[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Fwd: ThomasBSD Security Advisory #1

----------  Forwarded Message  ----------
Subject: ThomasBSD Security Advisory #1
Date: Sun, 1 Apr 2001 00:08:50 +0200
From: Thomas Lopatic <thomas_(_at_)_LOPATIC_(_dot_)_DE>

*** ThomasBSD Security Advisory #1 ***

                               Thomas Lopatic

                              April 1st, 2001


Having just found a serious security problem in the current release
 of a core Internet service, I have decided to start working on my
 own highly secure distribution of BSD, which will be called
 "ThomasBSD" for obvious reasons.

Features of ThomasBSD

ThomasBSD is based on OpenBSD, thus it is OpenBSD PLUS MORE,
mathematically making it (NetBSD PLUS MORE) PLUS MORE.

The epoch of ThomasBSD will be moved back from January 1st, 1970 to
January 1st, 1960. Whenever a security problem is found and fixed in
OpenBSD, this little shift will enable me to also correct the issue
 in ThomasBSD and then send mail to security-related mailing lists
 stating that "this was fixed in ThomasBSD about ten years ago."

In addition, ThomasBSD will, in a slightly different application of
the epoch shift, always be able to claim "Ten years without any local
or remote holes in any install, i.e. in the default install PLUS

My original plan was to only provide binary releases of ThomasBSD, so
that I would not have to go through the efforts of hiding
security-critical changes in the CVS tree by inventing harmless

Still, as binaries can be reverse engineered, ThomasBSD releases will
only consist of the MD5 sums of the binaries. However, people
 actively contributing to ThomasBSD will be able to obtain MD5 sums
 of the source code.

This will ensure that no other operating system can copy any fixes
from ThomasBSD, and thus provide ThomasBSD with a USP, i.e. a unique
security position.

Obtaining ThomasBSD

Binary releases will be regularly mailed to bugtraq. ThomasBSD 1.0 is


The mentioned security problem

This advisory will be the only time that I provide information on a
problem fixed in ThomasBSD that exceeds an MD5 sum of the diffs.

The problem is related to using isdigit() on user-supplied signed
characters without using isascii() first or type-casting the signed
character to an unsigned character or doing something similar.

Thus, the integer supplied as an argument to isdigit() may be
negative. If it is negative, the isdigit() function - depending on
 the operating system's implementation of libc - potentially performs
 look-ups outside its look-up table, since the provided integer is
 used as an index for the look-ups.

It has to be noted that some implementations of libc use a construct
along the lines of


unsigned short table[256 + 128];
unsigned short *base;


    base = table + 128;

    if (base[input] & ISDIGIT_MASK)
        return TRUE;

    return FALSE;


together with a table in which table[x] == table[x + 256] to be
tolerant to sloppy coding and return the same values for the inputs
 of -128 and 128, -127 and 129, -126 and 130, etc.

However, in vulnerable libraries isdigit() may return TRUE for at
least some negative inputs. As a further hint, consider the following
code snippet.


int i, v;
char in[MAX_INPUT_SIZE];


    for (i = v = 0; in[i] && isdigit(in[i]); i++)
        v = v * 10 + (in[i] - '0');


Also consider the fact that if the variable v was used as an index in
the program that this snippet belongs to, a programmer would
 typically rely on v - having been calculated in the way described -
 being non-negative and only validate the upper bound before using
 the index, i.e. compare v to the number of elements in the array, as


unsigned int array[ARRAY_SIZE];


    if (v >= ARRAY_SIZE)
        return -1;

    array[v] = [...]

The rest of the problem is left as an exercise to the reader. (Hint:
What happens if v is negative?)

Since vulnerabilities based on the described invalid use of the
isXXX() class of functions have up to now not been documented, I
 claim the right of assigning this class of security problems a name,
 the name of my choice being "giraffes". This will facilitate
 communication among security experts as in

             "Cool, I've found a new giraffe in OpenBSD!"


Visit your host, monkey.org