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But Nobody Does It Most common pattern: X509_NAME_get_text_by_NID (subj, NID_commonName, data, 1024); return data; Seen in Claws, Open1x, Wget, Bacula, Neon, OpenLDAP A CA based on X509_NAME_get_text_by_NID would only see/validate the first CN So What Would You Do? Wildcard policy Netscape has an unlimited wildcard policy – if you can get a cert for *, you win IE has a “chicken” wildcard policy – they’re only accepted two labels in (*.xxx.yyy) Three CN’s in one PKCS#10 Request CN= // for OpenSSL CN= // for IE CN=* // For Netscape But What Is A CN, Anyway? X.509 is written to ASN.1, something of a precursor to XML Designed to be very fast to parse Actually very fast to crash under fuzzing In 2002, the PROTOS project fuzzed SNMP and pretty much destroyed every router on the planet Every CA has an ASN.1 listener via PKCS#10 Should be based on a standard stack, hardened after 2002, but there’s random custom code all over the place out there Warning: Also a channel for SQL Injection Apparently, XKCD’s Little Bobby Tables caused some people to realize this might show up in a certificate (courtesy of Peter Guttmann): 125 40: SET { 127 38: SEQUENCE { 129 3: OBJECT IDENTIFIER commonName (2 5 4 3) 134 31: TeletexString Bob;DROP TABLE certificates;-- : } Names and Numbers In ASN.1, “Common Name” is not expressed by text, but by an “Object Identifer” or “OID” is the OID for Common Name How is this encoded? ASN.1 OIDs ASN.1 BER (Basic Encoding Rules) is a TLV (Tag-Length-Value) file format OIDs – Tagged 0x06 – have multiple numbers in a row, which may be larger than an individual byte Numbers are encoded in Base 128 – if the high bit is set(0x80) then the next number is part of this subdigit 06 = six 86 = six, and there’s another digit coming Simple OID (Common Name) T=06 (Object Identifier)L=03 (Length==3)V= 55: 2.5 // Don’t ask, really stupid compression  04: .4 03: .3 More Complex OID RSA