crypto/x509: reject misencoded empty EKU

[cpu]

RFC 5280 §4.2.1.12 when specifying the extended key usage (EKU) extension:

This extension indicates one or more purposes for which the certified
public key may be used

Further, the ASN.1 module defines ExtKeyUsageSyntax as:

ExtKeyUsageSyntax ::= SEQUENCE SIZE (1..MAX) OF KeyPurposeId

This is tested by x509-limbo's rfc5280::eku::ee-eku-empty testcase.

Go crypto/x509 should reject an empty EKU sequence when parsing a certificate. The x509.CreateCertificate function already gates emission of the extension on a non-empty list of usages to populate the sequence with.

Relates to #65085 & https://go-review.googlesource.com/c/go/+/783980

[gabyhelp]

Related Issues

• crypto/x509: reject CA w/ empty subject #79744
• crypto/x509: tighten up SAN criticality handling #79741
• crypto/x509: CreateCertificate should omit extensions if there aren't any #52319 (closed)
• crypto/x509: reject end-entity certs w/ name constraint ext. #79742
• crypto/x509: reject non-profile minimum/maximum in GeneralSubtree name constraints #77826
• crypto/x509: add ExtKeyUsage.OID #75325 (closed)
• crypto/x509: certificate with empty Authority Key Identifier extension considered invalid #70619
• crypto/x509: x509 certificate with issuerUniqueID and/or subjectUniqueID parse error #51754 (closed)

Related Code Changes

• crypto/x509: fix parsing of CSR with empty x509v3 extension attribute
• crypto/x509: improve Certificate validation avoiding empty issuers

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[mrkfrmn]

cc @golang/security