Cases6 cases

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1. IL-0226 Azure Front Door edge servers crashed worldwide on incompatible configuration metadata, causing timeouts and DNS errors

   Microsoft Azure2025

   Symptoms

   From 15:41 UTC on 29 October 2025, customers and Microsoft services using Azure Front Door (AFD) and Azure Content Delivery Network (CDN) got connection timeout errors and DNS resolution issues. Monitoring alerts started the investigation at 15:48 UTC.

   Intermittent timeoutsDNS resolution failuresService outageHigh latency

   Trigger

   At 15:35 UTC a specific sequence of valid customer configuration changes, made across two different AFD control plane build versions, produced incompatible customer configuration metadata that was deployed to the edge sites.

   Root cause

   Software defect

   Component

   CDN and edge networkAuthoritative DNS servers and records

   Contributing factor

   Gap in automated testsChange with a delayed effectIneffective rollbackExcessive blast radius

   Detection

   The configuration protection system detected the data plane impact and blocked configuration changes at 15:43 UTC, and monitoring alerts started the investigation at 15:48 UTC.

   Remediation

   • The configuration protection system automatically blocked all new and in-flight configuration changes worldwide
   • Removed the problematic customer configurations from the latest LKG snapshot by hand
   • Blocked all customer configuration changes from propagating to the data plane
   • Deployed the updated LKG configuration across the global fleet
   • Moved Azure Portal away from Azure Front Door
   • Rebalanced traffic by hand to a few healthy edge sites, then switched back to automatic traffic management
   • Blocked all AFD customer configuration changes at the Azure Resource Manager level until safeguards were in place

   Action items

   • Fix the control plane incompatibility and the data plane bug
   • Process each customer configuration completely and synchronously before production stages
   • Add stages to the phased configuration rollout, with longer bake time between them
   • Move configuration processing on data plane servers into isolated worker processes
   • Reduce configuration propagation time from 45 minutes to about 15 minutes
   • Test backwards compatibility with configurations generated by earlier control plane builds
   • Move critical first-party infrastructure to an active-active setup with fail away
   • Isolate customer configuration and traffic with micro cell segmentation of the AFD data plane
   • Restore customer configurations within one hour by using the local configuration cache
   • Cut data plane recovery further, to about 10 minutes
   • Remove delays in Azure Service Health alerts by raising resource thresholds
   • Extend automated Azure Service Health alerts to similar classes of service degradation
   • Fail over support request systems faster so Premier and Unified customers can always open requests

   Lessons

   Microsoft describes configuration propagation risk as inherent to a global Content Delivery Network, in which route, WAF and origin changes must be deployed quickly worldwide. Both this incident and the 9 October AFD incident came from that risk. This time the incompatible metadata passed a protection system that advances only on healthy data plane signals, because the crash came later from delayed asynchronous processing. Some learnings and repair items from the earlier incident applied here as well. Microsoft advises customers to build a resilient global HTTP ingress layer that stays available during regional or network disruptions, to manage resources through the REST API or PowerShell when management portals are unavailable, and to configure Azure Service Health alerts so the right people hear about service issues.

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2. IL-0341 Traffic between Cloudflare and AWS us-east-1 had high latency, packet loss and origin failures for almost four hours

   Cloudflare2025

   Symptoms

   From 16:27 UTC on 21 August 2025, users connecting to Cloudflare from servers in AWS us-east-1, or receiving connections from Cloudflare there, saw high latency, packet loss and failures to reach origins. Cloudflare's network team was alerted to internal congestion in Ashburn (IAD) at 16:44 UTC.

   Packet lossHigh latencyIntermittent timeouts

   Trigger

   At about 16:27 UTC a single customer began sending many requests from AWS us-east-1 to Cloudflare for objects in Cloudflare's cache, doubling the total traffic from Cloudflare to AWS.

   Root cause

   Capacity exhaustion

   Component

   Transit gateway or network interconnectBGP routingCDN and edge network

   Contributing factor

   No headroomHarmful manual actionExcessive blast radius

   Detection

   Cloudflare's network team was alerted to internal congestion in Ashburn (IAD) at 16:44 UTC, and an incident was raised for customer impact at 17:45 UTC.

   Remediation

   • Rate limited the single customer whose traffic caused the surge
   • Took further traffic engineering actions that fully resolved the congestion
   • Asked AWS to revert its BGP prefix withdrawals, which AWS finished at 20:07 UTC

   Action items

   • Build a mechanism to deprioritise a customer's traffic when it congests the network enough to affect others
   • Speed up the Data Center Interconnect upgrades to add network capacity
   • Work with AWS so the two companies' BGP traffic engineering actions do not conflict
   • Build a traffic management system that gives each customer a network resource budget and automates the manual actions used here

   Lessons

   Cloudflare concludes that it must isolate customers better so that no single customer can take over shared network resources and degrade the platform for others, and that its network needs enough capacity to meet demand. It describes the outage as the result of insufficient network congestion management during an unusual high-traffic event.

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3. IL-0555 Sentry's ingestion edge regions backlogged one after another and rejected traffic in three incidents in October 2024

   Sentry2024

   Symptoms

   On 21 October 2024 Sentry could not ingest data in US SaaS from 9:42 AM to 10:06 AM PT, and the status page reported elevated errors for ingestion. The same failure had also happened on 4 and 16 October.

   High error rateService outageAutoscaler misbehaviour

   Trigger

   In each of the three incidents an ingestion region saw an increase in backlogged traffic; the trigger was different each time.

   Root cause

   Capacity exhaustion

   Component

   CDN and edge networkAutomated capacity manager

   Contributing factor

   Automation amplificationExcessive blast radius

   Action items

   • Create more, smaller edge regions to reduce the chance of cascading failures, adding one region each in South America, North America and Europe
   • Change the autoscaling rules to use stronger signals than CPU load
   • Scale down more slowly so that iowait-based backlogs do not trigger scale-down events
   • Improve the metrics the edge produces to refine the autoscaling rules and their sensitivity

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4. IL-0339 Azure Front Door and Azure CDN users saw intermittent connection errors, timeouts and latency spikes for hours

   Microsoft Azure2024

   Symptoms

   From 11:45 UTC on 30 July 2024, a subset of customers got intermittent connection errors, timeouts and latency spikes when connecting to Microsoft services that use Azure Front Door (AFD) and Azure Content Delivery Network (CDN). The Azure Portal team saw the first service degradation at 11:47 UTC.

   Intermittent connection failuresIntermittent timeoutsHigh latencyPacket loss

   Trigger

   Around 11:45 UTC the Azure network DDoS protection service was disengaging after a routine DDoS mitigation and returning traffic to its default routing to AFD, but the routes at one site in Europe could not be updated because the network control plane could not reach that site during a local power outage.

   Root cause

   Configuration change

   Component

   CDN and edge networkDDoS protection serviceNetwork control plane (SDN)Data center power and cooling

   Contributing factor

   Overlapping failuresExcessive blast radiusMonitoring blind spot

   Detection

   The Azure Portal team detected service degradation at 11:47 UTC and began to investigate. At 12:10 UTC network monitoring correlated the Portal incident to a network issue at one site in Europe, and networking engineers joined.

   Remediation

   • Rerouted traffic under the standard congestion playbook, which removed most of the impact by 13:58 UTC
   • Ordered the network control plane to reissue its commands to the misbehaving device, which could not be reached
   • Removed the device that kept attracting traffic from the network and scanned the network for other issues

   Action items

   • Add the missing configuration on network devices so a DDoS mitigation issue in one geography cannot spread to another
   • Improve validation and monitoring in the Azure network to detect invalid configurations
   • Improve monitoring for when the DDoS protection service is unreachable from the control plane but still serving traffic

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5. IL-0491 Squarespace's CDN origin fleet fell below its healthy node threshold, and images, scripts and stylesheets failed to load

   Squarespace2019

   Symptoms

   At 12:10 AM ET on Tuesday, February 12, 2019, Squarespace's monitoring alerted the on-call engineer for the CDN origin fleet that the fleet had fallen below a safe number of healthy nodes. Requests to static.squarespace.com and static1.squarespace.com failed, so images, scripts and style sheets did not load for some sites, and the status page later reported elevated error rates serving media and stylesheets.

   High error rate

   Trigger

   At 10:30 PM ET on February 11, an engineer started a maintenance operation across the CDN origin fleet. The operation kept running in the background after the engineer believed it had been terminated.

   Root cause

   Operator error

   Component

   CDN and edge network

   Contributing factor

   Excessive blast radiusMonitoring blind spot

   Detection

   The monitoring system alerted the CDN origin fleet's on-call engineer at 12:10 AM ET when the fleet fell below a safe number of healthy nodes.

   Remediation

   • Restored the CDN origin fleet, partially by 2:15 AM ET and to normal behaviour at 2:20 AM ET

   Action items

   • Revise the incident-response playbooks with checks and actions for this fleet, starting with an assessment of customer impact and the immediate involvement of an Incident Commander
   • Improve external monitoring to assess customer impact of issues with the CDN partners and origin services more quickly
   • Revise operational policies and add safeguards for issuing maintenance commands, including limiting the share of the fleet a command can operate on
   • Reduce the need for direct manual maintenance operations
   • Create independent failure domains and route requests automatically around unhealthy parts of the system

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