Coder's unbounded memory allocation in provisioner file upload allows authenticated denial of service

Summary

NewDataBuilder in provisionersdk/proto/dataupload.go allocated a byte slice using the client-supplied FileSize from a DataUpload message without an upper-bound check. Although the DRPC wire limit is 4 MiB, the FileSize value itself was unconstrained

Impact

An authenticated user able to reach the provisioner daemon serve endpoint could send a roughly 50-byte message declaring a huge FileSize (for example 1 TiB), triggering an unrecoverable Go out-of-memory abort that terminates coderd. This is a single-message denial of service affecting the entire deployment.

Patches

The fix validates FileSize against an upper bound (MaxFileSize = 100 MiB) before allocation.

The fix was backported to all supported release lines:

Workarounds

Restrict access to the provisioner daemon serve endpoint to trusted provisioner daemon service accounts.

Resources

• Fix: #25710

Credits

Coder would like to thank Anthropic's Security Team (ANT-2026-22442) for independently disclosing this issue!