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This is the pull request that triggered today's issue [1]. They introduced a custom object that overrides Go's default ServeMux, such that their code won't collapse adjacent slashes, unlike Go's default.

They did this because previously, they used Go's default, which collapsed adjacent slashes -- this broke stuff until they troubleshooted it [2] and discovered that Cloudflare's OCSP responder, which they were using, actually documents that you shouldn't use the default ServeMux [3]. The commit that led to that condition only went in a month prior [4]. This is similar to an issue they had two years ago [9] that seems to have started this all.

Now the proposal is to strip the "leading slash" from the {ocsp-request} component of the incoming URI "{ocsp-uri}/{ocsp-request}" [5], but far better would be to perform path canonicalization on the {ocsp-uri} but not the {ocsp-request}. But it looks like they're relying on http.StripPrefix [6], which is an idiomatic Go way of hosting a server out of a subpath and returning 404 on any request not matching the prefix; this will be problematic without additional processing that gets slashes out of places they shouldn't be, while leaving alone where they should.

For more fun with slashes and OCSP, see this source code for NSS [7], this bugzilla issue [8], this code [10], and this mailing list thread [11].

[1] https://github.com/letsencrypt/boulder/pull/2748 [2] https://github.com/letsencrypt/boulder/issues/2728 [3] https://github.com/cloudflare/cfssl/blob/master/ocsp/respond... [4] https://github.com/letsencrypt/boulder/pull/2689/files [5] https://github.com/letsencrypt/boulder/issues/2774 [6] https://golang.org/src/net/http/server.go and search for "func StripPrefix" [7] https://chromium.googlesource.com/chromium/third_party/nss/+... and search for "slash" [8] https://bugzilla.mozilla.org/show_bug.cgi?id=1010594 [9] https://github.com/letsencrypt/boulder/issues/884 [10] https://github.com/r509/r509-ocsp-responder/blob/master/lib/... [11] https://sourceforge.net/p/openca/mailman/message/31630541/

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