An Analysis of TACACS+ Protocol Security
by Solar Designer
May 30, 2000
Last revised: July 7, 2024
This article presents an analysis of several vulnerabilities in Cisco's
TACACS+ protocol. Unfortunately, only some of the vulnerabilities
can be fixed without breaking the interoperability. Thus, the main
purpose of this article is to identify the weaknesses, to allow for
a conscious decision to be made on how much trust to place into the
encryption offered by TACACS+ - on a case by case basis.
Quoting the RFC draft, "TACACS+ provides access control for routers,
network access servers and other networked computing devices via one
or more centralized servers... TACACS+ improves on TACACS and XTACACS
by separating the functions of Authentication, Authorization and
Accounting and by encrypting all traffic between the NAS and the
daemon... TACACS+ uses TCP for its transport." Copies of the RFC
draft (from 1990s) and the final RFC (from 2020) can be obtained at:
Vulnerabilities
The attacks described here assume an attacker with access to the wire
but no knowledge of the encryption key, unless stated otherwise.
The first two vulnerabilities might seem obvious to those familiar
with the protocol. They are listed first to help simplify
understanding of the rest of the analysis, despite their relatively
minor impact.
1. Lack of integrity checking
Impact: accounting records can be altered while in transmission.
Almost no integrity checking exists in TACACS+. The only check
defined in the RFC draft is to make sure the sum of component lengths
matches the total size of the packet.
Combined with the MD5-based stream cipher used to encrypt TACACS+
packets, this lets an attacker with access to the wire flip most of
the bits in the packet (which affects the plaintext in the same way)
without the change getting detected. In particular, it is possible
to make meaningful changes to accounting packets, such as modifying
an elapsed_time from 9000 to 1000 with the flip of one bit.
2. Vulnerability to replay attacks
Impact: duplicate accounting records can be produced, possibly with
forged task_id fields to avoid detection.
TACACS+ lacks virtually any protection against replay attacks. The
only requirement is that packets have a correct sequence number.
Since all TACACS+ sessions start with a sequence number of 1 (not a
vulnerability in and of itself), the TACACS+ server will always
process a packet with seq_no set to 1. Packets from the middle of a
TACACS+ session can't always be replayed, as an attacker would need
to successfully get the session to the required seq_no first.
Especially easy to replay are accounting sessions, which consist of
only one packet sent to the server (with a seq_no of 1). Obviously,
it is also possible to replay the packets with certain bits flipped,
such as to get different task_id's in case a billing system is smart
enough to check for duplicate records.
The fact that TACACS+ uses TCP provides no security against replay,
as new TCP connections may be opened by an attacker for replaying
recorded TACACS+ sessions.
3. Forced session_id collisions
Impact: the encryption of reply packets can be compromised.
Due to its use of a stream cipher, the strength of TACACS+ encryption
depends heavily on unique session_id's for each session. If two
different packets happen to get the same session_id and the same
seq_no, they both become vulnerable to simple frequency analysis
attacks. Additionally, if there's known plaintext in one of the
packets, the corresponding parts of the other can trivially be
decrypted.
Unfortunately, it is possible to get the TACACS+ server to encrypt a
reply packet using a session_id of our choice. Combined with our
ability to replay packets sent to a TACACS+ server, this lets us
compromise the encryption of most of the packets on the way back.
This holds true for almost all packets with a seq_no of 2 (the first
reply packets in a session), as we're always able to make the server
at least have a look at and reply to our initial replayed packets
(seq_no of 1). In order to make sure the second reply is different
from the original one, we can flip a few bits in the request packet,
or indeed change anything in its cleartext header, before replay.
Luckily, passwords are typically only contained in packets travelling
to the server (which are not affected by this vulnerability) - not
on the way back. However, there are exceptions to this: passwords
for outbound PAP have to be sent to the remote end of a PPP link;
likewise, passwords for inbound CHAP used to be given to the NAS (in
minor_version 0 of the protocol, now deprecated). Information other
than user passwords may be of some use for an attacker as well; this
includes usernames and AV pairs.
4. The birthday paradox and session_id's
Impact: given enough sessions, encryption of many may be compromised.
Another problem with session_id's is that they're too small to be
unique if randomly chosen (as required by the RFC draft), and there's
no other way to keep them unique across multiple NAS'es and reloads.
Due to the birthday paradox, we can expect to see two different
sessions with the same session_id if we watch about 100,000 TACACS+
sessions. As separate sessions are used for authentication,
authorization, and accounting, and as multiple accounting records are
sent via TACACS+ at different stages of a user's session to the NAS,
this may correspond to only about 20,000 dialup sessions. Even for a
relatively small ISP, we can expect to see a match within one day.
If we watch for a month, we can get about 1000 matches, which might
give us a few hundred user passwords given the amount of luck (that
is, which packet types get the same session_id) and known plaintext
(such as attribute names) we can reasonably expect.
5. Lack of padding
Impact: the lengths of user passwords can be determined.
The RFC draft states that "there should be no padding in any of the
fields or at the end of a packet". Indeed, the implementations
follow this requirement.
The security implication, however, is that the lengths of variable
size data fields can often be determined from the packet sizes - an
attacker only needs a way to find out which packets contain the
information they are looking for. This task is simplified by the fact
that sequence numbers and packet types are transmitted in the clear.
In the case of determining password lengths, the corresponding
usernames can be obtained via finger to the NAS or similar approaches.
6. MD5 context leak
Impact: none practical; in pathological cases, a part of the packet
can be decrypted.
This vulnerability is only of theoretical value when applied to
TACACS+, and is included here for completeness' sake, as well as to
remind developers of the way MD5-like hashes should not be used. You
should be familiar with MD5 (RFC 1321) in order to understand this
short description.
The body of TACACS+ packets is encrypted by XOR'ing it with a series
of MD5 hashes (each 16 bytes long). The first two hashes (used to
encrypt first 32 bytes of the packet body) are as specified in the
RFC draft:
MD5_1 = MD5{session_id, key, version, seq_no}
MD5_2 = MD5{session_id, key, version, seq_no, MD5_1}
Now, let's assume this pathological scenario:
- the key is 49 bytes long
- we know or can guess first 16 bytes of the packet body
- first 9 bytes of MD5_1 are: 80 B8 01 00 00 00 00 00 00
(In practice, it would take about 272 packets until we see one with
the required bytes in MD5_1. This is clearly far too many.)
As we have the 16 bytes of known plaintext, we can determine the
entire MD5_1 (not just the 9 bytes we check for) from the encrypted
packet. This MD5_1 will match the normally unknown MD5 context from
within the calculation of MD5_2 (after processing of the first
64-byte block).
Now, MD5_2 becomes a function of MD5_1 only; we no longer need to
know the key in order to calculate MD5_2. Once we get MD5_2, the
decryption of the second 16 bytes of the packet body is trivial.
A way to make this attack impossible (and not just infeasible) would
be to define MD5_1 like this:
MD5_0 = 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
MD5_1 = MD5{session_id, key, version, seq_no, MD5_0}
7. Packet body length DoS and/or overflow
Impact: TACACS+ server DoS, TACACS+ client DoS, potential break-in.
Unlike other issues discussed here, this is an implementation defect.
However, the mistake is "essential" enough that both implementations
checked (the unsupported TAC_PLUS Developer's Kit vF4.0.3.alpha and
Cisco IOS 11.3(9)T) turned out to be vulnerable.
One of the fields in the 12-byte cleartext packet header is the body
length. The obvious way to read a packet off the socket is to read
the header first, allocate memory for the body, and then read the
body with another call. The "essential" mistake here is to not
sanity check the length field before allocating the memory.
Thus, a trivial DoS attack on a TACACS+ server is to make it run out
of memory by sending it a packet with a huge value in the length
field. Depending on the malloc(3) implementation, it may also be
required to actually fill the memory with data (which will require
some time for transmitting the data, but is still trivial).
It is also essential for implementations to allocate memory for both
the packet header and the body, and copy the header into that memory
before reading the body off the socket. The "essential" mistake here
is to not check for an integer overflow in calculating the total
memory size to allocate.
In tac_plus, this results in the ability to overflow the buffer with
our packet header data by up to 11 bytes. This is usually harmless,
but there might exist platforms where it's not.
Obviously, these two attacks require neither access to the wire, nor
knowledge of the key. The only requirement is the ability to connect
to a TACACS+ server.
Additionally, it is possible to cause tac_plus to at least misbehave
by exploiting the first two vulnerabilities mentioned here, together
with the lack of integer overflow checking in calculating the sum of
packet component lengths for the comparison.
It is likely that other attacks on tac_plus and the underlying OS are
possible when the encryption key is known, but these are outside the
scope of this analysis.
Similar attacks are possible against the clients; however, they
require either access to the wire, or the ability to do blind TCP
sequence number and timing prediction. In the case of Cisco IOS, it
is possible to allocate all of the available I/O memory for the
duration of the TCP connection.
Fixes
Cisco has fixed the tac_plus DoS/overflow in version F4.0.4.alpha of
their unsupported Developer's Kit.
This vulnerability is also fixed in tac+ia (a tac_plus clone) version 0.96.
Alternatively, you can apply this simple patch to an older version of
tac_plus:
--- tac_plus.F4.0.3.alpha.orig/packet.c Sat Apr 3 10:03:46 1999
+++ tac_plus.F4.0.3.alpha/packet.c Sun Nov 28 08:28:27 1999
@@ -446,6 +446,13 @@
/* get memory for the packet */
len = TAC_PLUS_HDR_SIZE + ntohl(hdr.datalength);
+ if ((ntohl(hdr.datalength) & ~0xffffUL) ||
+ len < TAC_PLUS_HDR_SIZE || len > 0x10000) {
+ report(LOG_ERR,
+ "%s: Illegal data size: %lu\n",
+ session.peer, ntohl(hdr.datalength));
+ return(NULL);
+ }
pkt = (u_char *) tac_malloc(len);
/* initialise the packet */
Recommendations
Note: these are general security recommendations on setting up
TACACS+ - they cannot fix some of the inherent protocol defects
discussed above.
1. Apply packet filtering where possible
In the simple case, you will have all the TACACS+ clients and servers
within your network. Make sure the servers are only accessible from
within your network, and preferably only by the IP addresses of the
clients (this may require a combination of filters on the server
systems themselves and anti-spoofing filters on your border routers).
The default TACACS+ server port is 49/tcp.
2. Choose strong encryption keys
Offline attacks against the encryption key are possible with only one
packet collected off the wire, and run much faster than similar
attacks against UNIX passwords do. Thus, a strong encryption key
should be larger than a typical user password. Keep in mind that if
the key becomes known, additional attacks against both TACACS+ server
and client systems become possible.
3. Avoid running tac_plus as root
Unfortunately, version F4.0.3.alpha has a few problems when running as
non-root, but it does support the TAC_PLUS_USERID and TAC_PLUS_GROUPID
defines at compile time. Be sure not to have any extra supplementary
groups when you start tac_plus, as it's not smart enough to drop
those. tac+ia-0.96 and later will have --enable-tacplus-username
"configure" option.
References
Credits and contact information
The analysis has been performed by Solar Designer <solar at openwall.com>.
I would like to thank Dug Song for reviewing the security advisory
(which this article is based on) and Damir Rajnovic of Cisco Systems PSIRT for
handling these vulnerabilities within Cisco.
Updated versions of this article will be made available at
https://www.openwall.com/articles/TACACS%2B-Protocol-Security.
Revision history
Release: May 30, 2000 (Openwall security advisory OW-001-tac_plus)
Updated: July 10, 2000 (revision 2 of the above)
Updated: June 6, 2010 (converted into article, better HTML formatting)
Updated: June 19, 2014 (dropped ftpeng.cisco.com links, which no longer work)
Updated: July 7, 2024 (updated RFC links)
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