Comment by yrro | Hacker News Reader

Comment by yrro | Hacker News Reader

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> An attacker with local access on a system where Microsoft Teams is installed could steal the tokens and use them to log into the victim's account.

Big deal? They can also read all sorts of stuff out of ~/.config, or my Kerberos tickets out of $KRB5CCNAME, or my tokens out of ~/.kube/config, or my MS Graph/Azure access tokens out of ~/.azure/accessTokens.json!

I guess Microsoft agrees:

> Vectra researchers discovered the problem in August 2022 and reported it to Microsoft. However, Microsoft did not agree on the severity of the issue and said that it doesn't meet the criteria for patching.

> With a patch unlikely to be released, Vectra's recommendation is for users to switch to the browser version of the Microsoft Teams client. By using Microsoft Edge to load the app, users benefit from additional protections against token leaks.

Hold on, can't the same attacker now read my access tokens out of ~/.mozilla/firefox or ~/.config/chromium?

> The researchers advise Linux users to move to a different collaboaration suite, especially since Microsoft announced plans to stop supporting the app for the platform by December.

While I wholeheartedly agree with anyone advising people to ditch the awful Microsoft Teams... why not just switch to the browser version, as already advised? This is what MS are advising users to do anyway...

[additional]

I'm still dreaming of a world where Flatpak runs every app as a separate UID and SELinux domain, to prevent the above attacks.

> Hold on, can't the same attacker now read my access tokens out of ~/.mozilla/firefox or ~/.config/chromium?

You are correct. There is no such thing as "multiple OS-level controls". In Google Chrome on Windows, the cookies in question are stored in an SQLite database at '%localappdata%\Google\Chrome\User Data\Default\Network\Cookies'. The cookie is named TSAUTHCOOKIE and contains an authentication token.

There is the same level of protection on this file, as there is on the Microsoft Teams Cookies database.

Source: I build forensics investigation tools for a living.

While that's true, they could be placed in secure credential storage instead. The Ripcord chat client for Discord and Slack uses Windows's secure credential storage, for example, so I can't easily extract its saved credentials and to clear them I go to the Credential Manager under Control Panel. Mozilla VPN and my GitHub credentials are both in Credential Manager instead of a dotfile as well.

The recommendation that a browser is safer seems pretty silly though. As far as I can tell no web browser uses secure credential storage.

This seems like a fundamental problem, if you store a secret locally, you can read it locally.

I was skeptical/curious how Windows's Credential Manager could have gotten around that, but Stackexchange says this (https://security.stackexchange.com/a/120001/76848):

> The Windows Credential Manager is anything but secure. It's "secure" at the user account level, which means that any process that the user ever runs and the user themselves must necessarily be trusted in order to call this system "secure" with a straight face.

So this should be about as secure as Chrome or Firefox's storage. A small layer of obfuscation to deter the least motivated script kiddies, but nothing that changes the nature of the problem. If you store secrets under your user account, then programs running under your user account can also read them back.

It’s true that protecting against processes running under the users account is hard, but exploiting that would require code execution

Accessing a secret in a file does not necessarily. It could be stored on a network share, included in a backup or archive or an attacker could confuse a program into handing them the file. Especially that used to be a very common attack against browsers.

While the Windows Credential Manager doesn’t really have any access controls below the user account level, the same is not true for the Mac Keychain. By default if an app tries to read a keychain entry that was created by another app then the user will be prompted to allow or deny access.

B-b-but the Windows Credential Manager asks me to authenticate before it shows me the value of any saved passwords! It must be secure, right?

What's 'secure credential storage'? I don't think the Windows Credential Management API counts. Any program running as my user can ask for credentials out of the store, can't it?

And even if there was such a secure storage location, any program running as my user can read the token out of /proc/<teams PID>/mem; or it can ptrace(2) Teams and extract the token from the communication with the credential storage API...

At least on macOS, the answer is no - a user alert indicating the application is asking to read the credentials will appear with a password field to grant access.

On Windows an application using the Data Protection API can force the decrypt procedure to show a dialog[1].

Though I haven't checked if the dialog is run in a secure context, like for example the UAC dialog is. And I don't know if the dialog contains any information identifying the processing asking.

If the dialog is just run in the normal context, or doesn't inform the user who wants to decrypt the data, it's not that useful for thwarting an attack I suppose.

[1]: https://docs.microsoft.com/en-us/windows/win32/api/dpapi/ns-...

the AWS CLI similarly stores your access credentials in ~/.aws/credentials. If you have logged-in local system access to a user's computer, there's a lot more you can get than their Teams token

Which is why one should avoid handing off AWS keys and secrets to people. Add SSO, then you will only have ephemeral keys.

Any good articles to start with?

Looks like you're using Kerberos in a corporate setting? Purely out of curiosity, what does that look like? Is the entire company's authentication infrastructure based on Kerberos? Is it used only for access to development infrastructure? Do you make use of the more "advanced" features of Kerberos like forwarding tickets for something like deployment/CI?

Until now, I genuinely thought Kerberos was used mostly only by MIT.

Active Directory uses Kerberos for authentication, it's safe to say it's pretty widely used outside of MIT. :)

Our user accounts live in an AD DS. We use FreeIPA (actually Red Hat Identity Management) to manage a second domain for Linux stuff. There is a trust between the domains so that users can log into systems with their AD credentials. It all works pretty well; most of my frustrations with the system are because of AD's limitations (chiefly, lack of built-in MFA; awful documentation; lack of decent tooling for creating keytabs & configuring resource-based constrained delegation, seriously, I can't believe how bad the AD admin tools are) and flaws in how we have configured AD (obsolete protocols and ciphers still in use; the trust is one-way so users on Windows machines can't authenticate to services in the FreeIPA domain).

This is for logging on to end user devices & accessing servers. Web apps we don't configure (any more) to use LDAP for authentication, rather OpenID Connect (against Azure AD, the confusingly similarly named service from MS, that syncs user accounts from AD DS). Where web apps want access to Kerberos-protected resources, we have some... interesting... setups to allow web apps to use constrained delegation to impersonate the users who have logged in (and this is another thing about AD DS that surprises me... it's incredibly easy to create a service principal that can impersonate any user when authenticating to a resource, including highly privileged users... there's nothing anywhere near as flexible as FreeIPA's Service Constrained Delegation mechanism, that can say "this service can impersonate anyone in group A to resource B")

CI/CD is a bit too advanced of a concept for our AD audience :), we just keep an eye on the platforms and products that our users use & make sure they're doing the right thing with respect to token handling, etc.

Thanks, this is wonderfully in-depth!

Windows has used Kerberos as its authentication mechanism since Windows 2000.

Exactly. I think the only time this matters if you have read permissions on a machine you shouldn't have and want to get access to other SSO-related network resources.

I think that the solution is to make it a non-issue: fingerprint the machine and make sure that the auth tokens are only usable on that specific machine... but this solution is both a privacy nightmare and a flimsy deterrent (especially on Linux, somewhat on Windows, might actually work on Macs though).

I agree with Microsoft that it's a non-issue: there's no concept of a separate user-inaccessible data compartment unlike with Android and iOS, so the best they could do is to store it in system-provided keychain (but usually it is often unlocked during log-in, defeating its purpose).

Actually there is Credential Guard which in concept does this. But Microsoft only use this for Kerberos/Domain stuff and it's only supported in Enterprise Edition.

They only don't support this because Windows is market segmented and Microsoft have fuck all product management and direction that isn't run by an army of crack smoking monkeys.

> there's no concept of a separate user-inaccessible data compartment

Would a TPM serve here, in some hypothetical world where folk were willing to put in the work to do this?

Or I guess I'm asking, naively, what are the functional differences between the TPMs on our desktop/laptop machines, and what smartphones have in them?

It could work but in Windows you generally need administrator privileges to touch TPM.

> Or I guess I'm asking, naively, what are the functional differences between the TPMs on our desktop/laptop machines, and what smartphones have in them?

A security processor on mobile devices are usually actually like TPM and Intel SGX combined together. I won't say it's better than PC TPMs, but have a very different design.

At some point Teams has to ask the TPM to unseal its encrypted copy of the access token. At that point an attacker can read the communication between Teams and the TPM, or it can read the token straight out of Teams' memory space.

Unless the TPM performs the cryptographic operations on the chip itself. TPM can do asymmetric crypto using a key in it's secure storage.

It would require Microsoft to use a different authentication scheme based on ephemeral signed tokens instead of OAUth to be beneficial for security. That is very unlikely to happen.

That scheme would also only reduce the access timeframe from whatever the OAuth token's lifetime is, to a timeframe constrained to the amount of time the computer (the key) is online.

Since on most systems the only two users are root/admin and the user's personal account, they could also just read it out of the process as well

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