[Am-info] Microsoft Internet Explorer Invalid SSL Certificate Chain Vulnerability

Tue, 13 Aug 2002 13:35:24 -0400

A flaw has been reported in the handling of SSL certificates by
Microsoft's Internet Explorer web browser. It may be possible for a
malicious party to create SSL certificates for arbitrary domains, which
will be treated as trusted by the vulnerable browser.

SSL certificates are normally granted and signed by a trusted root
authority, several of which are defined by default in most major web
browsers. It is possible, however, to create a chain of certificates. In
this case, the root certificate must be trusted, and intermediate
certificates should possess a Basic Constraints field which states the
certificate may be used as a signing authority.

Reportedly, Microsoft Internet Explorer does not require the Basic
Constraints field be properly defined. As a result, arbitrary=20
certificates may be used as intermediate authorities in a certificate=20
chain. A malicious party with one valid certificate may sign a new=20
certificate for an arbitrary domain.

The attacker may use the new certificate in order to impersonate a=20
domain. If the attacker is in a position to spoof the domain, or to=20
implement a man-in-the-middle attack, the malicious certificate may=20
allow the attack to go undetected.

Reportedly, Internet Explorer 6.0 will honor a Basic Constraints field
which is explicitely set to False. However, certificates without an
explicitely defined value for this field are still accepted as valid
intermediate authorities.

- --=20
Fred A. Miller
Systems Administrator
Cornell Univ. Press Services
fm@cupserv.org, www.cupserv.org
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