These extensions MUST be marked critical by the sections of the spec mentioned in the cryptowarnx(). That's determined by the ASN1_BOOLEAN that is extracted and ignored after the FIXME a few lines below each of the two hunks. Rather than getting the info from there, it's easier to use an API call that checks what was already parsed by d2i_X509().
Index: cert.c =================================================================== RCS file: /cvs/src/usr.sbin/rpki-client/cert.c,v retrieving revision 1.47 diff -u -p -r1.47 cert.c --- cert.c 5 Nov 2021 10:50:41 -0000 1.47 +++ cert.c 24 Dec 2021 23:40:55 -0000 @@ -588,6 +588,12 @@ sbgp_assysnum(struct parse *p, X509_EXTE int dsz, rc = 0, i, ptag; long plen; + if (!X509_EXTENSION_get_critical(ext)) { + cryptowarnx("%s: RFC 6487 section 4.8.11: autonomousSysNum: " + "extension not critical", p->fn); + goto out; + } + if ((dsz = i2d_X509_EXTENSION(ext, &sv)) < 0) { cryptowarnx("%s: RFC 6487 section 4.8.11: autonomousSysNum: " "failed extension parse", p->fn); @@ -890,6 +896,12 @@ sbgp_ipaddrblk(struct parse *p, X509_EXT ASN1_SEQUENCE_ANY *seq = NULL, *sseq = NULL; const ASN1_TYPE *t = NULL; int i; + + if (!X509_EXTENSION_get_critical(ext)) { + cryptowarnx("%s: RFC 6487 section 4.8.10: sbgp-ipAddrBlock: " + "extension not critical", p->fn); + goto out; + } if ((dsz = i2d_X509_EXTENSION(ext, &sv)) < 0) { cryptowarnx("%s: RFC 6487 section 4.8.10: sbgp-ipAddrBlock: "