Mozilla Firefox CVE-2026-100789

Exploitation requires a victim using an unpatched Firefox or ESR build (before Firefox 157, ESR 153.4, ESR 140.17, or ESR 115.42) to actively load attacker-controlled web content that exercises the Graphics Canvas2D rendering path - user interaction (UI:R) is the primary limiting factor. No authentication, no special non-default configuration, and no privileged access are needed (PR:N); Canvas2D is a standard always-enabled rendering feature, so no opt-in setting gates it. Realistic risk is bounded by the need to lure the victim to a malicious/compromised page and by Firefox's renderer sandbox (S:U), which would require an additional escape to reach the host OS.

This is a genuinely serious client-side vulnerability but not an active-exploitation emergency. The CVSS 3.1 vector (AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H = 8.8 High) reflects a network-reachable, low-complexity, unauthenticated memory-corruption flaw whose only gating factor is user interaction (the victim must load a malicious page). CWE-416 use-after-free in a JIT-adjacent rendering path like Canvas2D is the classic primitive for exploit chains leading to renderer RCE, which justifies the C/I/A:H rating. However, exploitation-likelihood signals are all absent or benign: no EPSS score is provided, there is no CISA KEV listing, and no public POC is referenced - the only sources are Mozilla's own bugzilla and MFSA advisories, meaning this was internally discovered/coordinated and disclosed with a fix rather than found in-the-wild. Net: real priority for patch-management (browser is ubiquitous, memory corruption, network-triggerable) but there is no evidence of widespread or targeted exploitation, so it ranks as an important routine-patch item rather than an emergency. Note the CVSS is scored S:U (unchanged) - consistent with a renderer-process compromise contained by Firefox's sandbox absent a further sandbox-escape.

An attacker crafts a malicious web page containing JavaScript/HTML that drives the Graphics Canvas2D component into a use-after-free state, then lures a victim running an unpatched Firefox/ESR build to visit it (via link, ad, or compromised site). By grooming the heap around the freed object, the attacker turns the dangling reference into controlled memory corruption, potentially achieving code execution in the renderer process. No public POC or exploit code is referenced at time of analysis.