A post on Stack Overflow seeks advice on the relative security between implementing a password reset mechanism that emails a temporary link vs. one that emails a temporary password. Stack Overflow questions typically attract high quality answers, which is a testament to the site’s knowledgeable readership and reputation system. Responses to this particular post didn’t fail.
Rather than retread the answers, let’s consider the underlying implication behind the question: it can be acceptable to send passwords via email.
Don’t do it. Don’t give users the expectation that passwords might be sent via email. Doing so is unnecessary and establishes a dangerous practice as possibly acceptable. If the site ever sends a password to a user, then the user might be conditioned to communicate in the other direction. An email purportedly from the site’s administrators might ask for the user’s password to “verify the account is active”, “prevent the account from being terminated”, or “restore information”. The email need not ask for a direct response. It may be an indirect prompt for the password using cautionary, concerned language that never the less sends the victim to a spoofed login page.
Site administrators will not elicit passwords via email or any other means. Sites won’t ask for passwords except at authentication barriers. Sites shouldn’t send passwords – however temporary they may be.
“Newt. My name’s Newt.”
Passwords – shared secrets ostensibly known only to the user and the web site – are a necessary evil of authentication. The password proves a user’s identity to a web site under the assumption that only the person who logs in as Newt, for example, knows the corresponding password to the account is rebecca.
A password’s importance to web security has a frustratingly indirect relationship to the amount of control a site is able to exert over its protection. Users must be trusted not only to use hard-to-guess words or phrases, but must be trusted not to share the password or otherwise actively divulge it through accident or ignorance. Only the luckiest of potential victims will click an emailed link and be brought to a friendly page:
“That’s it man, game over man, game over!”
Password reuse among sites poses another serious risk. The theft of a user’s credentials (email address and password) for a site like www.downloadwarez.free might seen relatively innocuous, but what if they’re the same credentials used for allof.my.friends or www.mostly.secure.bank? Even worse, what if the password matches the one for the Gmail, Hotmail, or Yahoo account? In that case
Nuke the entire site from orbit. It’s the only way to be sure.
“Seventeen days? Hey man, I don’t wanna rain on your parade, but we’re not gonna last seventeen hours!”
Use random, temporary links for password reset mechanisms via email. By this point you should be sufficiently warned off of the alternative of emailing temporary passwords. An obstinate few might choose to stick with passwords, arguing that there’s no difference between the two once they hit the unencrypted realm of email.
Instead, use a forward-thinking solution that tries to secure the entire reset process:
- Maintain the original password until the user changes it. Perhaps they’ll remember it anyway. This also prevent a mini-denial of service where attackers force users to change their passwords.
- Use a strong pseudo-random number generator to minimize the chance of success of brute force attacks guessing the link.
- Expire the link after a short period of time, perhaps in minutes or hours. This narrows the window of opportunity for brute force attacks that attempt to guess links. It’s no good to let an attacker initiate a password reset on a Friday evening and have a complete weekend to brute force links until the victim notices the email on a Monday morning.
“Maybe we got ‘em demoralized.”
For those of you out there who visit web sites rather than create them there’s one rule you should never ignore. If the site’s password recovery mechanism emails you the original password for your account, then stop using the site. Period. Full stop. Should you be forced to use the site under threat of violence or peer pressure, whichever is worse, at the very least choose a password that isn’t shared with any other account. Storing unencrypted passwords is lazy, bad security design, and a liability.
Quotes from the movie Aliens.
If you didn’t recognize them, go watch it now. It’s worth it. See you in about two hours.• • •
My book starts off with a discussion of cross-site scripting (XSS) attacks along with examples from 2009 that illustrate the simplicity of these attacks and the significant impact they can have. What’s astonishing is how little many of the attacks have changed.
Consider the following example, over a decade old, of HTML injection before the term XSS became so ubiquitous. The exploit also appeared about two years before the blanket CERT advisory that called attention to the insecurity of unchecked HTML (CA-2000-02)1.
We have just found a serious security hole in Microsoft’s Hotmail service (https://www.hotmail.com/) which allows malicious users to easily steal the passwords of Hotmail users.
The discoverers flouted the 90s trend to name vulns based on expletives or num3r1c characters and dubbed it simply the “Hot”Mail Exploit.
Disclosures of that era also tended to include greetz, typos, and self-aggrandizement about the hacker’s near-omnipotent skills. This disclosure skipped those aspects. However, the demo site satisfied an axiom of hacking culture by using a hacker handle that referenced pop culture, Blue Adept, a fantasy novel by Piers Anthony.
The attack required two steps. First, they set up a page on Geocities (a hosting service for web pages distinguished by being free before free was subsumed by the Web 2.0 label) that spoofed Hotmail’s login.
The attack wasn’t particularly sophisticated and it didn’t need to be. The login form collected the victim’s credentials and IP address, then mailed them to the newly-created Geocities account.
The second step involved executing the exploit against Hotmail by sending an email with HTML that contained a rather curious
imgtag. (Whitespace added for readability of the long, double-quoted string.):
Modern attacks might have more sophisticated obfuscation techniques and use tags other than the
imgelement, but it’s otherwise hard to distinguish what decade this payload is from.
What’s also entertaining is how timeless the “serious security concern” is from the original disclosure. They list four points, which I’ve only edited for a typo:
- The malicious code runs as soon as e-mail message is viewed
- The resources required to launch the attack are minimal and freely available.
- The malicious e-mail can be sent from virtually anywhere, including libraries, internet cafes, or classroom terminals
The reference to internet cafes gives away the era, but otherwise this description from 1998 sounds like it could be from today. (I also have an example from 1996.)
The problem of HTML injection, well known for over 10 years, remains a common vuln.
(Another edit from the future: XSS still remains a common vuln 25 years after this disclosure. Although I’m optimistic about its decline due to frameworks like React.)
• • •
To this day I dislike the framing of XSS as an input validation issue. Accept any characters you want – that’s not the issue! The flaw is output encoding and not handling those characters correctly for the context in which they’re being rendered. ↩
Sit and listen to Pink Floyd’s album, Wish You Were Here.
Can you tell a green field from a cold steel rail?
Yes? Could you tell a buffer overflow from a valid username in a web app? Yes again. What about SQL injection, cross-site scripting, directory traversal attacks, or appending “.bak” to every file? Once again: Yes.
Many of these attacks have common signatures that could be thrown into Snort or passed through a simple grep command when examining log files. These are the vulns that are reported most often on sites like www.cgisecurity.com or www.securityfocus.com. They pop up for good reason – they’re dangerous and can undermine an app.
On the other hand, a different category of attacks has not yet crept far enough into the awareness of security admins and developers – session attacks. There are several reasons for this relative obscurity. No automated tool does a proper job of identifying session vulns without customization to the app. There are no defined signatures to try, as opposed to the simpler single quote insertion for a SQL injection test. Most importantly, session state vulns vary significantly between apps.
The consequence of session attacks is no less severe than a good SQL injection that hits
xp_cmdshell. Consider last May’s Passport advisory in which an attacker could harvest passwords with tools no more sophisticated than a browser. The attack relies on selecting proper values for the
em(victim’s email) and
prefem(attacker’s email) parameters to the
emailpwdreset.srfpage. It should really be stressed that nowhere in this attack were invalid characters sent to the server. Both parameters were attacked, but the payload contained valid email addresses -– not email addresses with single quotes, long strings, Unicode encoded characters, or other nefarious items.
The attack succeeded for two major reasons. The email addresses are exposed to the client, rather than tracked on the server in a session object. The four steps ostensibly required for a user to complete the password reminder process did not have to be performed in sequential order, which demonstrates lack of a strong state mechanism.
It seems that most buffer overflow-style attacks against operating systems or Internet servers such as IIS or Apache have led to site defacements or denial of service attacks. Yet as e-commerce has grown, so have the vulns moved on from buffer overflows. Plus, attacks no longer just stick to site defacement, but have moved into identity or financial theft. Attacks that used to primarily affect only the site’s owners (and perhaps annoy users who can’t access the site for a while) now include more that directly affect users.
As admins and devs, we need to be aware of all the vulns that crop up in apps -– not just the easy (yet still important!) ones that currently populate the web vuln encyclopedia.
This was originally posted in July 2003 on the now-defunct vulns.com. Even several years later when I re-published it on the blog in 2008, no web app scanner could automatically identify such vulns in a reliable, accurate manner. Now 20 years later in 2023, many vulns still require human analysis.• • •