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An Analysis of the RADIUS Authentication Protocol
From: Joshua Hill <josh-radius () untruth org>
Date: Mon, 12 Nov 2001 17:30:14 -0800
An Analysis of the RADIUS Authentication Protocol
by Joshua Hill, InfoGard Laboratories [www.infogard.com]
Last Modified Mon Nov 12 16:12:53 PST 2001
A current HTML version of this paper can be found at:
Please send comments to <josh-radius () untruth org>
RADIUS is a widely used protocol in network environments. It is
commonly used for embedded network devices such as routers, modem
servers, switches, etc. It is used for several reasons:
* The embedded systems generally cannot deal with a large number of
users with distinct authentication information. This requires more
storage than many embedded systems possess.
* RADIUS facilitates centralized user administration, which is
important for several of these applications. Many ISPs have tens
of thousands, hundreds of thousands, or even millions of users.
Users are added and deleted continuously throughout the day, and
user authentication information changes constantly. Centralized
administration of users in this setting is an operational
* RADIUS consistently provides some level of protection against a
sniffing, active attacker. Other remote authentication protocols
provide either intermittent protection, inadequate protection or
non-existent protection. RADIUS's primary competition for remote
authentication is TACACS+ and LDAP. LDAP natively provides no
protection against sniffing or active attackers. TACACS+ is subtly
flawed, as discussed by Solar Designer in his advisory.
* RADIUS support is nearly omni-present. Other remote authentication
protocols do not have consistent support from hardware vendors,
whereas RADIUS is uniformly supported. Because the platforms on
which RADIUS is implemented on are often embedded systems, there
are limited opportunities to support additional protocols. Any
changes to the RADIUS protocol would have to be at least minimally
compatible with pre-existing (unmodified) RADIUS clients and
RADIUS is currently the de-facto standard for remote authentication.
It is very prevalent in both new and legacy systems.
This analysis deals with some of the characteristics of the base
RADIUS protocol and of the User-Password attribute. Depending on the
mode of authentication used, the described User-Password weaknesses
may or may not compromise the security of the underlying
authentication scheme. A complete compromise of the User-Password
attribute would result in the complete compromise of the normal
Username/Password or PAP authentication schemes, because both of these
systems include otherwise unprotected authentication information in
the User-Password attribute. On the other hand when CHAP or a
Challenge/Response system is in use, a complete compromise of the
User-Password attribute would only expose the underlying CHAP or
Challenge/Response information to additional attack, which may or may
not lead to a complete compromise of the authentication system,
depending on the strength of the underlying authentication system.
This analysis does not cover the RADIUS protocol's accounting
functionality (which is, incidentally, also flawed, but normally
doesn't transport information that must be kept confidential).
2 Protocol Summary
A summary of the RADIUS packet is below (from the RFC):
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
| Code | Identifier | Length |
| Authenticator |
| Attributes ...
The code establishes the type of RADIUS packet. The codes are:
12 Status-Server (experimental)
13 Status-Client (experimental)
The identifier is a one octet value that allows the RADIUS client to
match a RADIUS response with the correct outstanding request.
The attributes section is where an arbitrary number of attribute
fields are stored. The only pertinent attributes for this discussion
are the User-Name and User-Password attributes.
This description will concentrate on the most common type of RADIUS
exchange: An Access-Request involving a username and user password,
followed by either an Access-Accept, Access-Reject or a failure. I
will refer to the two participants in this protocol as the client and
the server. The client is the entity that has authentication
information that it wishes to validate. The server is the entity that
has access to a database of authentication information that it can use
to validate the client's authentication request.
2.1 Initial Client Processing
The client creates an Access-Request RADIUS packet, including at least
the User-Name and User-Password attributes.
The Access-Request packet's identifier field is generated by the
client. The generation process for the identifier field is not
specified by the RADIUS protocol specification, but it is usually
implemented as a simple counter that is incremented for each request.
The Access-Request packet contains a 16 octet Request Authenticator in
the authenticator field. This Request authenticator is a randomly
chosen 16 octet string.
This packet is completely unprotected, except for the User-Password
attribute, which is protected as follows:
The client and server share a secret. That shared secret followed by
the Request Authenticator is put through an MD5 hash to create a 16
octet value which is XORed with the password entered by the user. If
the user password is greater than 16 octets, additional MD5
calculations are performed, using the previous ciphertext instead of
the Request Authenticator.
Call the shared secret S and the pseudo-random 128-bit Request
Authenticator RA. The password is broken into 16-octet blocks p1, p2,
... pn, with the last block padded at the end with '0's to a 16-octet
boundary. The ciphertext blocks are c1, c2... cn.
c1 = p1 XOR MD5(S + RA)
c2 = p2 XOR MD5(S + c1)
cn = pn XOR MD5(S + cn-1)
The User-Password attribute contains c1+c2+...+cn, Where + denotes
2.2 Server Processing
The server receives the RADIUS Access-Request packet and verifies that
the server possesses a shared secret for the client. If the server
does not possess a shared secret for the client, the request is
Because the server also possesses the shared secret, it can go through
a slightly modified version of the client's protection process on the
User-Password attribute and obtain the unprotected password. It then
uses its authentication database to validate the username and
password. If the password is valid, the server creates an
Access-Accept packet to send back to the client. If the password is
invalid, the server creates an Access-Reject packet to send back to
Both the Access-Accept packet and the Access-Reject packet use the
same identifier value from the client's Access-Request packet, and put
a Response Authenticator in the Authenticator field. The Response
Authenticator is the is the MD5 hash of the response packet with the
associated request packet's Request Authenticator in the Authenticator
field, concatenated with the shared secret.
ResponseAuth = MD5(Code+ID+Length+RequestAuth+Attributes+Secret)
where + denotes concatenation.
2.3 Client Post Processing
When the client receives a response packet, it attempts to match it
with an outstanding request using the identifier field. If the client
does not have an outstanding request using the same identifier, the
response is silently discarded. The client then verifies the Response
Authenticator by performing the same Response Authenticator
calculation the server performed, and then comparing the result with
the Authenticator field. If the Response Authenticator does not match,
the packet is silently discarded.
If the client received a verified Access-Accept packet, the username
and password are considered to be correct, and the user is
authenticated. If the client received a verified Access-Reject
message, the username and password are considered to be incorrect, and
the user is not authenticated.
3 RADIUS Issues
The RADIUS protocol has a set of vulnerabilities that are either
caused by the protocol or caused by poor client implementation and
exacerbated by the protocol. The vulnerabilities that follow arose
during a somewhat shallow exploration of the protocol; this is not
expected to be a complete list of vulnerabilities of the RADIUS
protocol, these are merely the vulnerabilities that presented
themselves to the reviewer.
3.1 Response Authenticator Based Shared Secret Attack
The Response Authenticator is essentially an ad hoc MD5 based keyed
hash. This primitive facilitates an attack on the shared secret. If an
attacker observes a valid Access-Request packet and the associated
Access-Accept or Access-Reject packet, they can launch an off-line
exhaustive attack on the shared secret. The attacker can pre-compute
the MD5 state for (Code+ID+Length+RequestAuth+Attributes) and then
resume the hash once for each shared secret guess. The ability to
pre-compute the leading sections of this keyed hash primitive reduces
the computational requirements for a successful attack.
3.2 User-Password Attribute Cipher Design Comments
The User-Password protection scheme is a stream-cipher, where an MD5
hash is used as an ad hoc pseudorandom number generator (PRNG). The
first 16 octets of the stream cipher display the same properties as a
synchronous stream cipher. After the first 16 octets, the stream
cipher state integrates the previous ciphertext, and becomes more
accurately described as a self-synchronizing stream cipher.
The security of the cipher rests on the strength of MD5 for this type
of use and the selection of the shared secret. It is unclear what the
requirements for this cipher are, so it is unclear if the MD5 function
is appropriate for this use. MD5 is not designed to be a stream cipher
primitive, it is designed to be a cryptographic hash. This sort of
misuse of cryptographic primitives often leads to subtly flawed
3.3 User-Password Attribute Based Shared Secret Attack
Because of the selection of a stream cipher for protection of
the User-Password attribute, an attacker can gain information
about the Shared Secret if they can observe network traffic and
attempt an authentication. The attacker attempts to authenticate to
the client with a known password. The attacker then captures the
resulting Access-Request packet and XORs the protected portion of
the User-Password attribute with the password they provided to the
client. This results in the value of the MD5(Shared Secret + Request
Authenticator) operation. The Request Authenticator is known (it is
in the client's Access-Request packet), so the attacker can launch an
off-line exhaustive attack on the shared secret. Note, though, that the
attacker cannot pre-compute the MD5 state of the hash for the Request
Authenticator, because the Request Authenticator is hashed second.
3.4 User-Password Based Password Attack
The use of a stream cipher to protect the User-Password attribute
results in a vulnerability that allows an attacker to circumvent
any authentication rate limits imposed by the client. The attacker
first attempts to authenticate to the client using a valid username
and a known (and likely incorrect) user password. The attacker then
captures the resulting Access-Request packet and determines the
result of the MD5(Shared Secret + Request Authenticator) operation
(in the same way as in the previous attack). The attacker can then
replay modified Access-Request packets, using the same Request
Authenticator and MD5(Shared Secret + Request Authenticator) value,
changing the password (and the associated User-Password attribute) for
each replay. If the server does not impose user based rate limits,
this will allow the attacker to efficiently perform an exhaustive
search for the correct user password.
Note that the attacker can only use this method to attack passwords
that are 16 characters or less, as the User-Password protection
mechanism uses a chaining method that includes previous ciphertext
in the state after the first 16 octets of output.
Any sort of strong data authentication in the Access-Request packet
would make this attack impossible.
3.5 Request Authenticator Based Attacks
The security of RADIUS depends on the generation of the Request
Authenticator field. The Request Authenticator must be both unique
and non-predictable in order for the RADIUS implementation to be
secure. The RADIUS protocol specification does not emphasize the
importance of the Request Authenticator generation, so there are a
large number of implementations that use poor PRNGs to generate the
Request Authenticator. If the client uses a PRNG that repeats values
(or has a short cycle), the protocol ceases to provide the intended
level of protection.
The last two of these attacks require the attacker to cause the
client to produce a particular identifier value. This is generally not
particularly difficult, as identifiers were never meant as a security
feature. The actual method of identifier generation is not specified by
the protocol specification, but the most common method of generating
the identifier is to increment a one octet counter for each request,
and include the counter value as the identifier. Because the identifier
generation is normally deterministic, it often doesn't increase the
work factor very much at all. An attacker can insert a series of extra
requests to the client, forcing the desired identifier to reoccur much
more rapidly than it would normally. Even if the identifier were not
generated in a readily attackable way, it would still only increase
the work factor by 256 times.
3.5.1 Passive User-Password Compromise Through Repeated Request
If the attacker can sniff the traffic between the RADIUS client
and the RADIUS server, they can passively produce a dictionary of
Request Authenticators, and the associated (protected) User-Password
attributes. If the attacker observes a repeated Request Authenticator,
they can remove any influence of the Shared Secret from the first 16
octets of the passwords by XORing the first 16 octets of the protected
passwords together. This yields the first 16 octets of the two (now
unprotected) user passwords XORed together.
The impact of this attack varies according to how good the user
passwords are. If the users all chose random passwords of the same
length, the attacker can gain nothing because no information about
either password can be extracted. Unfortunately, this is a somewhat
unlikely occurrence. In reality, users choose passwords of varying
lengths (generally less than 16 characters) and of varying quality.
The easiest problem for the attacker to exploit is the case where the
two passwords are of different lengths. Ideally for the attacker, the
passwords are both less than 16 characters long and are significantly
different lengths. In this situation, one of the passwords has more
padding than the other, so the non-overlapping characters of the
longer password are XORed with '0' (the characters do not change).
This results in the non-overlapping characters of the longer password
being exposed to the attacker with no analysis.
More complex attacks are available if the attacker makes the assumption
that the users chose low-entropy passwords. In this situation,
the attacker can perform an intelligent dictionary attack guided by
statistical analysis of the overlapping region. This dictionary attack
can be further refined by noting the length of the two passwords and
the trailing portion of the longer password, and then only trying
passwords with this length and ending.
Even passwords longer than 16 characters are at risk from this attack,
because the attacker still gains information about the first 16
characters of the password. This provides a firm basis for later
attack, if nothing else.
3.5.2 Active User-Password Compromise through Repeated Request
The attacker can attempt to authenticate many times using
known passwords and intercept the generated Access-Request
packets, extracting the Request Authenticator and User-Password
attributes. The Attacker can then XOR the known password with the
User-Password attribute and be left with the MD5(Shared Secret +
Request Authenticator) value. The attacker generates a dictionary
of Request Authenticator values and associated MD5(Shared Secret +
Request Authenticator) values.
When the attacker sees a valid Access-Request packet that has a Request
Authenticator value that is in the attacker's dictionary, the attacker
can recover the first 16 octets from the protected region of the
User-Password field by looking up the associated MD5(Shared Secret +
Request Authenticator) value from the dictionary and XORing it with
the intercepted protected portion of the User-Password attribute.
3.5.3 Replay of Server Responses through Repeated Request Authenticators
The attacker can build a dictionary of Request Authenticators,
identifiers and associated server responses. When the attacker then
sees a request that uses a Request Authenticator (and associated
identifier) that is in the dictionary, the attacker can masquerade
as the server and replay the previously observed server response.
Further, if the attacker can attempt to authenticate, causing the
client to produce an Access-Request packet with the same Request
Authenticator and identifier as a previously observed successful
authentication, the attacker can replay the valid looking Access-Accept
server response and successfully authenticate to the client without
knowing a valid password.
3.5.4 DOS Arising from the Prediction of the Request Authenticator
If the attacker can predict future values of the Request Authenticator,
the attacker can pose as the client and create a dictionary of future
Request Authenticator values (with either the expected identifier,
or with every possible identifier) and associated (presumably
Access-Reject) server responses. The attacker can then masquerade as
the server and respond to the client's (possibly valid) requests with
valid looking Access-Reject packets, creating a denial of service.
3.6 Shared Secret Hygiene
The RADIUS standard specifically permits use of the same Shared Secret
by many clients. This is a very bad idea, as it provides attackers
with more data to work from and allows any flawed client to compromise
several machines. All RADIUS clients that possesses the same shared
secret can be viewed as a single RADIUS client for the purpose of all
these attacks, because no RADIUS protection is applied to the client
or server address.
Most client and server implementations only allow shared secrets
to be input as ASCII strings. There are only 94 different ASCII
characters that can be entered from a standard US style keyboard
(out of the 256 possible). Many implementations also restrict the
total length of the shared secret to 16 characters or less. Both of
these restrictions artificially reduce the size of the keyspace that
an attacker must search in order to guess the shared secret.
4.1 Summary Findings
The RADIUS protocol has several interesting issues that arise from
its design. The design and policy characteristics that seem to be
principally responsible for the security problems are as follows:
* The User-Password protection technique is flawed in many ways. It
should not use a stream cipher, and it should not use MD5 as a
cipher primitive. (note 3.2; attacks 3.3, 3.4, 3.5.1, 3.5.2)
* The Response Authenticator is a good idea, but it is poorly
implemented. (attack 3.1)
* The Access-Request packet is not authenticated at all. (attack
* Many client implementations do not create Request Authenticators
that are sufficiently random. (all attacks in 3.5)
* Many administrators choose RADIUS shared secrets with insufficient
information entropy. Many client and host implementations
artificially limit the shared secret key space. (note 3.6)
4.2 Suggested Protocol Additions
Selection of a well understood symmetric block cipher to protect
the user password would be good practice. A new User-Password like
attribute that uses an alternate encryption scheme should be created.
I suggest TDES (as specified in ANSI X9.52) used in CBC mode. If this
new attribute is used, the User-Password attribute should not be.
Ideally the block cipher would be keyed independently from the shared
secret, but this may prove unworkable for compatibility reasons.
Another option would be to key the cipher from some derived value
of the shared secret and the request authenticator. For instance
the cipher could be keyed from the output of an HMAC of the Request
Authenticator (where the HMAC is keyed by the shared secret) or by
seeding a cryptographic PRNG with the shared secret and the request
Instead of using an ad hoc keyed hash primitive in the Response
Authenticator, an accepted Message Authentication Code (MAC) should be
used. An HMAC would be an ideal choice for this primitive. In addition,
the Access-Request packet would benefit from authentication.
Though MD5 is a cryptographic hash that could be used in the HMAC
primitive, it has several significant attacks against it. The RADIUS
protocol would benefit from using SHA-1 instead of MD5 for HMACs.
In order to protect the Access-Request, Access-Accept and Access-Deny
packets, a new attribute should be created that contains a SHA-1-HMAC
of the entire RADIUS packet (with the SHA-1-HMAC attribute data set
to 0). If this attribute is present, the receiving client or server
should compute the HMAC for the entire RADIUS packet (with the HMAC
set to zeros) and verify that the result is the same as the stored
HMAC. If the result is not the same, the packet should be discarded.
When the server generates a RADIUS Access-Accept or Access-Reject
packet with a SHA-1-HMAC, it should set the Response Authenticator
to the associated Request Authenticator. If a client receives a
RADIUS Access-Accept or Access-Reject packet that has the SHA-1-HMAC
attribute, it should not test for the validity of the Response
When a client generates a RADIUS Access-Request packet, it should
include the SHA-1-HMAC attribute. When the server receives a RADIUS
Access-Request packet, it should verify the SHA-1-HMAC attribute.
4.3 Suggested Client Behavior Modifications
The RADIUS specification should require a strong cryptographic PRNG
for generation of the Request-Authenticator, such as the PRNG specified
in ANSI X9.17 appendix C or FIPS 186-2, appendix 3.
Shared Secret Behavior
The RADIUS specification should require each RADIUS client use a
different Shared Secret. It should also require the shared secret to
be a random bit string at least 16 octets long that was generated by
a strong cryptographic PRNG.
In order to facilitate entry of this bit string, clients and servers
should allow for input of arbitrary binary data. Quite likely, the
easiest solution is to allow for the entry of hexadecimal digits.
4.4 General Comments
Both servers and clients should support the base RADIUS protocol and
this extended RADIUS protocol. Both the server and the client should
allow the administrator to enable the use of these RADIUS extensions
on a client-by-client basis. This should be an explicit configuration
option, not just an automatic determination made by the server. An
automatic determination made by the server could lead to an attack
where the attacker attempts to force the client/server interactions
into the old RADIUS mode.
If it is not possible to change the RADIUS protocol, the system can
still be made much more secure by just following the suggestions
in section 5.3, which can all be implemented while still remaining
completely compliant with the existing RADIUS protocol.
4.5 Why Modify RADIUS?
So, why attempt to modify RADIUS at all? Why not just go to another
(presumably more modern, more secure) protocol? Well, for the most
part, the answer is "Because such a protocol doesn't currently exist."
In the near future, however, DIAMETER is likely to be released by
DIAMETER is the planned RADIUS replacement. The great majority
of all the protocol work that has gone into DIAMETER has been
directed to removing some of the functional limitations imposed by
the RADIUS protocol. Effectively no work has been done as relates
to the client/server security of the protocol. (CMS is defined,
but this is a security layer for the proxy to proxy interaction,
not the client to proxy/server interaction)
So, does this mean that they continue to use even RADIUS's ad hoc
system? No, they removed all security functionality from the protocol.
They did the protocol designer's equivalent of punting. Section 2.2
of the current DIAMETER protocol spec says: "Diameter clients, such
as Network Access Servers (NASes) and Foreign Agents MUST support IP
Security, and MAY support TLS. Diameter servers MUST support TLS,
but the administrator MAY opt to configure IPSec instead of using
TLS. Operating the Diameter protocol without any security mechanism
is not recommended."
So, all security aspects of the protocol are handled by IPSec and/or
TLS. From a security aspect, this strikes me as a very good idea. Both
IPSec and TLS are fully featured (sometimes too fully featured)
protocols that many people have reviewed. (That's already much better
than RADIUS ever did).
Examining this from a slightly different angle gives me some cause for
concern, however. It strikes me that the overhead imposed by a full
TLS/IPSec implementation is very significant for many current-day
embedded devices. This would seem to indicate that (at least in the
near future) manufactures are going to either continue to use RADIUS or
ignore the DIAMETER standard and perform DIAMETER without TLS or IPSec.
Because of this, I suspect that it would be advantageous to push for
at least minimal RADIUS protocol revision.
5 Previous Work
There has been some independent previous work with the RADIUS protocol:
Attacks 3.5.3 and 3.5.4 are likely the attacks referred to in the
The known password attack on the shared secret using the
Access-Request packet (attack 3.3) appears to have been first
observed in September, 1996 by Thomas H. Ptacek. Paper #1
The known password attack on the shared secret using the
Access-Request packet (attack 3.3), and the shared secret
attack on the Access-Reject and Access-Accept packets (attack
3.1) were independently observed in July, 1997 by Reilly
(rich.friedeman () ANIXTER COM) Shared Secret Recovery in RADIUS
RFC 2138, "Remote Authentication Dial In User Service
(RADIUS)", by C. Rigney, A. Rubens, W. Simpson, S. Willens.
The DIAMETER Base Protocol
DIAMETER CMS Security Application
The Handbook of Applied Cryptography, by Alfred J Menezes, Paul C. van
Ooschot, Scott A. Vanstone.
Chapter 5, chapter 6 and chapter 9. Most notably:
The MD5 based stream cipher as a synchronous stream cipher (6.1.1, ii)
The use of cryptographic functions in pseudorandom number generation
is discussed in section 9.2.6.
The use of a MDC in the creation of a MAC is discussed in 9.5.2.
An Analysis of the TACACS+ Protocol and its Implementations by Solar
- An Analysis of the RADIUS Authentication Protocol Joshua Hill (Nov 13)