Bugs item #2665432, was opened at 2009-03-05 16:14
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Category: misc
Group: None
Status: Open
Resolution: None
Priority: 5
Private: No
Submitted By: Olivier Hervieu (ohervieu)
Assigned to: Nobody/Anonymous (nobody)
Summary: auth_cookie generation is broken (channel_x11_req)
Initial Comment:
Hi guys,
When using libssh2_channel_x11_req, on linux, the auth_cookie is generated by
the following code :
unsigned char buffer[LIBSSH2_X11_RANDOM_COOKIE_LEN / 2];
libssh2_random(buffer, LIBSSH2_X11_RANDOM_COOKIE_LEN / 2);
for(i = 0; i < (LIBSSH2_X11_RANDOM_COOKIE_LEN / 2); i++) {
snprintf((char *) s + (i * 2), 2, "%02X", buffer[i]);
}
where s is a buffer of LIBSSH2_X11_RANDOM_COOKIE_LEN length.
When executing this, I always have a strange generated cookie. On the server
side, sshd says :
/usr/bin/X11/xauth: (stdin):2: key contains odd number of or non-hex characters
In fact the auth_cookie sent has a length of .... 1.
Refering to the man page of snprintf, it says : The functions snprintf() and
vsnprintf() write at most size bytes (including the trailing null byte
(’\0’)) to str.
So, if you a define a buffer : buffer[]="AAAAAAAAAAAAAAAA"
snprintf((char *)s +(i*2),2,"%02X", buffer[i]);
will always write '4\0' in the buffer for each loop of the for statement.
So, a correct cookie is generated by using :
snprintf((char *)s +(i*2),2+1 ,"%02X", buffer[i]);
Regards,
Olivier
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