(ITS#6460) SASL EXTERNAL fails with long certificate serial numbers

[Sergio.Gelato@astro.su.se]

Full_Name: Sergio Gelato
Version: 2.4.11 (problem also in HEAD)
OS: Debian GNU/Linux
URL: 
Submission from: (NULL) (85.225.123.197)


Using (X.509 certificate-based) SASL EXTERNAL authentication fails when the
serial number of the certificate is longer than 4 octets.

I have traced the problem to the use, by
libraries/libldap/tls.c:x509_cert_get_dn(),
of ber_get_int() to consume the serial number. This fails when the value to be
decoded
doesn't fit in a ber_int_t.

I discovered this bug with Debian's OpenLDAP 2.4.11-1+lenny1 but a look at the
OpenLDAP CVS indicates that the problem is still present in the current HEAD
(1.167).

I've successfully tested the following patch:
====================================================================================
RFC5280 §4.1.2.2 says "[c]ertificate users MUST be able to handle serialNumber
values up to 20 octets". ber_get_int() fails beyond 4 octets. Since we don't
actually care about the certificate serial number, let's just skip it.

--- a/libraries/libldap/tls.c
+++ b/libraries/libldap/tls.c
@@ -1673,7 +1673,8 @@
 	tag = ber_skip_tag( ber, &len );	/* Context + Constructed (version) */
 	if ( tag == 0xa0 )	/* Version is optional */
 		tag = ber_get_int( ber, &i );	/* Int: Version */
-	tag = ber_get_int( ber, &i );	/* Int: Serial */
+	tag = ber_skip_tag( ber, &len );		/* *Big* integer: Serial */
+	ber_skip_data( ber, len );
 	tag = ber_skip_tag( ber, &len );	/* Sequence: Signature */
 	ber_skip_data( ber, len );
 	if ( !get_subject ) {