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Fooling Go's X.509 Certificate Verification

58 points by hasheddan

by agwa

2 subcomments

The blog post provides a certificate chain that validates in OpenSSL but not in Go.
The reason it doesn't validate in Go is that the Subject field in the CA certificate uses a different string encoding than the Issuer field in the leaf certificate, so the fields are not byte-for-byte equal.
Go requires the Issuer and Subject to be byte-for-byte equal. This was permitted by older specs, but RFC 5280 changed the rules to require the use of RFC 4518 (LDAP stringprep) for comparing strings. This turned a simple memcmp into a complicated algorithm that requires implementing Unicode normalization, for virtually zero benefit. That's the last thing you want in your security-critical certificate verifier, so Go quite sensibly chose to follow the older specs in this regard. The CA/Browser Forum's Baseline Requirements also mandate byte-for-byte equality, so Go's behavior won't cause publicly-trusted certificates to be incorrectly rejected.
Note that LDAP stringprep is so complicated that OpenSSL doesn't even try to implement it properly and uses an approximation instead. So you would also be able to "fool" OpenSSL into rejecting certificate chains that RFC 5280 says are valid.
The blog post says that this is an "ongoing debate" in the Go project but I don't think that's accurate. I'd be shocked if they ever changed this behavior, given that crypto/x509 targets publicly-trusted certificates and the current behavior is so much simpler.

by fsmv

0 subcomment

I don't think that's an ongoing debate looks to me that it ended in 2019 https://github.com/golang/go/issues/31440#issuecomment-53724...

by briansmith

5 subcomments

This is uninteresting. CAs are well aware that they have to encode the subject DN and issuer DN identically to maximize interoperability. There are several implementations that require that.
If we were to make a new version of the spec for X.509 certificates, I would hope that we would eliminate all the non-UTF8 encodings so that this would be a non-issue.

by entrope

0 subcomment

Perhaps I overlooked something, but which part of this involved "fooling Go"? I would usually not call it "fooling" something to trigger a not-strictly-required rejection of a dubious trait, especially when best practice says to avoid that trait.

by woodruffw

0 subcomment

The title of this post is wildly misleading: Go’s behavior is correct here. But even if it wasn’t, failing closed isn’t generally a way one fools a X.509 path validator.
Source: having had the displeasure of implementing an RFC 5280 and CABF-conforming path validator.

by pqpdf

0 subcomment

This is a great example of how format complexity creates security problems.
The same thing shows up in PDFs: two tools can read the same file and disagree about what's actually in it because they interpret the structure differently.
X.509, PDFs, Office docs. Whenever multiple implementations are making judgment calls on the same bytes, those edge cases become very interesting from a security perspective.

by pseudohadamard

3 subcomments

This is what happens when kids today are unaware of history. This was a known problem 30+ years ago, and the Go kids have just rediscovered it for themselves. The most extreme case of this madness was imagining you could re-encode certificates into a DER blob from their stored components and the signature would still validate, something that OER (from memory) guys are now trying to do.
The rules for DNs are "there is only one encoding rule and that is memcpy(); there is only one matching rule and that is memcmp()". Given that Go has fallen into the decades-old trap of trying to re-encode strings, it's bound to be vulnerable to any number of other issues like evading excludedSubtrees through string-encoding tricks.