Should you update to Thunderbird ESR 153.6?
Yes, this week. Thunderbird ESR 153.6 is a security update Mozilla released on 27 Oct 2026 for Mac, Windows and Linux. It fixes 154 security flaws, 0 rated critical and 100 high by Mozilla.[1] None of them is known to be used in attacks.
It is the newest Thunderbird ESR 153 release, so it’s the one to have.
Should you update?
Yes, this week. It fixes 154 security flaws, and attackers study each release to work out what it fixed. Every security update closes holes that become public the moment it ships, so the risk of staying on the old version only grows. Waiting a week to see if others hit problems is a common habit; with a security update it’s the wrong trade.
Is Thunderbird ESR 153.6 safe to install?
Yes. It’s Mozilla’s own security release, and updates rarely go wrong; a backup makes it painless if one does. The one unsafe way to get it is from a link: Mozilla never sends updates by email, text or pop-up, so install it only from Thunderbird’s own About window, or thunderbird.net.
What it fixes
Mozilla lists 154 security flaws, 0 rated critical and 100 high by Mozilla.[1] The 100 critical and high ones are:
| Flaw | Severity | What Mozilla calls it |
|---|---|---|
CVE-2026-118174 | High | Displaying a message could load an attacker-chosen URL through an OpenPGP key attachment |
CVE-2026-118175 | High | Content of other messages could be included when replying to a crafted message |
CVE-2026-118176 | High | Bypass of remote UNC path restriction in mailbox: URLs |
CVE-2026-118177 | High | Calendar discovery accepted local file and remote network share locations |
CVE-2026-118178 | High | RSS feed could redirect its subscription to a local file or network share |
CVE-2026-117860 | High | Same-origin policy bypass in the DOM: Navigation component |
CVE-2026-117861 | High | Sandbox escape in the DOM: Navigation component |
CVE-2026-117862 | High | Privilege escalation due to incorrect boundary conditions in the Networking component |
CVE-2026-117863 | High | Information disclosure due to uninitialized memory in the Audio/Video component |
CVE-2026-117864 | High | Sandbox escape in the Form Manager component |
CVE-2026-117865 | High | Use-after-free in the JavaScript: GC component |
CVE-2026-117866 | High | Privilege escalation due to use-after-free in the Graphics: Canvas2D component |
CVE-2026-117867 | High | Use-after-free in the Layout component |
CVE-2026-117868 | High | Sandbox escape in the DOM: Networking component |
CVE-2026-117869 | High | Use-after-free in the DOM: Core & HTML component |
CVE-2026-117870 | High | Use-after-free in the JavaScript Engine component |
CVE-2026-117871 | High | Invalid pointer in the Graphics component |
CVE-2026-117872 | High | Use-after-free in the JavaScript: Standard Library component |
CVE-2026-117873 | High | Incorrect boundary conditions in the Audio/Video: Playback component |
CVE-2026-117874 | High | Use-after-free in the Internationalization component |
CVE-2026-117875 | High | Use-after-free in the Layout: Scrolling and Overflow component |
CVE-2026-117876 | High | Use-after-free in the Layout: Text and Fonts component |
CVE-2026-117877 | High | Use-after-free in the Layout: Text and Fonts component |
CVE-2026-117878 | High | Other issue, use-after-free in the Storage: IndexedDB component |
CVE-2026-117879 | High | Incorrect boundary conditions in the Layout: Tables component |
CVE-2026-117880 | High | Use-after-free in the XML component |
CVE-2026-117881 | High | Use-after-free in the DOM: Core & HTML component |
CVE-2026-117882 | High | Use-after-free in the Layout component |
CVE-2026-117883 | High | Use-after-free in the DOM: HTML Parser component |
CVE-2026-117884 | High | Use-after-free in the Networking component |
| and 70 more | ||
How to update, and how long it takes
A minute or two. Thunderbird restarts and your mail stays where it was.
- Open Thunderbird.On a Mac choose Thunderbird → About Thunderbird; on Windows open the ☰ menu → Help → About Thunderbird.
- Let it download the update.The About window checks for one by itself; when it says “Restart to update Thunderbird”, click it.
To check it worked: The same About Thunderbird window shows the version; an ESR version ends in “esr”. It should say 153.6 or later.
153.5.1 vs 153.6
Thunderbird ESR 153.5.1, released 20 Oct 2026, fixed 4 security flaws. Thunderbird ESR 153.6 fixes 154 security flaws and includes the fixes before it, so going straight to 153.6 gets you both.
- Before: Thunderbird ESR 153.5.1, 20 Oct 2026, 4 security flaws
- After: none yet. This is the newest.
- Thunderbird ESR updates, every release and the latest version
- Updates, the latest version of everything HARD tracks
References
- ^ Mozilla. “Mozilla Foundation Security Advisory MFSA 2026-122”, 27 Oct 2026.