Abstract:
---------
The ExpressPay stored-value card system used by FedEx Kinko's is
vulnerable to attack. An attacker who gains the ability to alter the
data stored on the card can use FedEx Kinko's services fraudulently
and anonymously, and can even obtain cash from the store.
Description:
------------
The FedEx Kinko's ExpressPay system, developed by enTrac Technologies
of Toronto, Ontario, is based on a Siemens / Infineon SLE4442 memory
chip card. The data stored on this card is freely rewritable once a
three-byte security code has been presented to the card's security
logic. Neither this security code nor the data stored on the card is
encrypted; anyone able to obtain the security code is free to rewrite
the data stored on the card using an inexpensive commercially
available smart card reader/writer.
The first thirty-two bytes of the memory chip card are writable and
subsequently permanently write-protectable (in this application, these
bytes are write-protected), and contain a header which identifies the
card as an ExpressPay stored-value card. Bytes 0x20 through 0x27
contain the value stored on the card, represented in IEEE 754
double-precision floating point format. Bytes 0x60 through 0x6A
contain the card's eleven-digit serial number stored as unsigned
zoned-decimal ASCII; digits 0x60 through 0x63 are the store number the
card was initially issued at, and the remaining seven digits are
assigned sequentially at the moment of first issue. A timestamp
indicating date and time of issue are located from 0x30 through 0x37,
and is repeated from 0xC7 through 0xCE.
In order to write to the card, a three-byte security code must be
presented in a specific sequence of commands as outlined by the
SLE4442's white paper. By soldering wires to the contact points of
the card and then connecting those wires to an inexpensive logic
analyzer, an attacker can sniff the three-byte code as the kiosk or a
card terminal prepares to write data to the card. This security code
appears to be the same across all FedEx Kinko's ExpressPay cards
currently in circulation.
Once the three-byte code is known to the attacker, the card's stored
value and serial number can be changed to any value. The ExpressPay
system appears to implicitly trust the value stored on the card,
regardless of what that value actually is. The system will also
accept cards with obviously fake serial numbers (e.g. a non-existent
store number followed by all nines). Using these altered cards,
xeroxes can be made from any machine with a card reader, and computers
can be rented anonymously and indefinitely. Most disturbing, however,
is that since stored-value cards can be cashed out by an employee at
the register at any time, an attacker could cash out altered cards
obtained at little or no monetary cost. If a card is cashed out, its
serial number does not appear to be invalidated in the system. If an
attacker were to clone a known good card and cash it out, the clone
would still be usable.
Tested Vendors:
---------------
- FedEx Kinko's
Suspected Vendors:
------------------
- Any client of enTrac Technologies who uses the ExpressPay
stored-value card system.
- Any company which uses a stored-value card system based on the SLE4442
Vendor and Patch Information:
-----------------------------
Proof-of-concept of the initial security vulnerability was achieved on
8 February 2006, with research into the ramifications continuing
through 12 February. Copies of this report were sent to both FedEx
Kinko's and enTrac Technologies on 15 February; a read receipt was
returned from enTrac on 19 February, while no receipt has yet been
received from FedEx Kinko's.
Solution:
---------
- Encrypt data before storing it on the SLE4442 card, or migrate to a
system which uses cards which have built-in encryption functionality.
- Verify that the stored value on the card does not significantly
differ from a reference value stored in a database.
- Do not allow the use of cards with invalid serial numbers.
- Invalidate serial numbers of cards that are cashed out.
Credits:
--------
Strom Carlson, Secure Science Corporation: Hardware Security Division
stromc@securescience.net
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