Certified Malice

One unfortunate (albeit entirely predictable) consequence of making HTTPS certificates “fast, open, automated, and free” is that both good guys and bad guys alike will take advantage of the offer and obtain HTTPS certificates for their websites.

Today’s bad guys can easily turn a run-of-the-mill phishing spoof:

HTTP Phish screenshot

…into a somewhat more convincing version, by obtaining a free “domain validated” certificate and lighting up the green lock icon in the browser’s address bar:

HTTPS Phish screenshot Phishing site on Android

The resulting phishing site looks almost identical to the real site:

Real and fake side-by-side

By December 8, 2016, LetsEncrypt had issued 409 certificates containing “Paypal” in the hostname; that number is up to 709 as of this morning. Other targets include BankOfAmerica (14 certificates), Apple, Amazon, American Express, Chase Bank, Microsoft, Google, and many other major brands. LetsEncrypt validates only that (at one point in time) the certificate applicant can publish on the target domain. The CA also grudgingly checks with the SafeBrowsing service to see if the target domain has already been blocked as malicious, although they “disagree” that this should be their responsibility. LetsEncrypt’s short position paper is worth a read; many reasonable people agree with it.

The “race to the bottom” in validation performed by CAs before issuing certificates is what led the IE team to spearhead the development of Extended Validation certificates over a decade ago. The hope was that, by putting the CAs name “on the line” (literally, the address line), CAs would be incentivized to do a thorough job vetting the identity of a site owner. Alas, my proposal that we prominently display the CAs name for all types (EV, OV, DV) of certificate wasn’t implemented, so domain validated certificates are largely anonymous commodities unless a user goes through the cumbersome process of manually inspecting a site’s certificates. For a number of reasons (to be explored in a future post), EV certificates never really took off.

Of course, certificate abuse isn’t limited to LetsEncrypt—other CAs have also issued domain-validated certificates to phishing sites as well:

comodophish

Who’s Responsible?

Unfortunately, ownership of this mess is diffuse, and I’ve yet to encounter any sign like this:

Blame the Browser

The core proposition held by some (but not all) CAs is that combatting malicious sites is the responsibility of the user-agent (browser), not the certificate authority. It’s an argument with merit, especially in a world where we truly want encryption for all sites, not just the top sites.

That position is bolstered by the fact that some browsers don’t actively check for certificate revocation, so even if LetsEncrypt were to revoke a certificate, the browser wouldn’t even notice.

Another argument is that browsers overpromise the safety of sites by using terms like Secure in the UI—while the browser can know whether a given HTTPS connection is present and free of errors, it has no knowledge of the security of the destination site or CDN, nor its business practices. Internet Explorer’s HTTPS UX used to have a helpful “Should I trust this site?” link, but that content went away at some point. Security wording is a complicated topic because what the user really wants to know (“Is this safe?”) isn’t something a browser can ever really answer in the affirmative. Users tend to be annoyed when you tell them only the truth– “This download was not reported as not safe.”

The obvious way to address malicious sites is via phishing and malware blocklists, and indeed, you can help keep other users safe by reporting any unblocked phish you find to the Safe Browsing service; this service protects Chrome, Firefox, and Safari users. You can also forward phishing messages to scam@netcraft.com and/or PhishTank. Users of Microsoft browsers can report unblocked phish to SmartScreen (in IE, click Tools > SmartScreen > Report Unsafe Website). Known-malicious sites will get the UI treatment they deserve:

Unfortunately, there’s always latency in block lists, and a phisher can probably turn a profit with a site that’s live less than one hour. Phishers also have a whole bag of tricks to delay blocks, including cloaking whereby they return an innocuous “Site not found” message when they detect that they’re being loaded by security researchers’ IP addresses, browser types, OS languages, etc.

Blame the Websites

Some argue that websites are at fault, for:

Relying upon passwords and failing to adopt unspoofable two-factor authentication schemes which have existed for decades
Failing to adopt HTTPS or deploy it properly until browsers started bringing out the UI sledgehammers
Constantly changing domain names and login UIs
Emailing users non-secure links to redirector sites
Providing bad security advice to users

Blame the Humans

Finally, many lay blame with the user, arguing user education is the only path forward. I’ve long given up much hope on that front—the best we can hope for is raising enough awareness that some users will contribute feedback into more effective systems like automated phishing block lists.

We’ve had literally decades of sites and “experts” telling users to “Look for the lock!” when deciding whether a site is to be trusted. Even today we have bad advice being advanced by security experts who should know better, like this message from the Twitter security team which suggests that https://twitter.com.access.info is a legitimate site.

Email from Twitter Security Team

Where Do We Go From Here?

Unfortunately, I don’t think there are any silver bullets, but I also think that unsolvable problems are the most interesting ones. I’d argue that everyone who uses or builds the web bears some responsibility for making it safer for all, and each should try to find ways to do that within their own sphere of control.

Following is an unordered set of ideas that I think are worthwhile.

Signals

Historically, we in the world of computers have been most comfortable with binary – is this site malicious, or is it not? Unfortunately, this isn’t how the real world usually works, and fortunately many systems are moving more toward a set of signals. When evaluating the trustworthiness of a site, there are dozens of available signals, including:

HTTPS Certificates
Age of the site
Has the user visited this site before
Has the user stored a password on this site before
Has the user used this password before, anywhere
Number of visitors observed to the site
Hosting location of the site
Presence of sensitive terms in the content or URL
Presence of login forms or executable downloads

None of these signals is individually sufficient to determine whether a site is good or evil, but combined together they can be used to feed systems that make good sites look safer and evil sites more suspect. Suspicious sites can be escalated for human analysis, either by security experts or even by crowd-sourced directed questioning of ordinary users. For instance, see the heuristic-triggered Is This Phish? UI from Internet Explorer’s SmartScreen system:

The user’s response to the prompt is itself a signal that feeds into the system, and allows more speedy resolution of the site as good or bad and subject to blocking.

One challenge with systems based on signals is that, while they grow much more powerful as more endpoints report signals, those signal reports may have privacy implications for individual users.

Reputation

In the real world, many things are based on reputation—without a good reputation, it’s hard to get a job, stay in business, or even find a partner.

In browsers, we have a well-established concept of bad reputation (your site or download appears on a block list) but we’ve largely pushed back against the notion of good reputation. In my mind, this is a critical failing—while much of the objection is well-meaning (“We want a level playing field for everyone”), it’s extremely frustrating that we punish users in support of abstract ideals. Extended Validation certificates were one attempt at creating the notion of good reputation, but they suffered from the whims of CAs’ business plans and the legal system that drove absurdities like:

While there are merits to the fact that, on the web, no one can tell whether your site is a one-woman startup or a Fortune 100 behemoth, there are definite downsides as well.

Owen Campbell-Moore wrote a wonderful paper, Rethinking URL bars as primary UI, in which he proposes a hypothetical origin-to-local-brand mapping, which would allow browsers to help users more easily understand where they really are when interacting with top sites. I think it’s a long-overdue idea brilliant in its obviousness. When it eventually gets built (by Apple, Microsoft, Google, or some upstart?) we’ll wonder what took us so long. A key tradeoff in making such a system practical is that designers must necessarily focus on the “head” of the web (say, the most popular 1000 websites globally)– trying to generalize the system for a perfectly level playing field isn’t practical– just like in the real world.

Assorted

Browsers could again make a run at supporting unspoofable authentication methods natively.
Browsers could do more to take authentication out from the Zone of Death, limiting spoofing attacks.
New features like Must-Staple will allow browsers to respond to certificate revocation information without a performance penalty.
Automated CAs could deploy heuristics that require additional validation for certificates containing often-spoofed domains.For instance, if I want a certificate for paypal-payments.com, they could either demand additional information about me personally (allowing a visit from Law Enforcement if I turn out to be a phisher) or they could even just issue a certificate with a validity period a week in the future. The certificate would be recorded in CertificateTransparency logs immediately, allowing brand monitoring firms to take note and respond immediately if phishing is detected when the certificate became valid. Such systems will be imperfect (you need to handle BankOfTheVVest.com as a potential spoof of BankOfTheWest.com, for instance) but even targeting a few high-value domains could make a major dent.

Fight the phish!

-Eric

Update: Chris wrote a great post on HTTPS UX in Chrome.

The Line of Death

When building applications that display untrusted content, security designers have a major problem— if an attacker has full control of a block of pixels, he can make those pixels look like anything he wants, including the UI of the application itself. He can then induce the user to undertake an unsafe action, and a user will be none the wiser.

In web browsers, the browser itself usually controls the top of the window, while pixels under the top are under control of the site. I’ve recently heard this called the line of death:

Line of death below omnibox — The dotted red line delineates the line of death

If a user trusts pixels above the line of death, the thinking goes, they’ll be safe, but if they can be convinced to trust the pixels below the line, they’re gonna die.

Unfortunately, this crucial demarcation isn’t explicitly pointed out to the user, and even more unfortunately, it’s not an absolute.

For instance, because the area above the LoD is so small, sometimes more space is needed to display trusted UI. Chrome attempts to resolve this by showing a little chevron that crosses the LoD:

…because untrusted markup cannot cross the LoD. Unfortunately, as you can see in the screenshot, the treatment is inconsistent; in the PageInfo flyout, the chevron points to the bottom of the lock and the PageInfo box overlaps the LoD, while in the Permission flyout the chevron points to the bottom of the omnibox and the Permission box only abuts the LoD. Sometimes, the chevron is omitted, as in the case of Authentication dialogs, and as of Chrome 70, the chevron appears to have been removed entirely for all UI.

Alas, even when it was in Chrome, the chevron is subtle, and I expect most users will fall for a faked chevron, like some sites have started to use¹:

The bigger problem is that some attacker data is allowed above the LoD; while trusting the content below the LoD will kill your security, there are also areas of death above the line. A more accurate Zones of Death map might look like this:

In Zone 1, the attacker’s chosen icon and page title are shown. This information is controlled fully by the attacker and thus may consist entirely of deceptive content and lies.

In Zone 2, the attacker’s domain name is shown. Some information security pros will argue that this is the only “trustworthy” component of the URL, insofar as if the URL is HTTPS then the domain correctly identifies the site to which you’re connected. Unfortunately, your idea of trustworthy might be different than the experts’; https://paypal-account.com/ may really be the domain you loaded, but it has no relationship with the legitimate payment service found at https://paypal.com.

The path component of the URL in Zone 3 is fully untrustworthy; the URL http://account-update.com/paypal.com/ has nothing to do with Paypal either, and while spoofing here is less convincing, it also may be harder for the good guys to block because the spoofing content is not found in DNS nor does it create any records in Certificate Transparency logs.

Zone 4 is the web content area. Nothing in this area is to be believed. Unfortunately, on windowed operating systems, this is worse than it sounds, because it creates the possibility of picture-in-picture attacks, where an entire browser window, including its trusted pixels, can be faked:

Paypal window is fake content from evil.example.com

When hearing of picture-in-picture attacks, many people immediately brainstorm defenses; many related to personalization. For instance, if you run your OS or browser with a custom theme, the thinking goes, you won’t be fooled. Unfortunately, there’s evidence that that just isn’t the case.

Story time

Back in 2007 as the IE team was launching Extended Validation (EV) certificates, Microsoft Research was publishing a paper calling into question their effectiveness. A Fortune 500 financial company came to visit the IE team as they evaluated whether they wanted to go into the EV Certificate Authority business. They were excited about the prospect (as were we, since they were a well-known-name with natural synergies) but they noted that they thought the picture-in-picture problem was a fatal flaw.

I was defensive– “It’s interesting,” I conceded, “but I don’t think it’s a very plausible attack.“

They retorted “Well, we passed this screenshot around our entire information security department, and nobody could tell it’s a picture-in-picture attack. Can you?” they slid an 8.5×11 color print across the table.

“Of course!” I said, immediately relieved. I quickly grew gravely depressed as I realized the implications of the fact that they couldn’t tell the difference.

“How?” they demanded.

“It’s a picture of an IE7 browser running on Windows Vista in the transparent Aero Glass theme with a page containing a JPEG of an IE7 browser running on Windows XP in the Luna-aka-Fisher Price theme?” I pointed out.

“Oh. Huh.” they remarked as they squinted and reexamined the picture.

My thoughts of using browser personalization as an effective mitigation died that day.

Other mitigations were proposed; one CA built an extension where hovering over the EV Lock Icon (“Trust Badge”) would dim the entire screen except for the badge. One team proposed using image analysis to scan the current webpage for anything that looked like a fake EV badge.

Personally, my favorite approach was Tyler Close’s idea that the browser should use PetNames for site identity– think of them as a Gravatar icon for salted certificate hashes– not only would they make every HTTPS site’s identity look unique to each user, but this could also be used as a means of detecting fraudulent or misissued certificates (in a world before we had certificate transparency).

The Future is Here … and It’s Worse

HTML5 adds a Fullscreen API, which means the Zone of Death looks like this: zodfullscreen

The Metro/Immersive/Modern mode of Internet Explorer in Windows 8 suffered from the same problem; because it was designed with a philosophy of “content over chrome”, there were no reliable trustworthy pixels. I begged for a persistent trustbadge to adorn the bottom-right of the screen (showing a security origin and a lock) but was overruled. One enterprising security tester in Windows made a visually-perfect spoofing site of Paypal, where even the user gestures that displayed the ephemeral browser UI were intercepted and fake indicators were shown. It was terrifying stuff, mitigated only by the hope that no one would use the new mode. (Example POC)

Virtually all mobile operating systems suffer from the same issue– due to UI space constraints, there are no trustworthy pixels, allowing any application to spoof another application or the operating system itself. Historically, some operating systems have attempted to mitigate the problem by introducing a secure user gesture (on Windows, it’s Ctrl+Alt+Delete) that always shows trusted UI, but such measures tend to confuse users (limiting their effectiveness) and often get “optimized away” when the UX team’s designers get ahold of the product.

It will be interesting to see how WebVR tries to address this problem on an even grander scale.

Update: PWA Window Controls Overlay

As of Microsoft Edge version 105, Web Applications that are installed as Progressive Web Applications can utilize a new feature whereby the app’s author controls nearly every pixel of the window, much like a traditional native code app:

You can read more about this capability in the Edge team’s Window Overlay Controls blog post. From an attacker’s point-of-view, this new PWA capability has a pretty high barrier to entry– to abuse it, you must already have convinced the user to “install” your malicious app, and if you could do that, you likely could’ve gotten them to undertake other dangerous operations instead.

Beyond Browsers

Of course, other applications have the concept of a LoD as well, including web applications. Unfortunately, they often get this wrong. Consider Outlook.com’s rendering of an email:

When Outlook has received an email from a trusted sender, they notify the user via a “This message is from a trusted sender.” notice. Which appears directly inside a Zone of Death:

Enterprising phishers have taken advantage of this and generate their own fake “trusted sender” notifications atop their phishing content. Similar attacks exist against email-signing mechanisms.

Security UI is hard.

December 2022 Update: Chrome Security UX boss Emily Stark provides thoughts on this topic in her blog post.

-Eric

¹ “Why would they fake a permission prompt? What would they gain?” you ask? Because for a real permission prompt, if you click Block, the browser’s restrictions mean that the site can never ask you again, while with a fake prompt, they can spam you as often as they like. As soon as you click Allow in the fake prompt, a scammer will immediately cause the browser to present the real prompt.

Client Certificates on Android

Recently, this interesting tidbit crossed my Twitter feed:

Sure enough, if you visited the site in Chrome, you’d get a baffling prompt.

My hometown newspaper shows the same thing:

No Certificates Found prompt on Android

Weird, huh?

Client certificates are a way for a browser to supply a certificate to the server to verify the client’s identity (in the typical case, a HTTPS server only sends its certificate so that the client can validate that the server is what it says it is.

In the bad old days of IE6 and IE7, the default behavior was to show a similar prompt, but what’s going on with the latest Chrome on modern Android?

It turns out that this is a limitation of the Android security model, to which Chrome on Android is subject. In order for Chrome to interact with the system’s certificate store, the operating system itself shows a security prompt.

If your server has been configured to accept client certificates (in either require or want mode), you should be sure to test it on Android devices to verify that it behaves as you expect for your visitors (most of whom likely will not have any client certificates to supply).

-Eric

HTTPS Only Works If You Use It – Tipster Edition

Convoy with three armored tanks and one pickup truck

It’s recently become fashionable for news organizations to build “anonymous tip” sites that permit members of the public to confidentially submit tips about stories of public interest.

Unfortunately, would-be tipsters need to take great care when exploring such options, because many organizations aren’t using HTTPS properly to ensure that the user’s traffic to the news site is protected from snoopers on the network.

If the organization uses any non-secure redirections in loading its “Tips” page, or the page pulls any unique images or other content over a non-secure connection, the fact that you’ve visited the “Tips” page will be plainly visible to your ISP, employer, fellow coffee shop patron, home-router-pwning group, etc.

NYTimes call for Tips, showing non-secure redirects

The New Yorker Magazine call for Tips, showing non-secure redirects

Here are a few best practices for organizations that either a) anonymous tips online or b) use webpages to tell would-be leakers how to send anonymous tips via Tor or non-electronic means:

Use HTTPS for all resources
Enable HTTP Strict Transport Security with preload for all domains
Enable Content-Security-Policy with Upgrade-Insecure-Requests and block-all-mixed-content
Ensure that you collect tips from the same domain that users use to read articles (avoid hostname giveaways like https://tips.example.com because HTTPS doesn’t keep hostnames private)
Avoid redirectors and clicktrackers
Scour your pages for non-secure links

For end users:

Consider using Tor or other privacy-aiding software.
Don’t use a work PC or any PC that may have spyware or non-public certificate roots installed.

Stay private out there!

-Eric

Security UI in Chrome

The combined address box and search bar at the top of the Chrome window is called the omnibox. The icon and optional verbose state text adjacent to that icon are collectively known as the Security Chip:

The security chip can render in a number of states, depending on the status of the page:

	Secure – Shown for HTTPS pages that were securely delivered using a valid certificate and not compromised by mixed content or other problems.
	Secure, Extended Validation – Shown for Secure pages whose certificate indicates that Extended Validation was performed.
	Neutral – Shown for HTTP pages, as well as Chrome’s built-in pages, like `chrome://extensions`, as well as pages containing passive mixed content, or delivered using a policy-allowed SHA-1 certificates.
	Not Secure – Shown for HTTP pages that contain a password or credit card input field. Learn more.
	Not Secure (Red) – What Chrome will eventually show for all HTTP pages. You can configure a flag (`chrome://flags/#mark-non-secure-as`) to Always mark HTTP as actively dangerous today to get this experience early.
	Not Secure, Certificate Error – Shown when a site has a major problem with its certificate (e.g. it’s expired).
	Dangerous – Shown when Google Safe Browsing has identified this page as a source of malware or phishing attacks.

The flyout that appears when you click the security chip is called PageInfo or Website Settings; it shows the security status of the page and the permissions assigned to the origin website:

The text atop the pageinfo flyout explains the security state of the page:

Clicking the Learn More link on the flyout for valid HTTPS sites once opened the Chrome Developer Tools’ Security Panel, but now it goes to a Help article. To learn more about the HTTPS state of the page, instead press F12 and select the Security Panel:

The View certificate button opens the Windows certificate viewer and displays the current origin’s certificate. Reload the page with the Developer Tools open to see all of the secure origins in the Secure Origins List; selecting any origin allows you to view information about the connection and examine its certificate.

The floating grey box at the bottom of the Chrome window that appears when you over over a link is called the status bubble. It is not a security surface and, like IE’s status bar, it is easily spoofed.

Navigation to sites with severe security problems is blocked by an interstitial page.

(A list of interstitial pages in Chrome can be found at chrome://interstitials/ ).

Clicking on the error code (e.g. ERR_CERT_AUTHORITY_INVALID in the screenshot below) will show more information about the certificate of the blocked site:

Clicking the Advanced link shows more information, and in some cases will show an override link that allows you to disregard the security protection and proceed to the site anyway.

If a site uses HTTP Strict Transport Security, the Proceed link is hidden and the user has no visible option to proceed.

In current versions of Chrome, the user can type a fixed string (sometimes referred to as a Konami code) to bypass HSTS, but this option is deliberately undocumented and slated for removal from Chrome.

If a HTTPS problem is sufficiently bad, the network stack will not connect to the site and will show a network error page instead.

-Eric

PS: There exists a developer reference to Chrome Security UI across platforms, but it’s somewhat outdated.