A [critical vulnerability](https://www.mozilla.org/en-US/security/advisories/mfsa2020-49/)was discovered in the JavaScript engine of *Firefox* and *TorBrowser*.
Until Tails 4.13 (November 17), we recommend all users to set the [[security level of *TorBrowser*|doc/anonymous_internet/Tor_Browser#security-level]] to *Safer* or *Safest*.
The *Safer* or *Safest* security level of *TorBrowser* are not affected because the feature of JavaScript that is affected, the *[[!wikipedia just-in-time compilation]]*, is disabled at these security levels.
The details of the vulnerability were not disclosed, making it harder to exploit, and we are not aware of any use of this vulnerability against actual users.
