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Useless OpenSSH resources exhausion bug via GSSAPI
From: Adam Zabrocki <zabrocki () cern ch>
Date: Mon, 1 Aug 2011 23:11:10 +0200

Name:                 Useless OpenSSH resources exhausion bug via GSSAPI
Author:               Adam Zabrocki (<pi3 () itsec pl>)
Date:                 2008-2009 (old useless bug ;P)


   Description:

        OpenSSH is a FREE version of the SSH connectivity tools
that technical users of the Internet rely on. Users of telnet,
rlogin, and ftp may not realize that their password is transmitted
across the Internet unencrypted, but it is. OpenSSH encrypts all
traffic (including passwords) to effectively eliminate eavesdropping,
connection hijacking, and other attacks. Additionally, OpenSSH
provides secure tunneling capabilities and several authentication
methods, and supports all SSH protocol versions. 

The Generic Security Services Application Program Interface (GSSAPI,
also GSS-API) is an application programming interface for programs
to access security services.
The GSSAPI, by itself, does not provide any security. Instead,
security service vendors provide GSSAPI implementations usually
in the form of libraries installed with their security software. 


   Details:

        It is possible to provide any value to the xmalloc()
function, which is simple wrapper to the malloc() function.
This open an windows to force an application to alocate a huge
amount of the memory (4GB?) and naturally exhause avaibale
resources. Repeating this attack, by simple open many session,
can kill the server.

Implementation of the xmalloc() function can be found here:

"xmalloc.c"
void *
xmalloc(size_t size)
{
        void *ptr;

        if (size == 0)
                fatal("xmalloc: zero size");
        ptr = malloc(size);
        if (ptr == NULL)
                fatal("xmalloc: out of memory (allocating %lu bytes)",
(u_long) size);
        return ptr;
}

This wrapper check what is the return value from the malloc()
- error supports, and check if it is not passed 0 (zero) value
for the argument. Its very clever because a lot of bugs are
based on misscalculation and passing 0 value for the argument
can be exploited in some situation. Especially this is exploitable
situation at Windows systems.

The bug exists in this code:

"./gss-serv.c"
static OM_uint32
ssh_gssapi_parse_ename(Gssctxt *ctx, gss_buffer_t ename, gss_buffer_t
name)
{
        u_char *tok;
        OM_uint32 offset;
        OM_uint32 oidl;

        tok = ename->value;
...
...

        if (ename->length < 6 || memcmp(tok, "\x04\x01", 2) != 0)
                return GSS_S_FAILURE;

...
...

        oidl = get_u16(tok+2); /* length including next two bytes */
        oidl = oidl-2; /* turn it into the _real_ length of the variable
OID */

...
...
        if (tok[4] != 0x06 || tok[5] != oidl ||
            ename->length < oidl+6 ||
            !ssh_gssapi_check_oid(ctx, tok+6, oidl))
                return GSS_S_FAILURE;

        offset = oidl+6;

        if (ename->length < offset+4)
                return GSS_S_FAILURE;

 [1]    name->length = get_u32(tok+offset);
        offset += 4;

 [2]    if (ename->length < offset+name->length)
                return GSS_S_FAILURE;

 [3]    name->value = xmalloc(name->length+1);
 [4]    memcpy(name->value, tok+offset, name->length);
 [5]    ((char *)name->value)[name->length] = 0;

        return GSS_S_COMPLETE;
}

It is possible to set a huge value at line [1] which can
bypass len check at line [2] and cause to allocate our
value +1 at line [3]. Function xmalloc() protect from
the situation described before about passing 0 (zero)
value to the argument a make nice working exploit via
calling at line [4] and [5]. Anyway it is still possible
to force an resources exhausion attack.

Going furhter, definition of the structure which is
interested for us is here:

"/usr/include/gssapi/gssapi.h"
      size_t length;
      void *value;
} gss_buffer_desc, *gss_buffer_t;

Function ssh_gssapi_parse_ename() is called here:

"./gss-serv.c"
/* Privileged (called from accept_secure_ctx) */
OM_uint32
ssh_gssapi_getclient(Gssctxt *ctx, ssh_gssapi_client *client)
{
        int i = 0;

        gss_buffer_desc ename;

       ...
       ...

       if ((ctx->major = ssh_gssapi_parse_ename(ctx,&ename,
            &client->exportedname))) {
                return (ctx->major);
        }

       ...
       ...
}

and this function is called here:

"./gss-serv.c"
OM_uint32
ssh_gssapi_accept_ctx(Gssctxt *ctx, gss_buffer_desc *recv_tok,
    gss_buffer_desc *send_tok, OM_uint32 *flags)
{
        OM_uint32 status;
        gss_OID mech;

 [1]    ctx->major = gss_accept_sec_context(&ctx->minor,
            &ctx->context, ctx->creds, recv_tok,
            GSS_C_NO_CHANNEL_BINDINGS, &ctx->client, &mech,
            send_tok, flags, NULL, &ctx->client_creds);

        if (GSS_ERROR(ctx->major))
                ssh_gssapi_error(ctx);

        if (ctx->client_creds)
                debug("Received some client credentials");
        else
                debug("Got no client credentials");

        status = ctx->major;

        /* Now, if we're complete and we have the right flags, then
         * we flag the user as also having been authenticated
         */

        if (((flags == NULL) || ((*flags & GSS_C_MUTUAL_FLAG) &&
            (*flags & GSS_C_INTEG_FLAG))) && (ctx->major ==
GSS_S_COMPLETE)) {
 [2]            if (ssh_gssapi_getclient(ctx, &gssapi_client))
                        fatal("Couldn't convert client name");
        }

        return (status);
}

What is important here? Call for the vuln code is at line [2].
Whole this code is reponsible for authentication. At line [1]
is function which do REAL authentication. So vuln code is called
directly after REAL auth. So (un)fortunately this bug is directly
post-auth :( This is the reason why this bug is useless in fact
and public now ;)

Going furhter:

"auth2-gss.c"
static void
input_gssapi_token(int type, u_int32_t plen, void *ctxt)
{
   ...

   ...
   ...
        maj_status = PRIVSEP(ssh_gssapi_accept_ctx(gssctxt, &recv_tok,
            &send_tok, &flags));
   ...
   ...
}

Next:

/*
 * We only support those mechanisms that we know about (ie ones that we
know
 * how to check local user kuserok and the like)
 */
static int
userauth_gssapi(Authctxt *authctxt)
{
   ...

   ...
   ...
        dispatch_set(SSH2_MSG_USERAUTH_GSSAPI_TOKEN,
&input_gssapi_token);
   ...
   ...
}

and this methos is registered here:

Authmethod method_gssapi = {
        "gssapi-with-mic",
        userauth_gssapi,
        &options.gss_authentication
};


   Affected Software:

        Any OpenSSH server which support "gssapi-with-mic" auth.
In fact every modern Linux distro support it by default.


   Greetz for Markus Friedl (openssh team).


   Disclosure Timeline

*) 01 Aug,  2011  -  release advisory
*) 01 Aug,  2011  -  release patch
*) 28 Jul,  2011  -  contact with vendor
*) xx xxx,  2008  -  found bug


--
http://pi3.com.pl
http://blog.pi3.com.pl/?p=159

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