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Comment by rauhl | Hacker News Reader

Sadly, I believe that Firefox Accounts — upon which Firefox Sync is built — are insecure in execution. The security of the whole system relies on keeping your password secret from Mozilla, but as you can see when accessing https://accounts.firefox.com/oauth/signin?scope=profile&clie... your browser downloads the JavaScript it will use to derive your authentication token from Mozilla.

There’s nothing stopping Mozilla from serving all users malicious JavaScript which sends the password in plaintext; there’s nothing stopping Mozilla from serving 1% of users, or even a single targeted user. They could be forced to do so by a government with control over them; presumably some number of their employees could likewise be suborned by a state or criminal enterprise.

This should all be done by code compiled into the browser and triggered from the UI, rather than JavaScript run within a web page. Yes, Mozilla could also serve malicious Firefox binaries, or even publish malicious Firefox source, but the odds of someone in the world noticing that are significantly higher than a single targeted individual or small number of targeted individuals noticing.

Or maybe I’m missing something? I don’t think I am, though: that Firefox Account page is just HTML served from accounts.firefox.com; it’d be easy enough to just make it a simple form submission and capture the user’s password. And with the password, the security of the system fails completely: it’s simplicity itself to decrypt all stored data with it.

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Hi, Firefox Accounts developer here. You're correct in your understanding that that login flow is ultimately driven by a webpage, and this is a deliberate trade-off that we made in the interests of reach and usability of the system.

It's certainly a trade-off that not everyone is comfortable with, but we're confident it's the right one for the majority of our users. You can read some previous discussions on the topic in these bugs (and additional suggestions/feedback therein is definitely welcome):

  * https://bugzilla.mozilla.org/show_bug.cgi?id=1034526
  * https://bugzilla.mozilla.org/show_bug.cgi?id=1447656

>The reason the login form is delivered as web content is to increase development speed and agility

You saved some sprints but invalidated the purpose of the project. Very agile.

>Ultimately I think we can have web content from accounts.firefox.com be just as trustworthy as, say, a Mozilla-developed addon which might ship in the browser by default, which is a pretty high bar. We're not there yet, but it seems worth pursuing to try to get the best of both worlds.

The safety of the default installation is crowdsourced across all users and can't be targeted. The safety of the JS I load from Mozilla is not and I would have to verify its safety every time. Unless I'm misunderstanding something it can never be as trustworthy.

A more complete comment from rfk on the bug tracker:

> The reason the login form is delivered as web content is to increase development speed and agility. You're right that web content has a larger potential attack surface than code that's built into the browser, but using web content also brings other kinds of security benefit that may not be obvious. That agility meant that during the incident in [1] we were able to respond quickly and effectively to protect users data, and to roll out an updated login flow containing an email confirmation loop. It means that when we ship two-factor authentication over the coming weeks, it will be immediately available to all users on all platforms. It means we can address Bug 1320222 in a single place and be confident we won't lock out older devices. And it means we can easily bring new Firefox apps like Lockbox into the Firefox Accounts ecosystem.

> Our approach has been to embrace the benefits of web content while trying to reduce the potential attack surface as much as possible. That includes some simple things like hosting the web content on its own server to reduce exposure to application server bugs, and shipping default HSTS and HPKP settings for the accounts.firefox.com domain. It also includes some in-browser measures to prevent interference with FxA web content, such as (the currently private) Bug 1415644. As a future step I'd like to see us implement content-signing for accounts.firefox.com and have it enforced by the browser, following the example of things like Bug 1437671.

> Ultimately I think we can have web content from accounts.firefox.com be just as trustworthy as, say, a Mozilla-developed addon which might ship in the browser by default, which is a pretty high bar. We're not there yet, but it seems worth pursuing to try to get the best of both worlds.

"Every time" for this use case is once per browser install, at the moment you perform the authentication with Firefox Sync, which is the same as the number of times you'd want to verify the binary right before authenticating.

The tradeoff they made here has essentially zero impact on the number of times you need to verify their code, it's just a matter of whether you'd have to verify browser native authentication code or authentication code delivered through a website written in JS, at the moment you authenticate.

A concern like the one raised in this thread is certainly valid for websites that have expiring sessions, where you can switch accounts and log in and out of. And we certainly do need better tools around signature verification and version pinning for websites like we do for binaries (content-addressed networks like IPFS may have good answers there).

But for this use case, it's not a practical concern by any measure, and all this alarmism seems really misdirected.

You're still not addressing the ease with which a targeted attack can be directed at a single user.

In order to compromise firefox native code, they would have to compile malicious code and ship it to everyone. My distro maintainers would need to include the malicious binary in their repos, including a signed hash of the compromised binary, and I'd need to install it, where my package manager would verify the hash.

In order to compromise a single user's browser session, they'd simply need to fingerprint the user's browser and then serve them different content than everyone else gets. No hashes or signatures on javascript, no safety in numbers, etc.

if someone is using a package-manager that uses code signing then indeed, the binary is harder to attack than the JS. (only because the package-manager would need to collude).

However, a lot of people get their software from downloaded .exe's or auto-upgrading installations. For them, JS or binary are equally vulnerable. (All it takes is a mozilla signature)

Besides, it is undeniably better to only be vulnerable to an active attack from mozzila, than to be vulnerable against a passive attack from them.

Most distributions disable auto-upgrade in Firefox, for many reasons (security and auditability being one of the main ones) so you won't get auto-upgrade from a distribution.

And even the "download .exes from the internet" usecase is precisely as secure as downloading JS from the internet that is verified once per install. To attack someone who has an auto-updating Firefox and downloaded it from the internet, you need to intercept and attack TLS -- but only when the upgrade happens which is a fairly limited opportunity. The JS attack has the exact same properties if the above comment (that it only gets downloaded once per install) is true.

So therefore it is strictly less secure in the optimal case, and it is no more secure in the sub-optimal case. So security really isn't a strong argument (the real argument is that it allows for more "agile development" -- which is an understandable argument if you cop to that being the only reason for such a design).

If you can attack TLS, game is over, you can't trust anything. A huge majority of Firefox users use built-in update mechanisms. Making life harder for majority of users to improve security of the selected few is a questionable decision. And if you're really insisting, you can always install some addon which will calculate hashsum of JavaScript resources.

> Making life harder for majority of users to improve security of the selected few is a questionable decision

I agree in theory, though as an aside this isn't true for distribution packages because usually they are GPG signed with keys that are in TPMs on the release machines. Of course any other internet communication relies on TLS not being broken.

But another attack would be modifying one of Firefox's mirrors to host malicious Firefox (not a TLS attack but an attack of a specific mirror). GPG detached signatures for distribution packages protect against this and many other such problems (obviously some attacks against the build servers of a distribution would be problematic, but the same applies for any software project).

Though to be fair, I don't know if Firefox's auto-updater uses an update system like TUF or a distribution-style update system (which is mostly equivalent in terms of security) which would protect against these sorts of problems.

> Making life harder for majority of users to improve security of the selected few is a questionable decision.

I don't understand how logins being built-in to the browser is making life harder for the majority of users. It wouldn't make a difference to them. It would make a difference to the development team, but one could easily argue that the development team should be willing to make life slightly harder for themselves in order to make Firefox users more secure.

> So therefore it is strictly less secure in the optimal case, and it is no more secure in the sub-optimal case. So security really isn't a strong argument

I agree. I was arguing for having some form of e2e encryption (like Firefox currently has) as opposed to not having e2e encryption. I wanted to argue against the idea that, because the e2e was implemented in JS, one might as well not have it.

Then, regarding the gap between e2e in JS vs e2e in binary, my point was that JS is just as good in most cases.

> Most distributions disable auto-upgrade in Firefox, for many reasons (security and auditability being one of the main ones) so you won't get auto-upgrade from a distribution.

Does that mean that the code is only signed by the package distributor, and not mozzilla? Because in that case, the package manager becomes a single point of failure. Then again, I guess that is always the case. Still, it would be weird that, as far as mozzilla trust goes, a signed exe from internet is better than a signed package from your preferred package manager.

In openSUSE our build system can be configured to auto-check the signatures of the source archives used for building, so you can check the builds to make sure that we are building from an official source releases (assuming the GPG detach-sign their source tarballs -- something I recommend any software release manager do).

But most distributions do their own builds, and without reproducible builds being universally available -- not to mention that distributions usually have minimal compiler hardening flag requirements as well as patches in some cases -- you couldn't reuse signatures for the final binary. Also the entire package is getting signed, so the binary's signature wouldn't be sufficient (and checking it on-install would be quite complicated as well).

> Still, it would be weird that, as far as Mozilla trust goes, a signed exe from internet is better than a signed package from your preferred package manager.

I think that has always been the general case, since distributions are an additional layer of people maintaining a downstream copy of a project. But don't get me wrong, most distributions have processes that allow you to verify that the source used for builds of large projects like Firefox are built using the real sources.

There's also usually several layers of human and automated review before a package upgrade can actually land in a distribution.

The vast majority of Firefox users receive updates from Mozilla via the auto-update mechanism, which would also be vulnerable to the compromise in a similar way.

(A Linux distribution could also be compromised and used in a targeted way of course)

>> then serve them different content than everyone else gets

To help my understanding, to achieve an attack like this, would the attacker need to circumvent SSL on the client, or takeover the script serving web server? Or is there another attack vector that I'm not seeing?

The attacker in this case would be Mozilla itself. No need for an MITM. In this hypothetical, a government agency contacts Mozilla and says "Here is a canvas/HSTS/other fingerprint. Please serve this malicious code when this fingerprint accesses the login."

The point is that Mozilla can single out individual users for targeted attacks, whereas they could not do that if they had to put the malicious code into Firefox itself.

Right I see. So the barrier with Firefox itself, is that the malicious code wouldn't get built into the product and served as an update. However, in that scenario, Firefox could serve a malicious update to a single user, only that it's harder to fingerprint that.

> You saved some sprints but invalidated the purpose of the project. Very agile.

This sums it up indeed quite keenly and with an amount of snark I personally appreciate. Thanks

How did you install Firefox in the first place?

You can verify hash with others or compile it yourself.

Did you personally, and at least one other trusted party, sign off on every single commit, or are you trusting Mozilla?

Where did you get the hash you're comparing against?

Firstly, no matter what you're trusting the developers of the software you're running on your computer.

Secondly, the software (and/or its hash), just like this JavaScript, is delivered to you in a verifiably secure fashion i.e. SSL.

What's the difference?

Sure, this JS can change. Do you have automatic updates running for Firefox, or any piece of software on your computer?

With attack vectors it's also about ease of exploitation. In this case, the ease is high. If the person you are responding to compiles their own browser, the bar to put an exploit in there is already much higher. Yes, there are still attack vectors. And there always will be. The point is they're harder to access.

Your initial comment was pretty adamant that Mozilla had really messed up by delivering the code as JS. However, what is the attack vector that they've introduced by taking this approach?

It sounds to me like you're referring to a man-in-the-middle style attack. However, to be best of everyone's current knowledge, that's simply not possible with SSL.

It's only possible if the attack vector includes having already compromised the user's computer and installed a root certificate. At which point this is all pretty moot.

I think you have me confused with someone else. I have made no points except the ones in the post you are responding to.

In this case it looks like you're missing the fact that you can change the JS on the server with a high amount of ease and a low discoverability (it can be changed just for you and it won't show anywhere else).

> I think you have me confused with someone else. I have made no points except the ones in the post you are responding to.

My apologies, that's what I get for reading on my mobile.

> In this case it looks like you're missing the fact that you can change the JS on the server with a high amount of ease and a low discoverability (it can be changed just for you and it won't show anywhere else).

You raise a reasonable point. It is indeed something everyone should be aware of. It's mostly a matter of trust, not security.

However, the same is equally true of someone you trust changing the binaries, source and/or hashes that are delivered to you; whether you got those from Mozilla, or somewhere else.

For example, the relatively recent Handbrake release compromise - https://news.ycombinator.com/item?id=14281808

“That’s simply not possible with SSL”

I agree that we don’t currently know of easy attacks on SSL if you’re pinning certs (which it sounds like Mozilla does here). But all you need is a rogue CA to MITM SSL if you’re not pinning certs, so I don’t think “simply not possible” is an accurate description of SSL as generally used by the broad web-dev community.

The question is how hard it is to detect tampering. My linux distribution builds firefox from source and signs the build. The builds are also checked to be reproducible.

Raising the bar is a good thing.

I wasn't aware that any distribution (besides Tor Browser) was building Firefox (or anything really) reproducibly.

There's debian's https://reproducible-builds.org/ effort, but I thought that wasn't making much progress lately, nor was it deployed.

Could you provide more info on what distro you're using, or how they're doing this?

S/he may be referring to Gentoo Linux.

> Do you have automatic updates running for Firefox …

No.

> … or any piece of software on your computer?

Also, no. But even did I, there’s a world of difference between automatic updates from e.g. Debian and automatic updates from Mozilla.

> there’s a world of difference between automatic updates from e.g. Debian and automatic updates from Mozilla.

In what way?

This is obviously somewhat anecdotal, but...

I'm the developer of Heimdall. Software that flashes firmware onto Samsung phones. The software quite literally has the ability to replace almost every piece of software running on your phone. If it were compromised, it could not only own a user's phone, but also potentially everything a user accesses on said phone.

Sure my software is open-source, and I encourage anyone interested to inspect the code, I'm sure there are bugs. However, the `heimdall-flash` package in the official Debian repositories... I didn't make it, and I have no connection with whoever did. Now, don't be alarmed, despite being several years out of date, to the best of my knowledge it's a perfectly good package, and I'm thankful that the maintainer went to the effort. However, it would be so easy for someone to have published a malicious package. This is pretty powerful software, it has significantly more power than root on your mobile phone.

I love Debian, both philosophically and in practice. But does it really deserve your trust more than Mozilla?

It's perfectly normal for Debian packages to be maintained by other people that the original developers of that piece of software, isn't it? Debian has more than 60000 packages but doesn't have 60000 package maintainers – the roles are quite separate.

For example, Linus Torvalds doesn't maintain the Debian kernel packages. If whoever does were to put malicious code in the kernel packages, that would be very bad, just as if Heimdall were compromised, which is why Debian has a relatively small set of trusted package maintainers and doesn't let just anyone put code in the official distribution.

> Debian has a relatively small set of trusted package maintainers and doesn't let just anyone put code in the official distribution

There are presently 2619 official Debian maintainer GPG keys[1].

Considering the scope, that's not ridiculous, but I wouldn't call it small.

[1] http://ftp.debian.org/debian/pool/main/d/debian-keyring/

Dist repo?

This is very interesting and thanks for clarifying, but if you concede that there is a security trade-off here for the sake of usability, then isn't this, by definition, not "Private by Design".

As in: you chose other principals to guide your design other than privacy?

Nobody purely chooses privacy or security to guide their design. An implementation of Firefox sync that was purely, 100% private by design would be airgapped, it wouldn't sync over a network.

Arguably, a private by design implementation of Firefox sync wouldn't even exist. You significantly increase your number of attack vectors by making your session available on multiple devices. What happens if your Android phone is compromised? Better to only have your session on one device.

Obviously I'm being hyperbolic here, but the point I'm getting at is that security isn't black and white, and you will always be making tradeoffs for usability, no matter what the context is.

What that means for "private by design", I dunno. Maybe it's just a buzzword. Maybe it's just a matter of degree. Other people can debate that if they really want to. But I do know that the moment you put doors on your house, it's less secure than it used to be.

The actual valuable question is, "is Mozilla's tradeoff good enough for usability that it justifies the decrease in security?" I'm not sure whether the answer to that is yes or no.

The privacy is at least verifiable. In the sense that users can at least look at the implementation themselves, and (granted with some difficult) potentially detect changes.

This is much better than simply sending your password off to a third-party and having to trust that the company is doing what they say they're doing.

Sorry, but you (mozilla) deliberately crippled your system to the point where mozilla or certain key personnel of mozilla/whoever operates the data centers or anybody running some MITM middlebox - if they wanted to/where compelled to - could easily target specific users or large groups of users rather stealthily, by your servers (and any successful MITM) having the capability to underhand compromised code on a request-by-request basis that could sneak out the actual password thus compromising all the user's data you store.

Then you go ahead and publish articles how your system is "Safe" and "Private by Design" underpinned by "here is SOME MATH to prove it!".

And just now, you state that you "traded off" the "Safe" and "Private by Design" properties for "<unspecific marketing speech, something about reach and usability>", but are somehow "confident" that falsely advertising your broken implementation as having properties it does not possess is the right thing to do (for most users). wat?!

Why is the code that derives stuff from the password and the UI for entering the password not in the browser itself? Surely not for the merit of the users, as there is no real usage/UX difference for them between "https://accounts.firefox.com/" and "chrome://firefox-account/".

Why not ship the password handling logic in the Firefox binary?

Could this be solved by creating a WebExtension that completely replaces the frontend with a bundled copy and then including it in Firefox? If you trust the browser and therefore the bundled extension, you don't need to trust the server at all.

This response reminds me of this discussion https://news.ycombinator.com/item?id=17804916

If you have that much distrust you shouldn't run Firefox at all since every page you load would be at risk. I doubt that any browser would match such an high bar

There was a substantive suggestion for improvement... to include the javascript calls to grab the authentication inside the source code, rather than a remote call.

Maybe an intermediate solution would be to use integrity hashes on the remote calls.

> Maybe an intermediate solution would be to use integrity hashes on the remote calls.

That was my immediate thought upon reading your comment and realizing that the bar to hit is really to be just as secure as if it's baked in the binary, but on reading the bugzilla links that _rfk posted in a comment to your original comment, I'm not sure it really works for their goals (whether or not you think those goals are correct or match yours).

If I maybe understand what they are working towards, it's having the flexibility to provide fixes/features quickly while still providing security. I'm not sure the features portion of that is as important as the credit they give it, given we're talking about security here, but quick fixes is good. I imagine they are looking at a way to provide a specialized context or something similar to lock down what can and cannot be done if the served content is malicious, but I'm not entirely sure what that would look like.

I don't think the Javascript for authentication can be client-side. Authentication of passwords must be done on the server.

Did you read the article? The system is designed so that an authentication token is derived from the password, and this token, rather than the password itself, is authenticated against on the server. Since the encryption key can not be derived from the authentication token, this shields the content of the encrypted data the server stores from itself.

GP rightly noted that deriving the authentication token with code served by the server at log-in time renders most of the security architecture of the system rather moot, unless user has the time to verify the JavaScript they are served every time they log in.

> unless user has the time to verify the JavaScript they are served every time they log in.

Which is once per install, the same amount of times then user would have to verify the native code

There's actually a great difference, because for binary distribution it simply suffices that the user verify the cryptographic hash of the binary they downloaded. Then they can be confident that they were served the correct publicly distributed binary, and not a subverted version.

JavaScript by its very nature can not be easily, if at all, verified. Even if you verify that you are being served the same code as other users and the code is not subverted at one point in time, it's very much expected that the code may change at any time at the web developer's discretion. This is also probably the very reason why Mozilla implemented it like this (so that they can update the Sync experience branding without updating the browser.)

Even if you hard-coded the signature of the JavaScript library file responsible for the cryptographic operations in the binary, browser DOM by itself would require modifications so that any other UI logic script in the page would not be able to sniff your password as you enter it..

> JavaScript by its very nature can not be easily, if at all, verified.

Yes it can, in the same way you mentioned in your previous paragraph. Mozilla post the hash of the JS source file, and any user who wants to can verify the hash of the source file they have can do so in exactly the same way they verify a binary. It's literally calling sha1sum on the js file isntead of the binary.

> This is also probably the very reason why Mozilla implemented it like this (so that they can update the Sync experience branding without updating the browser.)

I'd partially agree here - it's implemented like this so that they can update sync without updating the browser. If there's an issue found, mozilla can fix it, change the JS that is being served, and update the hash of the file. Sync branding (to me) has nothing to do with it.

> Even if you hard-coded the signature of the JavaScript library file responsible for the cryptographic operations in the binary, browser DOM by itself would require modifications so that any other UI logic script in the page would not be able to sniff your password as you enter it..

Yes, but this is moving the goalposts from the user verifying the js. This is true whether the code is in the binary or not. If the code goes in the binary, and is shown as part of the DOM, you need to modify the DOM logic to ensure nothing else can see it.

Not really, since one can verify the Firefox code oneself, or hire others to do it, or rely on the fact that it’s a matter of public record and do forensics analysis after the fact should one be compromised.

But a target JavaScript file sent only to oneself a single time … that’s practically impossible to validate or prove.

Most people download Firefox binaries directly from Mozilla. They can't verify the code (even if they knew how).

On larger Linux distributions, the trust is less with Mozilla and more with the distro-package maintainers - use of dynamically remote-loaded JavaScript here shifts the trust back to Mozilla; if you trust your maintainers but not Mozilla, then this is a problem.

You really need to trust both. A browser vendor has so many layers to attack you, if they're malicious, that there's no way around it. Same for package maintainers.

So are you suggesting to use a private API to login to accounts.firefox.com? And what about using accounts.firefox.com in other browsers?

How could this even possibly be mitigated, though? At some point you have to trust something, right? This argument could extend all the way down to your OS, chipset, whatever. Unless you've built every part of your machine from scratch it's always going to rely on trusting a component you know nothing about.

If you include this code in firefox itself, you can review the code and verify that the official firefox binaries use that code.

But what about the code you run to verify the binaries? You have to trust that. Or are you reviewing every bit by hand?

Trusting the binary is a one-time effort. Here you have to trust the JS everytime you authenticate.

Even one-time, comparing the hash of a git commit or an installer is annoying but practically doable. I'd have no idea how to safely do something like this for a website. And I for sure ain't able to audit those ~200kb js by myself.

But my point is you also have to trust that your underlying OS, or chipset, or even some other software on your computer doesn't have some way to thwart the entire effort. If the thing you're using the do your verification is itself compromised, then you're just as screwed.

So you never update Firefox? For most users it automatically updates itself.

For this use case, authenticating is a one-time effort as well, per browser. If you're authenticating again, you're almost certainly doing it on a different device/browser, at which point you'd have to verify the new browser binary anyways.

Sign-ins to Firefox Accounts (used to access Mozilla services as web pages) don’t expire after some time? I didn’t know that.

If Mozilla wants your passwords there's nothing to stop them adding code to Firefox itself that detects when you are typing your password in and sends it to them. It could be limited to just you in that case too.

This is really to protect against two things:

1. Legal requests for your data. It's actually not really been tested whether or not the government can force an entity to insert a backdoor for a specific user or not. I think everyone hopes that can't do that but does anyone really know?

2. Data breaches.

> If Mozilla wants your passwords there's nothing to stop them adding code to Firefox itself that detects when you are typing your password in and sends it to them. It could be limited to just you in that case too.

Users who consume Firefox from distributions, rather than directly from Mozilla, are much better protected from this. "Nothing to stop" is not true in this case.

Sure but then you're just trusting a different party (the packagers). Now you have to trust two groups of people.

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