[xmlsec] Certificate priority in verifying signatures

[Andrea Ieri]

Hallo, I am writing a thesis on AAI security and have been using xmlsec1
for verifying the signatures of the federation metadata.

If I understand the output correctly, verifying the signed metadata
works fine (besides warnings regarding the self-signed certificate):

shiblenny:~/idem# xmlsec1 --verify --pubkey-cert-pem signer_bundle.pem
signed-metadata.xml
func=xmlSecOpenSSLX509StoreVerify:file=x509vfy.c:line=360:obj=x509-store:subj=X509_verify_cert:error=4:crypto
library function failed:subj=/C=IT/O=GARR/CN=GARR Certification
Authority;err=19;msg=self signed certificate in certificate chain
func=xmlSecOpenSSLX509StoreVerify:file=x509vfy.c:line=408:obj=x509-store:subj=unknown:error=71:certificate
verification failed:err=19;msg=self signed certificate in certificate chain
OK
SignedInfo References (ok/all): 1/1
Manifests References (ok/all): 0/0

Next, I've tried to throw away the valid signature, sign the bare
metadata with a different certificate, embed it and verify the tampered
metadata. Verification works fine either by specifying the new
certificate or letting xmlsec1 retrieve it from the xml file.

shiblenny:~/idem# xmlsec1 --verify tamperedmetadata.xml
func=xmlSecOpenSSLX509StoreVerify:file=x509vfy.c:line=360:obj=x509-store:subj=X509_verify_cert:error=4:crypto
library function failed:subj=/C=AU/ST=Some-State/O=Internet Widgits Pty
Ltd;err=18;msg=self signed certificate
func=xmlSecOpenSSLX509StoreVerify:file=x509vfy.c:line=408:obj=x509-store:subj=unknown:error=71:certificate
verification failed:err=18;msg=self signed certificate
OK
SignedInfo References (ok/all): 1/1
Manifests References (ok/all): 0/0

shiblenny:~/idem# xmlsec1 --verify --pubkey-cert-pem roguecert.pem
tamperedmetadata.xml
func=xmlSecOpenSSLX509StoreVerify:file=x509vfy.c:line=360:obj=x509-store:subj=X509_verify_cert:error=4:crypto
library function failed:subj=/C=AU/ST=Some-State/O=Internet Widgits Pty
Ltd;err=18;msg=self signed certificate
func=xmlSecOpenSSLX509StoreVerify:file=x509vfy.c:line=408:obj=x509-store:subj=unknown:error=71:certificate
verification failed:err=18;msg=self signed certificate
OK
SignedInfo References (ok/all): 1/1
Manifests References (ok/all): 0/0


The problem is the following:

shiblenny:~/idem# xmlsec1 --verify --pubkey-cert-pem signer_bundle.pem
roguemetadata.xml
func=xmlSecOpenSSLX509StoreVerify:file=x509vfy.c:line=360:obj=x509-store:subj=X509_verify_cert:error=4:crypto
library function failed:subj=/C=AU/ST=Some-State/O=Internet Widgits Pty
Ltd;err=18;msg=self signed certificate
func=xmlSecOpenSSLX509StoreVerify:file=x509vfy.c:line=408:obj=x509-store:subj=unknown:error=71:certificate
verification failed:err=18;msg=self signed certificate
OK
SignedInfo References (ok/all): 1/1
Manifests References (ok/all): 0/0

Apparently, the embedded certificate takes precedence over the one
specified in the command line!
Since I am new to concepts related to xml signing, there may be
something I'm overlooking here, but if my analysis is correct, this is a
serious issue as users would be misled into thinking that
roguemetadata.xml is signed by signer_bundle.pem while it is not.

Thanks,
Andrea Ieri