OpenSSH Security Advisory (adv.option)

[Markus Friedl <markus () openbsd org>]

Weakness in OpenSSH's source IP based access control
for SSH protocol v2 public key authentication.

1. Systems affected:

        Versions of OpenSSH between 2.5.x and 2.9.x using
        the 'from=' key file option in combination with
        both RSA and DSA keys in ~/.ssh/authorized_keys2.

2. Description:

        Depending on the order of the user keys in
        ~/.ssh/authorized_keys2 sshd might fail to apply the
        source IP based access control restriction (e.g.
        from="10.0.0.1") to the correct key:

        If a source IP restricted key (e.g. DSA key) is
        immediately followed by a key of a different type
        (e.g. RSA key), then key options for the second key
        are applied to both keys, which includes 'from='.

3. Impact:

        Users can circumvent the system policy
        and login from disallowed source IP addresses.
        
4. Solution:

        Apply the following patch.

        This bug is fixed in OpenSSH 2.9.9

5. Credits:

        None.

Appendix:

Index: key.c
===================================================================
RCS file: /cvs/src/usr.bin/ssh/key.c,v
retrieving revision 1.31
retrieving revision 1.32
diff -u -p -IRCSID -r1.31 -r1.32
--- key.c       2001/09/17 20:50:22     1.31
+++ key.c       2001/09/19 13:23:29     1.32
@@ -358,7 +358,7 @@ write_bignum(FILE *f, BIGNUM *num)
        return 1;
 }
 
-/* returns 1 ok, -1 error, 0 type mismatch */
+/* returns 1 ok, -1 error */
 int
 key_read(Key *ret, char **cpp)
 {
@@ -413,7 +413,7 @@ key_read(Key *ret, char **cpp)
                } else if (ret->type != type) {
                        /* is a key, but different type */
                        debug3("key_read: type mismatch");
-                       return 0;
+                       return -1;
                }
                len = 2*strlen(cp);
                blob = xmalloc(len);