Daily Archives: August 12, 2009

Open Source, Technology

Snaxx 21

HaxxYes!

It is now time to once again leave your dark and dusty corners of your office or closet, bring yourself up to speed on what currency we’re using in this country and then unite with fellow hackers and technologists in Stockholm City during a fine September evening. The entire Haxx team is delighted to inform that Snaxx-21 is about to happen…

Monday, September 28th 2009

Time: around 18:30

Where: see the snaxx site!

As usual we’re informal, and as our friends you’re of course allowed and encouraged to bring other friends who are similar in spirit and who you think would appreciate an event such as this.

When you’ve decided to show up, please email me and say so.

There might even be free t-shirts involved this time!

Oh, and if you are a Stockholmer and didn’t get this invite by mail already, let me know and I’ll add you to the list of people who get this notice by the old trusty RFC822 way.

cURL and libcurl, Security

curl fooled by null-prefix

We’ve just now released a security advisory on curl and libcurl regarding how a forger can trick libcurl to verify a forged site as having a fine certificate if you just had a CA create one for you with a carefully crafted embedded zero…

I think this flaw brings the light so greatly on the problems we deal with to maintain code to be safe and secure. When writing code, and as in this case using C, we might believe we’re mostly vulnerable to buffer overflows, pointer messups, memory leaks or similar. Then we see this fascinatingly imaginative “attack” creep up…

The theory in short and somewhat simplified:

A server certificate is always presented by a server when a client connects to it using SSL. The certificate contains the servers name. The client verifies that A) the cert is signed by the correct authority and B) that the cert has the correct name inside.

The A) thing works because servers buy their cert from a CA authority that has its public signature in all browsers, and thus we can be “cryptographically safe” when we see a match.

This last flaw was in the naming part (B). Apparently someone managed to trick a CA to hand out a cert to them using an embedded zero byte. Like if haxx.se would buy the cert, we’d get it with an embedded zero like:

“example.com\0.haxx.se”

Now, this works fine in certificates since they store the string and its length separately. In the language C we’re used to have strings that are terminated with a trailing zero… so, if we would take over the “example.com” HTTPS server we could put our legitimately purchased certificate on that server and clients would use strcmp() or the equivalent to check the name in the certificate against the host name they try to connect to.

The embedded zero makes strcmp(host, certname) return MATCH and the client was successfully fooled.

curl is no longer vulnerable to this trick since 7.19.6, and we have released a boatload of patches for older versions in case upgrading is not an option.

cURL and libcurl

curl 7.19.6 is here!

Yet again we strike back with an update to the popular download tool curl and the transfer library libcurl.

Noticeable changes this time include:

  • A security related fix, for the flaw named CVE-2009-2417.
  • CURLOPT_FTPPORT (and curl’s -P/–ftpport) support port ranges
  • Added CURLOPT_SSH_KNOWNHOSTS, CURLOPT_SSH_KEYFUNCTION, CURLOPT_SSH_KEYDATA so that both the library and the curl tool now understand and work with OpenSSH style known_hosts file (if built with libssh2 1.2 or later)
  • CURLOPT_QUOTE, CURLOPT_POSTQUOTE and  CURLOPT_PREQUOTE can be told to ignore error responses when used with FTP. Handy if you want to run custom commands that may fail, but still enjoy persistent connections properly.

Let me just mention that the known_host support will make the SCP and SFTP transfers done with curl one step more secure. My work on this feature (both in libssh2 and in libcurl) was sponsored by a well-known company that shall remain unidentified at their request.

cURL