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fix(middleware): reject empty CSRF tokens in v4 - #3189
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Backport #3135 as defense in depth. Reject empty cookie or client tokens before constant-time comparison, retaining normal valid-token behavior. Co-authored-by: Harry Tran <hoang@platphormcorp.com>
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Backport of #3135 by @hoangtaiki to v4.
Reject an empty cookie token or client token before constant-time comparison. Two empty strings currently compare equal and permit an unsafe request when an empty CSRF cookie and empty client token are supplied. This is defense in depth: an attacker able to overwrite the cookie can also choose a matching nonempty token, so this change does not establish an additional exploitable vulnerability and does not warrant a security advisory.
Reuse the original regression with v4's existing test harness. Validation on macOS / Go 1.27.3: the regression fails before the production change; root and middleware package tests pass with
-race, andgo vet ./..., formatting andgit diff --checkpass afterward. No API or dependency changes.Original contributor credit is preserved in the commit trailer.