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Add FAQ entry on how Cygwin counters install and update MITM attacks

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* faq-setup.xml: Document how Cygwin secures installation and
	update against man-in-the-middle (MITM) attacks.  Note that
	setup embeds a public key to check the signature of setup.ini,
	and that setup.ini includes SHA-512 cryptographic hashes.

Signed-off-by: David A. Wheeler <dwheeler@dwheeler.com>
This commit is contained in:
David A. Wheeler
2015-04-02 14:04:55 -04:00
committed by Corinna Vinschen
parent 383ff5fc47
commit f33e34f333
2 changed files with 127 additions and 1 deletions
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7
winsup/doc/ChangeLog
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@@ -1,3 +1,10 @@
2015-04-02 David A. Wheeler <dwheeler@dwheeler.com>
* faq-setup.xml: Document how Cygwin secures installation and
update against man-in-the-middle (MITM) attacks. Note that
setup embeds a public key to check the signature of setup.ini,
and that setup.ini includes SHA-512 cryptographic hashes.
2015-03-31 Jon TURNEY <jon.turney@dronecode.org.uk>
* misc-funcs.xml (cygwin_internal): Correct return type.

121
winsup/doc/faq-setup.xml
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@@ -156,6 +156,120 @@ and that installing the older version will not help improve Cygwin.
</para>
</answer></qandaentry>
<qandaentry id="faq.setup.install-security">
<question><para>How does Cygwin secure the installation and update process?</para></question>
<answer>
<para>
Here is how Cygwin secures the installation and update process to counter
<ulink url="https://en.wikipedia.org/wiki/Man-in-the-middle_attack">man-in-the-middle (MITM) attacks</ulink>:
</para>
<orderedlist>
<listitem><para>The Cygwin website provides the setup program
(<literal>setup-x86.exe</literal> or <literal>setup-x86_64.exe</literal>)
using HTTPS (SSL/TLS).
This authenticates that the setup program
came from the Cygwin website
(users simply use their web browsers to download the setup program).
You can use tools like Qualsys' SSL Server Test,
<ulink url="https://www.ssllabs.com/ssltest/"/>,
to check the HTTPS configuration of Cygwin.
The cygwin.com site supports HTTP Strict Transport Security (HSTS),
which forces the browser to keep using HTTPS once the browser has seen
it before (this counters many downgrade attacks).
</para></listitem>
<listitem><para>The setup program has the
Cygwin public key embedded in it.
The Cygwin public key is protected from attacker subversion
during transmission by the previous step, and this public
key is then used to protect all later steps.
You can confirm that the key is in setup by looking at the setup project
(<ulink url="http://sourceware.org/cygwin-apps/setup.html"/>)
source code file <literal>cyg-pubkey.h</literal>
(the key is automatically generated from file <literal>cygwin.pub</literal>).
</para></listitem>
<listitem><para>The setup program downloads
the package list <literal>setup.ini</literal> from a mirror
and checks its digital signature.
The package list is in the file
<literal>setup.bz2</literal> (compressed) or
<literal>setup.ini</literal> (uncompressed) on the selected mirror.
The package list includes for every official Cygwin package
the package name, cryptographic hash, and length (in bytes).
The setup program also gets the relevant <literal>.sig</literal>
(signature) file for that package list, and checks that the package list
is properly signed with the Cygwin public key embedded in the setup program.
A mirror could corrupt the package list and/or signature, but this
would be detected by setup program's signature detection
(unless you use the <literal>-X</literal> option to disable signature checking).
The setup program also checks the package list
timestamp/version and reports to the user if the file
goes backwards in time; that process detects downgrade attacks
(e.g., where an attacker subverts a mirror to send a signed package list
that is older than the currently-downloaded version).
</para></listitem>
<listitem><para>The packages to be installed
(which may be updates) are downloaded and both their
lengths and cryptographic hashes
(from the signed <literal>setup.{bz2,ini}</literal> file) are checked.
Non-matching packages are rejected, countering any attacker's
attempt to subvert the files on a mirror.
Cygwin currently uses the cryptographic hash function SHA-512
for the <literal>setup.ini</literal> files.
</para></listitem>
</orderedlist>
<para>
Cygwin uses the cryptographic hash algorithm SHA-512 as of 2015-03-23.
The earlier 2015-02-06 update of the setup program added support for SHA-512
(Cygwin previously used MD5).
There are no known practical exploits of SHA-512 (SHA-512 is part of the
widely-used SHA-2 suite of cryptographic hashes).
</para>
</answer></qandaentry>
<qandaentry id="faq.setup.increase-install-security">
<question><para>What else can I do to ensure that my installation and updates are secure?</para></question>
<answer>
<para>
To best secure your installation and update process, download
the setup program <literal>setup-x86.exe</literal> (32-bit) or
<literal>setup-x86_64.exe</literal> (64-bit), and then
check its signature (using a signature-checking tool you trust)
using the Cygwin public key
(<ulink url="https://cygwin.com/key/pubring.asc"/>).
This was noted on the front page for installing and updating.
</para>
<para>
If you use the actual Cygwin public key, and have an existing secure
signature-checking process, you will counter many other
attacks such as subversion of the Cygwin website and
malicious certificates issued by untrustworthy certificate authorities (CAs).
One challenge, of course, is ensuring that
you have the actual Cygwin public key.
You can increase confidence in the Cygwin public key by checking older copies
of the Cygwin public key (to see if it's been the same over time).
Another challenge is having a secure signature-checking process.
You can use GnuPG to check signatures; if you have a trusted Cygwin
installation you can install GnuPG.
Otherwise, to check the signature you must use an existing trusted tool or
install a signature-checking tool you can trust.
</para>
<para>
Not everyone will go through this additional effort,
but we make it possible for those who want that extra confidence.
We also provide automatic mechanisms
(such as our use of HTTPS) for those with limited time and
do not want to perform the signature checking on the setup program itself.
Once the correct setup program is running, it will counter other attacks
as described in
<ulink url="https://cygwin.com/faq/faq.html#faq.setup.install-security"/>.
</para>
</answer></qandaentry>
<qandaentry id="faq.setup.virus">
<question><para>Is Cygwin Setup, or one of the packages, infected with a virus?</para></question>
<answer>
@@ -197,8 +311,13 @@ disk if you are paranoid.
</orderedlist>
<para>This should be safe, but only if Cygwin Setup is not substituted by
something malicious, and no mirror has been compromised.
something malicious.
See also
<ulink url="https://cygwin.com/faq/faq.html#faq.setup.install-security"/>
for a description of how the
Cygwin project counters man-in-the-middle (MITM) attacks.
</para>
<para>See also <ulink url="https://cygwin.com/faq/faq.html#faq.using.bloda"/>
for a list of applications that have been known, at one time or another, to
interfere with the normal functioning of Cygwin.