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fix(middleware): reject empty tokens in CSRF validation - #3135
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subtle.ConstantTimeCompare treats two empty byte slices as equal, so
validateCSRFToken("", "") returns true. Extractors in extractor.go
treat a present-but-empty query/form/header value as successfully
extracted, so an attacker can supply an empty client token with no
special access. If the stored cookie token is ever empty (e.g. an
app clears it to "" instead of expiring it, or CookieSecure is false
and the cookie is overwritten over plain HTTP), CSRF validation is
bypassed entirely. Reject empty tokens before the comparison.
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Reviewed the empty-token guard and regression test. Approving as defense in depth. Cookie-write capability also permits setting a matching nonempty token, so the empty case does not establish an additional exploitable vulnerability and will not receive a security advisory.
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Merged as defense in depth. The v4 backport is open at #3189 with the original contributor credit and regression coverage. No security advisory is being created for this empty-token case. |
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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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Summary
validateCSRFTokencompares tokens withsubtle.ConstantTimeCompare, which returns equal for two empty byte slices. Extractors inextractor.gotreat a present-but-empty form/query/header value as a successfully extracted client token, so if the stored cookie token is ever an empty string, any request supplying an empty token passes CSRF validation without knowing the real token.token == "" || clientToken == ""before the constant-time compare invalidateCSRFToken.This can't happen from Echo's own default config alone (tokens are always 32 random bytes), but it is reachable if the stored cookie token ever becomes empty - for example an application clearing the CSRF cookie to
""instead of expiring it, orCookieSecure: false(the default) allowing the cookie to be overwritten over plain HTTP by a network attacker. Rejecting empty tokens outright removes this edge case entirely.Test plan
go build ./...go vet ./...go test ./...TestCSRFWithConfig/nok,_POST_empty_cookie_token_+_empty_client_token_is_blocked) and verified it fails without the fix and passes with it