CVE-2026-100804

Sandbox escape due to use-after-free in the Preferences: Backend component

Risk score: 66/100
CVSS: 9.6 v3.1
Exploit chance (30 days): 0.3%
Published: 29 Sep 2026

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What to know

What it is: Sandbox escape due to use-after-free in the Preferences: Backend component. This vulnerability was fixed in Thunderbird 157 and Firefox 157.
Affected: Firefox
Fix: Apply the vendor's security update; see the linked advisory.
Exploitation: No known exploitation

Story map

Story map: the flaws, the fix, how it unfolded and what it connects to. Boxes link to the record or article.

At a glance

Firefox: Amber, patch this week. Invalid pointer in the JavaScript: WebAssembly component. This vulnerability was fixed in Firefox ESR 153.4, Thunderbird 157, Thunderbird 140.17, Thunderbird 153.4, Firefox 157, and Firefox ESR 140.17.

The flaws

CVE-2026-100804: Sandbox escape due to use-after-free in the Preferences: Backend component (CVSS 9.6 · EPSS 0%)
CVE-2026-100788: Invalid pointer in the JavaScript: WebAssembly component (CVSS 9.8 · EPSS 0%)
CVE-2026-100775: Sandbox escape in the Graphics component (CVSS 9.6 · EPSS 0%)
CVE-2026-100781: Sandbox escape due to incorrect boundary conditions in the Graphics: WebRender component (CVSS 9.6 · EPSS 0%)

The fix

Fix: Apply the vendor's security update; see the linked advisory.

Who is exposed: Firefox

Where it stands

No known exploitation

Connects to

Same weakness: Use-after-free (CWE-416). 399 other warnings in 30 days
Same product: Earlier Firefox warnings. 73 in the last 90 days

Why it is Amber

CVSS 9.6 critical
Remotely exploitable without logging in

References

Original description

Sandbox escape due to use-after-free in the Preferences: Backend component. This vulnerability was fixed in Thunderbird 157 and Firefox 157.

Last activity 29 Sep 2026. Patch Forecast updates every hour.