mailing list archives
Password Recovery (long) was Re: "Forgot Password" function
From: Charles Miller <cmiller () pastiche org>
Date: Sat, 19 Oct 2002 21:42:32 +1000
On Saturday, October 19, 2002, at 07:44 AM, Brecrost Jones wrote:
First off, thanks to everyone for their responses!
The above outlines perfectly the issue I am worried about, and that is
the inherent insecurity of any email-based solution.
This is the start of a more formal examination of the topic. A lot of
it is reiterating things from other emails, but I thought I'd collect
them all together.
Work in progress:
By: Charles Miller, acknowledging input from David Bullock, Kevin
Spett, Haroon Meer, Jeroen Latour, Chris Shepherd, Bill Smith and
Password recovery becomes necessary when the user of a system is no
able to authenticate themselves because they have lost or forgotten
password. Any systems that require authentication will need to have some
policy or procedure for password recovery.
The simplest policy is: "Password recovery will not be performed." When
combined with a policy that locks inactive accounts, it is also the
most secure approach. Users who lose their password are required to
repeat the registration process and acquire a new identity. The user
will lose their previous identity and any saved information, and the
cost of annoying your more careless users must be offset against any
benefits in security and simplicity.
If the "no recovery" policy is impossible, then there are two forms of
recovery mechanism that may be implemented in some combination:
o Communicating a secret with the user over some pre-arranged
channel, and having them use that to re-authenticate.
o Providing a secondary means of authentication for the purpose of
recovering or resetting the first.
In a controlled environment, the second is the most popular. In a
corporate environment, the user presents themselves to their manager or
IT department, who verifies they are who they say they are, and resets
their password. (Of course there's also the "secure channel" of how the
IT department communicates the new password to the user, but that's
trivial once the authentication has occurred.)
I will assume here that the password recovery is being applied in a
situation where the users of the system are customers or clients, and
can not be readily identified in this manner.
o When recovering a password, always assign, or require the user to
choose a new password.
- You shouldn't be storing unhashed passwords anywhere anyway
- If the recovery procedure is compromised, the real user will
be unable to log in. This makes it easier to detect fraud.
o (obviously) log all recovery attempts
o Only allow recovery once within a particular time-period (three
months) Subsequent recovery attempts will result in a direction
to call customer service, where a rep will manually perform the
- When the user calls, the rep can guarantee that the previous
recovery attempt was genuine.
- It's an opportunity to educate the user.
Things to Consider:
o Is the process automated, or does it require human intervention?
o If the process requires human contact, are you ready to deal with
users from other countries, or who do not speak the same language
o To what degree can you inconvenience your users before they decide
not to bother? Is it a problem if they do?
o What steps can you take to make sure the users don't forget their
passwords in the first place?
Obviously, the best thing to do is to have the user physically present
themselves to you with 100 points worth of photographic identification.
This is a form of secondary-authentication recovery, and is very
secure. Like all optimal solutions, this is generally impossible in a
web environment, and is a particular inconvenience to users.
That said, if you need security, and you have the luxury of physical
offices near the vast majority of your customers (I'm thinking of banks
and government departments here), this is worth thinking about.
o Best, legally defensible security.
o In case of fraud, you might remember what they look like
o Requires human intervention in all cases.
o Requires user to take time out (during business hours) to do the
o May be difficult or impossible dependent on user's distance from
Another form of secondary authentication, the user does not present
themselves, but transmits a facsimile of some kind of official
identification such as a passport or drivers license.
o Keep a copy of the identification on file from signup-time, to be
compared in the event of recovery.
- ID documents look different from state to state and country
to country. Unless you have some idea what it _should_ look
like, you can easily be fooled by a competent graphic artist.
o So long as the identification is on file, this is pretty secure,
an attacker must be a competent forger, and know the correct
numbers for the documents. If not, this is easily defeated by a
o Requires human intervention in all cases.
o User must have access to a fax machine.
o Digital forgery is possible, and masked by the quality of the fax.
o Identity documents are replaced or renewed, so some secure
for replacing the filed ID is necessary.
o Does not cover how to get the new password to the user after
identified themselves over the fax.
e-mailing the password to the user's registered address is the most
common form of recovery on public websites. It is a weak form of
"secure channel" recovery, where it is assumed the path between the
server and the user's email client is secure. Of course, this
assumption isn't particularly defensible, but the process requires
almost no human intervention, and is "secure enough" for most
o Send the user a one-time password, based on the user's password
a timestamp. That way, the recovery password can only be used
main password is changed, and can be expired if not used for 24
- The email does not become a security problem if it's archived.
o highly automated
o familiar, and very easy for the user to understand
o Attacks rely on compromising some part of the communications chain
between server and user. Thus, targetting an attack against a
individual requires some effort.
o vulnerable at every point that the email could be intercepted
o users may not understand that by sharing their email account with
family members, they are making passwords available
o susceptible to opportunistic attacks - once an attacker has
any mailserver, they can methodically break the accounts of all
of that server.
o if the mailserver or gateway from which the recovery mails
compromised, it's game over for everybody.
A secure channel method, sending an email encrypted with some secret
only known to the customer is possible, but is sufficiently impractical
that it only deserves one sentence here.
Question and Answer
Another form of secondary authentication. When registering the account,
the user records some personal information. To recover the password,
the user must answer questions based on this personal information,
either to a web page, or over the phone to a representative.
The questions and answers become a shared secret, a form of secondary
password. Remember how we warn people not to base their passwords on
their names, pets names or dates of birth? The security of this
technique varies based on the choice of questions: the ease of an
attacker guessing the questions, and the ease of researching the
answers. It's amazing how many institutions believe that your date of
birth and your mother's maiden name are sufficiently obscure to protect
your bank account.
One implementation would be to request the information that was
supplied during registration, such as address, phone number and
credit-card details. This sort of information may, however, be quite
easy for an attacker to come across or socially engineer.
Properly managed, this technique is more secure than e-mailing the
password in the case of generalised or opportunistic attacks. However,
due to its susceptibility to research, it is less secure against
o Have a large pool of questions, of which the user only has to
answer a subset during signup/recovery.
- The more questions in the pool, the more research an attacker
- On the other hand, having a set of 50 personal questions in the
sign-up process will scare people away.
o Some systems allow the user to choose the questions as well. This
is a bad idea, as users don't understand security, and will
things too easy for an attacker to guess, or too hard for
to work out what the hell they were thinking, six months hence.
o If the process is not automated, and for some insane reason you
have the original password on file, "What did you think the
was?" can be an effective question.
o Less susceptible to opportunistic attacks than mailed passwords.
o Still able to be automated, or performed by untrained employees
following scripts and exercising next to no personal judgement.
o Hard to come up with a good set of questions.
o The better the attacker knows the target, the less secure it is.
Unless the questions got _very_ personal, my brother could
answer 90% of them for me.
o Users may consider personal questions to be an intrusion.
o The answers may not be easy to match automatically, leading to
Another secure channel method. The user makes a request for a new
password, and some human being from the company calls the user on their
registered phone number, and delivers the new password over the phone.
This method can be combined with "Question and Answer" for a pretty
effective recovery system. It mostly limits attacks to friends and
family (who have access to the telephone and the personal information),
and since the time of the call is logged and the attacker has to talk
to the representative, it's easier to trace fraud.
o When combined with Question and Answer, is probably the most
effective method after faxed identification.
o Company bears the cost of phone calls, which might be difficult
have international users.
o Quite resource-intensive.