[UNIX] OpenSSH Buffer Management Vulnerability

From: SecuriTeam (support_at_securiteam.com)
Date: 09/17/03

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      OpenSSH Buffer Management Vulnerability
    ------------------------------------------------------------------------

    SUMMARY

    OpenSSH is a suite of network connectivity tools that can be used to
    establish encrypted connections between systems on a network and can
    provide interactive login sessions and port forwarding, among other
    functions.

    The OpenSSH team has announced a bug which affects the OpenSSH buffer
    handling code. This bug has the potential of being remotely exploitable.

    DETAILS

    Versions affected:
    All versions of OpenSSH's sshd prior to 3.7.1 contain buffer management
    errors. It is uncertain whether these errors are potentially exploitable,
    however, we prefer to see bugs fixed proactively.

    Other implementations sharing common origin may also have these issues.

    Solution:
    Upgrade to OpenSSH 3.7.1 or apply the following patch.

    Appendix A: patch for OpenSSH 3.6.1 and earlier
    Index: buffer.c
     ===================================================================
    RCS file: /cvs/src/usr.bin/ssh/buffer.c,v
    retrieving revision 1.16
    retrieving revision 1.18
    diff -u -r1.16 -r1.18
    --- buffer.c 26 Jun 2002 08:54:18 -0000 1.16
    +++ buffer.c 16 Sep 2003 21:02:39 -0000 1.18
    @@ -23,8 +23,11 @@
     void
     buffer_init(Buffer *buffer)
     {
    - buffer->alloc = 4096;
    - buffer->buf = xmalloc(buffer->alloc);
    + const u_int len = 4096;
    +
    + buffer->alloc = 0;
    + buffer->buf = xmalloc(len);
    + buffer->alloc = len;
       buffer->offset = 0;
       buffer->end = 0;
     }
    @@ -34,8 +37,10 @@
     void
     buffer_free(Buffer *buffer)
     {
    - memset(buffer->buf, 0, buffer->alloc);
    - xfree(buffer->buf);
    + if (buffer->alloc > 0) {
    + memset(buffer->buf, 0, buffer->alloc);
    + xfree(buffer->buf);
    + }
     }
     
     /*
    @@ -69,6 +74,7 @@
     void *
     buffer_append_space(Buffer *buffer, u_int len)
     {
    + u_int newlen;
       void *p;
     
       if (len > 0x100000)
    @@ -98,11 +104,13 @@
         goto restart;
       }
       /* Increase the size of the buffer and retry. */
    - buffer->alloc += len + 32768;
    - if (buffer->alloc > 0xa00000)
    +
    + newlen = buffer->alloc + len + 32768;
    + if (newlen > 0xa00000)
         fatal("buffer_append_space: alloc %u not supported",
    - buffer->alloc);
    - buffer->buf = xrealloc(buffer->buf, buffer->alloc);
    + newlen);
    + buffer->buf = xrealloc(buffer->buf, newlen);
    + buffer->alloc = newlen;
       goto restart;
       /* NOTREACHED */
     }
    Index: channels.c
     ===================================================================
    RCS file: /cvs/src/usr.bin/ssh/channels.c,v
    retrieving revision 1.194
    retrieving revision 1.195
    diff -u -r1.194 -r1.195
    --- channels.c 29 Aug 2003 10:04:36 -0000 1.194
    +++ channels.c 16 Sep 2003 21:02:40 -0000 1.195
    @@ -228,12 +228,13 @@
       if (found == -1) {
         /* There are no free slots. Take last+1 slot and expand the array.
    */
         found = channels_alloc;
    - channels_alloc += 10;
         if (channels_alloc > 10000)
           fatal("channel_new: internal error: channels_alloc %d "
               "too big.", channels_alloc);
    + channels = xrealloc(channels,
    + (channels_alloc + 10) * sizeof(Channel *));
    + channels_alloc += 10;
         debug2("channel: expanding %d", channels_alloc);
    - channels = xrealloc(channels, channels_alloc * sizeof(Channel *));
         for (i = found; i < channels_alloc; i++)
           channels[i] = NULL;
       }
     ===================================================================

    Appendix B: patch for OpenSSH 3.7
    Index: buffer.c
     ===================================================================
    RCS file: /cvs/src/usr.bin/ssh/buffer.c,v
    retrieving revision 1.17
    retrieving revision 1.18
    diff -u -r1.17 -r1.18
    --- buffer.c 16 Sep 2003 03:03:47 -0000 1.17
    +++ buffer.c 16 Sep 2003 21:02:39 -0000 1.18
    @@ -23,8 +23,11 @@
     void
     buffer_init(Buffer *buffer)
     {
    - buffer->alloc = 4096;
    - buffer->buf = xmalloc(buffer->alloc);
    + const u_int len = 4096;
    +
    + buffer->alloc = 0;
    + buffer->buf = xmalloc(len);
    + buffer->alloc = len;
       buffer->offset = 0;
       buffer->end = 0;
     }
    @@ -34,8 +37,10 @@
     void
     buffer_free(Buffer *buffer)
     {
    - memset(buffer->buf, 0, buffer->alloc);
    - xfree(buffer->buf);
    + if (buffer->alloc > 0) {
    + memset(buffer->buf, 0, buffer->alloc);
    + xfree(buffer->buf);
    + }
     }
     
     /*
    Index: channels.c
     ===================================================================
    RCS file: /cvs/src/usr.bin/ssh/channels.c,v
    retrieving revision 1.194
    retrieving revision 1.195
    diff -u -r1.194 -r1.195
    --- channels.c 29 Aug 2003 10:04:36 -0000 1.194
    +++ channels.c 16 Sep 2003 21:02:40 -0000 1.195
    @@ -228,12 +228,13 @@
       if (found == -1) {
         /* There are no free slots. Take last+1 slot and expand the array.
    */
         found = channels_alloc;
    - channels_alloc += 10;
         if (channels_alloc > 10000)
           fatal("channel_new: internal error: channels_alloc %d "
               "too big.", channels_alloc);
    + channels = xrealloc(channels,
    + (channels_alloc + 10) * sizeof(Channel *));
    + channels_alloc += 10;
         debug2("channel: expanding %d", channels_alloc);
    - channels = xrealloc(channels, channels_alloc * sizeof(Channel *));
         for (i = found; i < channels_alloc; i++)
           channels[i] = NULL;
       }

    ADDITIONAL INFORMATION

    The original advisory can be downloaded from:
    <http://www.openssh.com/txt/buffer.adv>
    http://www.openssh.com/txt/buffer.adv.

    The information has been provided by OpenSSH.

    ========================================

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