Critical

Same-origin policy bypass in Firefox/Thunderbird Networking: Cookies

IdentifiersCVE-2026-12304CWE-346· Origin Validation Error

CVE-2026-12304 is a same-origin policy bypass vulnerability in the Networking: Cookies component of Mozilla Firefox and Thunderbird. The provided advisory content identifies the flaw only at a high level and does not include the specific vulnerable function, code path, or triggering condition. Mozilla states that the issue was fixed in Firefox 152, Firefox ESR 140.12, Thunderbird 152, and Thunderbird ESR 140.12. Based on the available information, exploitation would allow web content to bypass browser same-origin restrictions enforced around cookie-related networking behavior.

For your environment

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ANALYST BRIEF

Impact, mitigation & remediation

What it means. What to do now. Patch path, mitigations, and the assume-compromise checklist.

Impact

What an attacker gets, and what they’ve been doing with it.

Successful exploitation could bypass the browser same-origin policy, potentially allowing unauthorized cross-origin access to cookie-related data or enabling actions that should be restricted to the legitimate origin. Depending on the surrounding application context, this could expose sensitive session information, weaken origin isolation, and facilitate account compromise or unauthorized actions in web applications accessed through the affected browser.

Mitigation

If you can’t patch tonight, do this now.

No vendor-provided workaround is described in the supplied content. Until updates can be applied, reduce exposure by limiting access to untrusted websites and web content, enforcing least privilege for end users, and applying browser hardening and endpoint protections where feasible. Patching is the primary mitigation.

Remediation

Patch, then assume compromise.

Upgrade affected Mozilla products to fixed versions: Firefox 152 or later, Firefox ESR 140.12 or later, Thunderbird 152 or later, and Thunderbird ESR 140.12 or later. Debian users should install the updated firefox-esr package version 140.12.0esr-1~deb13u1 or later where applicable.

PUBLIC EXPLOITS

Exploits

No public exploits tracked yet. Mallory keeps watching.

VALID 0 / 0 TOTALView more in app

No public exploit code observed for this vulnerability.

EXPOSURE SURFACE

Affected products & vendors

Products and vendors Mallory has correlated with this vulnerability. Open in Mallory to drill down to specific CPE configurations and version ranges.

VendorProductType

MozillaFirefoxapplication

MozillaFirefox Esrapplication

Vendor-confirmed product mapping. Mallory continuously reconciles this list against your asset inventory.

ACTIVITY FEED

Recent activity

7 sources tracked across advisories and community write-ups. News coverage will land here when it surfaces.

7 SOURCESView more in app

No news coverage yet. Advisories and community discussion only.

What this page doesn’t show

The version that knows your environment.

This page is what’s public. Mallory adds the parts that aren’t: which of your assets are affected, which adversaries are exploiting it right now, which detections to deploy, and what to do tonight.

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Exposure mapping

Query your assets running an affected version, and investigate the blast radius.

Threat actor evidence

Every observed campaign linking this CVE to a named adversary.

Associated malware

Malware families riding this exploit, with evidence and IOCs.

Detection signatures

YARA, Sigma, Snort, and vendor rules, auto-deployed to your SIEM.

Vendor-by-vendor mapping

Cross-references every affected SKU, including bundled OEM variants.

Social activity2

Community discussion across Reddit, Mastodon, and other social sources.

EXTERNAL REFERENCES

MITRE CVE

cve.org · CVE-2026-12304

NVD

nvd.nist.gov/vuln/detail/CVE-2026-12304

MITRE CWE · CWE-346

Origin Validation Error

bugzilla.mozilla.org

bugzilla.mozilla.org/show_bug.cgi?id=2034944

mozilla.org

mozilla.org/security/advisories/mfsa2026-57

mozilla.org

mozilla.org/security/advisories/mfsa2026-58

mozilla.org

mozilla.org/security/advisories/mfsa2026-60

mozilla.org

mozilla.org/security/advisories/mfsa2026-61