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Date: Thu, 13 Dec 2018 13:53:55 +0100
From: Salva Peiró <speirofr@...il.com>
To: solar@...nwall.com
Cc: oss-security@...ts.openwall.com
Subject: Re: CVE Request: mini-httpd (<= v1.30) is affected by
 a response discrepancy information exposure (CWE-204)

I totally agree its a robustness bug, not a vulnerability,
therefore, it is not worth assigning a CVE for this issue,
I did not have enough evidences at the time I requested the CVE.

Thanks,
--
salva

On Thu, Dec 13, 2018 at 11:21 AM Solar Designer <solar@...nwall.com> wrote:

> Hi,
>
> Thank you for this additional detail.
>
> On Thu, Dec 13, 2018 at 08:46:56AM +0100, Salva Peir?? wrote:
> > The htpasswd password for the "user" is generated by htpasswd from
> > apache2-utils:amd64 (= 2.4.25-3+deb9u6) on Debian
> >
> > # Generate password "user" for "user"
> > $ /usr/bin/htpasswd -c auth/.htpasswd user
> > New password: <user>
> > Re-type new password: <user>
> > Adding password for user user
> >
> > $ cat  auth/.htpasswd
> > user:$apr1$5.vGoLoA$OrxfML2lNUHvhMJrIC7lP.
> >
> > Then a request is made to mini-httpd:
> >
> > $ curl http://user@....0.0.1:8000/auth/
> >
> > This causes the mini-httpd to invoke crypt(3) with the following
> arguments
> > cryptpass = crypt(key, salt), I've added printf's to mini_httpd.c to
> report
> > the actual
> > arguments being passed and the value returned by crypt():
> >
> > $ mini_httpd -D -p 8000 -h 127.0.0.1 -l /dev/stderr
> > key "" salt $apr1$Eh4Xgu3L$YIbNfgDcC1bRGBQWKMS.A1 cryptpass (null)
> > errno 22 strerror Invalid argument
>
> This tells us that mini_httpd isn't compatible with Apache httpd's
> htpasswd.  mini_httpd uses system-provided crypt(3), whereas Apache
> httpd's htpasswd by default generates its own password hashes that are
> generally not supported by system-provided crypt(3).
>
> > Then mini_httpd.c receives a SIGSEGV when performing strcmp() on the NULL
> > cryptpass at mini_httpd.c:2407. The cause of the NULL return value is
> that
> > the salt given to crypt() is invalid as show by errno=EINVAL.  So
> crypt(3)
> > is setting
> > errno=EINVAL to report that the htpasswd file generated by apache2-utils
> is
> > not
> > valid for being used with mini_httpd.
>
> Exactly.  That's an interoperability issue and a robustness bug.  But to
> call it a vulnerability is a stretch, in my opinion.
>
> It doesn't allow for the attack you had described ("remotely enumerate
> valid htpasswd usernames").  It only allows to detect existence of
> usernames that are listed with unsupported hash types (or with otherwise
> incorrect hash encoding strings), which is server-side misconfiguration.
>
> Plenty of other projects had to add checks that crypt(3) return value is
> non-NULL, and would crash on a NULL return before.  We didn't treat most
> of these as vulnerabilities, and didn't assign CVE IDs.
>
> Two notable exceptions are Cyrus SASL CVE-2013-4122 and PostgreSQL
> CVE-2014-0066.  I couldn't easily find any reasoning why they got CVE
> IDs, but these cases might in fact be special: the password hash
> encoding string might be provided by the remote system.  If so, this
> opens up other issues as well, where maliciously high cost settings
> encoded in there would allow for remote DoS, which is worse than the
> local DoS possible via .htpasswd files.  But that's a separate topic,
> being discussed e.g. in:
>
> Consider introducing limits on resource usage by maybe-rogue hash encodings
> https://github.com/besser82/libxcrypt/issues/54
>
> There was a tiny bit of discussion of the Cyrus SASL issue here:
>
> https://www.openwall.com/lists/oss-security/2013/07/15/1
>
> in which Sebastian Krahmer suggested that the missing NULL return check
> could potentially allow for an authentication bypass if a thread could
> consume so much heap address space that a valid allocation at the NULL
> address would exist.  I think that while an issue like this might exist
> on some system, the possibility of such allocation succeeding should be
> treated as the vulnerability.  Otherwise we'd have to treat every NULL
> dereference bug anywhere as a vulnerability, which isn't practical and
> distracts attention from making such allocations impossible.
>
> To summarize:
>
> I think your finding needs a fix, and the fix you propose is correct and
> sufficient, but it isn't a vulnerability and doesn't need a CVE ID.
>
> Thanks again,
>
> Alexander
>

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