExpMatch(host, "outlook-cba.office365.us") || shExpMatch(host, "outlook-cba.partner.outlook.cn")) return "DIRECT";
• else return "PROXY 10.0.0.1:8080";
• Firewall outbound rule:
• Permit outbound TCP 443 to FQDN: outlook‑cba.office365.com (and the other CBA hostnames). If firewall cannot use FQDN rules, consult vendor guidance for allowed IP ranges and accept that IP allowlisting may require frequent updates.

These examples are illustrative; always adapt to your configuration language and vendor.

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When to escalate to Microsoft or SEG vendor support​

Escalate to Microsoft support when:

• You have validated that direct connections (bypassing SEG) succeed but SEG‑proxied connections fail.
• Client certificate is valid and presented by the device, but Exchange Online rejects it with server‑side errors.
• You observe anomalous behavior confined to Microsoft’s CBA hostnames that you cannot resolve through gateway configuration.

Escalate to SEG vendor when:

• The appliance cannot negotiate TLS 1.3 or fails to forward SNI.
• Client certificates are stripped or not forwarded through the gateway.
• Appliance firmware lacks a documented way to bypass or permit these hostnames without breaking inspection for other traffic.

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Final recommendations and best practices​

• Treat the change as both a security improvement and an operational project. Don’t assume it will be invisible in environments that inspect or redirect Exchange traffic.
• Prefer hostname allowlisting and SNI‑aware policies rather than brittle IP allowlists.
• Coordinate with SEG and network vendors early; confirm TLS 1.3 and client certificate passthrough support.
• Implement a phased pilot and robust monitoring to catch issues early and minimize end‑user impact.
• If your gateway cannot be updated quickly, configure explicit bypasses for the CBA hostnames to preserve functionality while you remediate the appliance.
• Document all changes and maintain rollback steps to revert gateway rules if unexpected failures occur.

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Conclusion​

Routing Exchange ActiveSync Certificate‑Based Authentication traffic to dedicated, TLS 1.3‑capable endpoints is a security‑forward move that improves protocol resilience and isolates client‑certificate flows. For the majority of organizations and modern clients, the change will be seamless. For environments that inspect, proxy, or guard ActiveSync connections with SEGs and TLS‑intercepting appliances, this change requires careful preparation: update firewall and proxy policies to permit the new hostnames, verify TLS 1.3 and client certificate passthrough support, and run a controlled pilot before broad rollout. With clear inventory, vendor coordination, and a phased approach, organizations can realize the security benefits while avoiding operational disruption.

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Source: Microsoft Exchange Team Blog Exchange Online ActiveSync Certificate-based Authentication endpoint changes | Microsoft Community Hub