1771972 - With TLS 1.3 disabled in the config settings, Firefox still connects to websites with TLS 1.3

Status: RESOLVED FIXED

(Core :: Networking, defect, P2)

Description

[only1ryan]

User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:91.0) Gecko/20100101 Firefox/91.0

Steps to reproduce:

In Firefox I went to about:config and set

security.tls.version.max = 3

which disabled TLS 1.3 and left TLS 1.2 as the highest security setting. I checked the browser at ssllabs.com, which confirmed TLS 1.2 was the highest available protocol and listed the appropriate cipher suites.

Actual results:

A number of websites (e.g. Google, Facebook, Youtube) connected with TLS 1.3 and a new cipher suite. That is what I learned when I clicked on the lockbox in the address bar.

Expected results:

Why did Firefox not respect the new setting? I have toggled

security.tls.version.fallback-limit

between 3 and 4, but it does not seem to make any difference. Also, how does a person choose the cipher suites for TLS 1.3? They do not seem to be listed under

security.ssl3

in the about:config settings. Thanks.

Comment 1

[Hani Yacoub, Desktop QA]

I managed to reproduce this only on youtube.com from the examples provided above on Firefox 100.0.2, Firefox 102.0b2 and on Firefox 103.0a1, and it was connected with TLS 1.3 instead of TLS 1.2.
On Firefox 91.10.0esr I managed to reproduce the issue also on google.com.
I'm not sure is "Security" component is the right one, please feel free to change it if you think otherwise.

Comment 2

[Dana Keeler (she/her) [:keeler]]

Did you clear the cache before connecting to those sites?

Comment 3

[only1ryan]

Each time I start Firefox I make it a point to go to the Privacy and Security settings and clear the Cookies and Site Data.

Comment 4

[Dana Keeler (she/her) [:keeler]]

What about the network cache, though? (it's called "Cache" in that UI)

Comment 5

[only1ryan]

Sorry about that. I also clear the one called "Cached Web Content."

Comment 6

[Dana Keeler (she/her) [:keeler]]

I think what's going on is we're getting an h3 alt-svc header and necko is using that when it probably shouldn't, if TLS 1.3 is disabled.

Comment 7

[Kershaw Chang [:kershaw]]

I think it should not be too difficult to add a check in necko to see if TLS 1.3 is enabled.
Put this to our priority queue candidate and wait for review.

Comment 8

[Frederik Braun [:freddy]]

The reporter asked through [e-mail] to nominate this for the bug bounty program. We will make our decision only after the bug has been fixed. However, given the information that is currently present and given that this is not considered a security bug as per https://wiki.mozilla.org/Security_Severity_Ratings/Client, it is likely that this will not be rewarded a bounty. (Final decision at the discretion of the bug bounty committee.)

Comment 9

[Greg Hess]

Dragana can you please review and see if this is for Necko.

Comment 10

[Dragana Damjanovic [:dragana]]

The prefs are handled by PSM. Moving to the right component.

Comment 11

[Dana Keeler (she/her) [:keeler]]

PSM handles the prefs, but Necko needs to do the appropriate thing based on the values of the perfs (namely, disregard h3 alt-svc headers if TLS 1.3 isn't enabled).

Comment 12

[Dragana Damjanovic [:dragana]]

Attachment: Disable HTTP/3 if TLS 1.3 is disabled

Comment 13

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/d4ebe7f7af0f
Disable HTTP/3 if TLS 1.3 is disabled r=necko-reviewers,valentin

Comment 14

[Marian-Vasile Laza]

https://hg.mozilla.org/mozilla-central/rev/d4ebe7f7af0f