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CVE-2026-74986 is a site-isolation vulnerability in Mozilla’s CSS Parsing and Computation component. The flaw can undermine browser site-isolation boundaries, potentially permitting unintended cross-site data access or a security-boundary bypass within the browser context. Mozilla fixed the issue in Firefox 154, Firefox ESR 153.1, Thunderbird 154, and Thunderbird 153.1.
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What it means. What to do now. Patch path, mitigations, and the assume-compromise checklist.
What an attacker gets, and what they’ve been doing with it.
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Patch, then assume compromise.
No public exploits tracked yet. Mallory keeps watching.
No public exploit code observed for this vulnerability.
Products and vendors Mallory has correlated with this vulnerability. Open in Mallory to drill down to specific CPE configurations and version ranges.
Vendor-confirmed product mapping. Mallory continuously reconciles this list against your asset inventory.
14 sources tracked across advisories, community write-ups, and news. New activity surfaces here as Mallory finds it.
Site-isolation weakness in Firefox CSS parsing and computation.
A site-isolation flaw in Firefox CSS parsing and computation.
A site isolation issue in Firefox's CSS Parsing and Computation component.
Unknown
Query your assets running an affected version, and investigate the blast radius.
Every observed campaign linking this CVE to a named adversary.
Malware families riding this exploit, with evidence and IOCs.
YARA, Sigma, Snort, and vendor rules, auto-deployed to your SIEM.
Cross-references every affected SKU, including bundled OEM variants.
Community discussion across Reddit, Mastodon, and other social sources.