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Attack of the Week: Group Messaging in WhatsApp and Signal

blog.cryptographyengineering.com

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Re: Attack of the Week: Group Messaging in WhatsApp and Signal

#2

Post by aplorbust »

"This means the privacy of your end-to-end encrypted group chat is only guaranteed if you actually trust the WhatsApp server."

"This undermines the entire purpose of end-to-end encryption."

"And yet, the entire point of end-to-end encryption is to remove the server from the trusted computing base."

"The challenge here is that since WhatsApp itself determines who the administrators are, this isn't quite so simple."

Not only does this system require trusting a third party, that party is none other than Facebook. Its business is built on learning about the lives of users and selling ads, not serving as a naive broker of "secure messages".

Re: Attack of the Week: Group Messaging in WhatsApp and Signal

#3

Post by problems »

So they didn't bother to check if the add to group message was from someone in the group?

I mean no offense to the developers, but this seems like a fairly basic oversight and quite concerning that respected and popular products didn't get this level of review until now.

No crazy cryptographic mess involving improper ordering of authentication or weird random number generation, this is a simple logic bug. One that I'm sure many of us would have considered if we were implementing it, things like this do get missed too of course, but enough eyes on the design could have caught this.

Re: Attack of the Week: Group Messaging in WhatsApp and Signal

#6

Post by yuliyp »

aplorbust wrote: post #2

"This means the privacy of your end-to-end encrypted group chat is only guaranteed if you actually trust the WhatsApp server." "This undermines the entire purpose of end-to-end encryption." "And yet, the entire point of end-to-end encryption is to remove the server from the trusted computing base." "The challenge here is that since WhatsApp itself determines who the administrators are, this isn't quite so simple." No…

Hyperbolic much?

This is a weakness that not only requires WhatsApp to be malicious but to not care about the trivial visibility of an exploit (the client would still display the extra person being added to the group chat).

Re: Attack of the Week: Group Messaging in WhatsApp and Signal

#7

Post by arkadiyt »

problems wrote: post #3

So they didn't bother to check if the add to group message was from someone in the group? I mean no offense to the developers, but this seems like a fairly basic oversight and quite concerning that respected and popular products didn't get this level of review until now. No crazy cryptographic mess involving improper ordering of authentication or weird random number generation, this is a simple logic bug. One that I'…

> One that I'm sure many of us would have considered if we were implementing it, things like this do get missed too of course, but enough eyes on the design could have caught this.

It wasn't a design flaw, it was an insecure-direct-object-reference implementation flaw. IDORs are extremely common, but since the group id is an unguessable 128 bits, the bug can only be used by someone who was already in the group previously to rejoin the group. I'm sure it'll get patched shortly, if it hasn't been already.

For the WhatsApp case, a malicious WhatsApp server could add someone to your group, but everyone in the group would see it.

These bugs are not big deals. The real harm comes from regular people reading articles like the Wired one or the famously wrong Guardian one and switching to much worse alternatives, like SMS or Telegram.

Re: Attack of the Week: Group Messaging in WhatsApp and Signal

#8

Post by rickycook »

yuliyp wrote: post #6
aplorbust wrote: post #2

"This means the privacy of your end-to-end encrypted group chat is only guaranteed if you actually trust the WhatsApp server." "This undermines the entire purpose of end-to-end encryption." "And yet, the entire point of end-to-end encryption is to remove the server from the trusted computing base." "The challenge here is that since WhatsApp itself determines who the administrators are, this isn't quite so simple." No…

Hyperbolic much? This is a weakness that not only requires WhatsApp to be malicious but to not care about the trivial visibility of an exploit (the client would still display the extra person being added to the group chat).

not malicious per se, just with non obvious ulterior motives

Re: Attack of the Week: Group Messaging in WhatsApp and Signal

#9

Post by v1nc »

jgunsch wrote: post #5

Does anyone know what the disclosure story is here? Have WhatsApp and Signal had a chance to respond?

As said here: https://www.wired.com/story/whatsapp-security-flaws-encrypti...

"The researchers say that Open Whisper Systems, the non-profit that runs and maintains Signal, nonetheless responded to their work, saying that it's currently redesigning how Signal handles group messaging. Open Whisper Systems declined to comment on the record to WIRED about the Ruhr researchers' findings."

"In a phone call with WIRED, a WhatsApp spokesperson confirmed the researchers' findings. [...]And the WhatsApp spokesperson also noted that preventing the Ruhr University researchers' attack would likely break a popular WhatsApp feature that allows anyone to join a group simply by clicking on a URL." So I don't think WhatsApp will fix it.

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