CVE-2026-55438: Coder's workspace app CORS origin check can be bypassed via UUID-based subdomain spoofing

Summary

Coder's subdomain-based workspace app proxy allowed the same-owner CORS check to be bypassed. When a workspace-name subdomain segment parsed as a UUID, the workspace was resolved by ID without confirming the URL's username matched the real owner, while the CORS middleware trusted the unverified username in the hostname.

Note: Practical exploitation requires subdomain app routing (wildcard hostname) enabled and a victim who visits the attacker's crafted app URL while authenticated.

Impact

An authenticated user could craft a subdomain encoding their own workspace UUID and a victim's username. If the victim visited the attacker's URL, the attacker's JavaScript could issue credentialed cross-origin fetch() requests to the victim's workspace apps and read the responses, exfiltrating data accessible through those apps.

Patches

The fix validates the subdomain username against the resolved workspace's actual owner and bases the same-owner CORS decision on the authoritative owner identity.

The fix was backported to all supported release lines:

| Release line | Patched version | |---|---| | 2.34 | v2.34.2 | | 2.33 | v2.33.8 | | 2.32 | v2.32.7 | | 2.29 (ESR) | v2.29.17 |

Workarounds

None.

Resources

- Fix: #26086, #26085

Credits

Coder would like to thank Anthropic's Security Team (ANT-2026-22434) for independently disclosing this issue!