Onderwerp: Re: [bugtraq] Security Flaw with Digital signatures in Microsoft Outlook

Bonjour,

Hodie VIII Kal. Apr. MMV est, Roberto Franceschetti scripsit:
> The following one has been "hacked" so that the sender now appears to be "Hackers Franceschetti" (hackers@logsat.com). Note that Outlook states that the email is absolutely valid, and that the certificate is Valid and Trusted. This is most definitely no
t the case, as I've altered the original message to make it appear as a different person actually sent it. Imagine the scenario where a digital signature is supposed to unequivocally identify a sender, but now this email that appears to be sent by "hacker
s" appears legitimate, and a poor victim will trust it and send the hacker any confidential information he is asked for... (follow the hyperlinks for the email's source):

It is clearly indicated "Signed by: roberto@logsat.com", what's the
problem? (see below)

> Screenshot at http://www.logsat.com/Signatures/Hacked1.gif
> Email's source at http://www.logsat.com/Signatures/Hacked1.msg

It's not an email, it's a binary message that can be opened only by
Microsoft Outlook. Could you please provide pure text messages? Same
request for your conversation between MS, CERT, and you.

> This 3rd email is yet another variation showing how a digitally signed email can further be forget without Outlook ever raising warning flags (follow the hyperlinks for the email's source):

In your 2 examples, you aparently fail to notice that the envelope of
the message is not signed *at all*. What you're modifying in precisely
this envelope. What is really signed is the *body* of the message,
that's all. If you change the "From" address, or the subject, or the
sending date, that won't invalidate the signature.

I don't like to say this, but here, Microsoft did something useful for
the end user, by clearly displaying the identity of the signer, along
with the declared identity of the sender. If you want them to do more,
that's something else. But cryptographically speaking, the signatures
haven't been invalidated by your manipulations.

> The full emails with the conversations between myself, Microsoft and CERT can be found here (http://www.logsat.com/Signatures/emails.asp). I hope that by making this information public all the users who rely on digital signatures will be aware of this s
evere security flaw in Microsoft Outlook, and will take other precautions to ensure the identity of users in digitally signed emails they receive.

Could you reformat your web page? It's difficult to read, and .msg
files don't fit my Linux machine.

--
Erwann ABALEA <erwann.abalea@keynectis.com>

This pops up some interesting questions.
Who is the average user going to believe the email is from?
The name in the 'from" field, or the name in the 'signed by' field?

Couple in some recipient confidence in one of the 'from' values, and the
signature means little or nothing, imho.

We still have enough problem with phishing emails, let alone training =
people
abut a new, but very rarely used feature in email.

Lyal

-----Original Message-----
From: Erwann Abalea [mailto:eabalea@certplus.com] On Behalf Of Erwann =
ABALEA
Sent: Saturday, 26 March 2005 3:50 AM
To: Roberto Franceschetti
Cc: bugtraq@securityfocus.com
Subject: Re: [bugtraq] Security Flaw with Digital signatures in =
Microsoft
Outlook


Bonjour,

Hodie VIII Kal. Apr. MMV est, Roberto Franceschetti scripsit:
> The following one has been "hacked" so that the sender now appears to=20
> be "Hackers Franceschetti" (hackers@logsat.com). Note that Outlook=20
> states that the email is absolutely valid, and that the certificate is =

> Valid and Trusted. This is most definitely not the case, as I've=20
> altered the original message to make it appear as a different person=20
> actually sent it. Imagine the scenario where a digital signature is=20
> supposed to unequivocally identify a sender, but now this email that=20
> appears to be sent by "hackers" appears legitimate, and a poor victim=20
> will trust it and send the hacker any confidential information he is=20
> asked for... (follow the hyperlinks for the email's source):

It is clearly indicated "Signed by: roberto@logsat.com", what's the =
problem?
(see below)

> Screenshot at http://www.logsat.com/Signatures/Hacked1.gif
> Email's source at http://www.logsat.com/Signatures/Hacked1.msg

It's not an email, it's a binary message that can be opened only by
Microsoft Outlook. Could you please provide pure text messages? Same =
request
for your conversation between MS, CERT, and you.

> This 3rd email is yet another variation showing how a digitally signed =

> email can further be forget without Outlook ever raising warning flags =

> (follow the hyperlinks for the email's source):

In your 2 examples, you aparently fail to notice that the envelope of =
the
message is not signed *at all*. What you're modifying in precisely this
envelope. What is really signed is the *body* of the message, that's =
all. If
you change the "From" address, or the subject, or the sending date, that
won't invalidate the signature.

I don't like to say this, but here, Microsoft did something useful for =
the
end user, by clearly displaying the identity of the signer, along with =
the
declared identity of the sender. If you want them to do more, that's
something else. But cryptographically speaking, the signatures haven't =
been
invalidated by your manipulations.

> The full emails with the conversations between myself, Microsoft and=20
> CERT can be found here (http://www.logsat.com/Signatures/emails.asp).=20
> I hope that by making this information public all the users who rely=20
> on digital signatures will be aware of this severe security flaw in=20
> Microsoft Outlook, and will take other precautions to ensure the=20
> identity of users in digitally signed emails they receive.

Could you reformat your web page? It's difficult to read, and .msg files
don't fit my Linux machine.

--=20
Erwann ABALEA <erwann.abalea@keynectis.com>

Roberto writes:

> This 3rd email is yet another variation showing how a digitally
> signed email can further be forget without Outlook ever raising
> warning flags (follow the hyperlinks for the email's source):

Digitally-signed messages cannot be forged. However, only the body of a
digitally-signed message is actually included in the text covered by the
signature; the headers are excluded. That's not an Outlook
idiosyncrasy, it's just the way signed e-mail works.

In every screenshot you provide, Outlook correctly identifies the party
that created the digital signature. That's what a security-conscious
user will check. And the text of the message has not been changed, so
the signature is still valid, and no forgery has occurred.

I'm afraid I don't see any problem here. Yes, it's inconvenient that
one can forge the "From" line of a message, but in secure e-mail, one
doesn't rely on the "From" line, anyway, precisely because it can be so
easily forged. I suppose it might be nice if Outlook made the
discrepancy between the "From" line and the signer's authenticated
identity a bit more obvious, but that's not a security breach, just an
ergonomic issue.

--
Anthony

In-Reply-To: <20050325202052.15663.qmail@www.securityfocus.com>

Mr Roberto managed to change email headers – “from address”.

Security Professionals know the email headers can't be trusted and can be easily forged.
Most Outlook users *do not*.

The “signed by” and the certificate signer remain valid – so its issue of educating users to look at the "signed by" field , and not the from address.

This "Feature / Flaw" can be the source of some problematic phishing scams:
if i buy a certificate for some-user@company.com
and send a forged email with "from" helpdesk@company.com, signed by that some-user@company.com
I'm sure it can get the passwords from most of the users.

I'm sure some ppl will take this further and we are going to see this trend in phishing evolving quickly.

Dori Fisher,
We! Consulting group.