Issue description
Date announced: January 29, 2026
We have identified a critical issue affecting queries that specify a collation and target a sharded cluster, which can result in duplicate or stale results. You may be impacted by these issues if you:
Specify a collation other than “simple” for your queries AND
Use sharding
The latest patch versions of MongoDB contain a fix for this issue.
Impact
This issue can only manifest if ALL of the following are true:
You perform queries that use a collation with a locale specified other than “simple”;
You query a sharded collection, and
Chunk migration has recently moved a document your query matches between shards. This can occur during shard balancing, shard addition or removal, or resharding.
If your application meets the criteria above, we recommend that you:
Upgrade to the latest version of MongoDB
Fixes and remediation
For MongoDB Atlas Customers: Your Atlas clusters already have been upgraded to versions containing fixes for these issues, unless you have configured a maintenance window. If you have a configured maintenance window, the change will apply during the next scheduled window, or you may apply the change immediately using the Begin Maintenance Now button.
For more details, see SERVER-94315. If you have any questions, open a support case or start a chat with the Atlas Support team.