CVE-2017-7820: Mozilla Firefox vulnerability

CVE-2017-7820: The "instanceof" operator can bypass the Xray wrapper mechanism.

The "instanceof" operator can bypass the Xray wrapper mechanism. When called on web content from the browser itself or an extension the web content can provide its own result for that operator, possibly tricking the browser or extension into mishandling the element. This vulnerability affects Firefox < 56.

UnknownCVSS not scoredNot KEV-listedUpdatedAug 5, 2024, 16:12 UTC (UTC+00:00)

Security readout for executives and security teams

CVE-2017-7820 is a Firefox browser isolation flaw. A web page could influence how privileged browser or extension code evaluates an object, potentially causing it to treat untrusted content incorrectly. The provided sources identify Firefox versions before 56 as affected, but do not provide CVSS scoring or confirmed exploitation. Exposure is limited to environments still running Firefox versions earlier than 56. Current managed fleets should verify browser inventory rather than assume absence, especially on legacy systems, kiosks, embedded desktops, or unmanaged endpoints. Treat this as a legacy-browser hygiene issue unless inventory shows Firefox before 56 in use. Prioritize remediation where vulnerable browsers handle internet content or business-critical workflows. Mitigation focus: Upgrade Firefox to version 56 or later according to Mozilla guidance.; Remove or isolate unsupported Firefox versions before 56 from enterprise endpoints.; Review browser extension governance for legacy Firefox deployments..

Prepared: Jun 7, 2026, 06:44 UTC (UTC+00:00)

Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.

Potential ATT&CK relevance

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Vulnerability profileCVE Program record

Severity: Unknown
CVSS: Not scored
Known Exploited: No
Published: Jun 11, 2018, 21:00 UTC (UTC+00:00)

Official CVE source material

CNA and ADP enrichment extracted from CVE v5

These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.

0CVSS vectors

0Timeline events

0ADP providers

3Source links

CVSS and timeline data

No CVSS vectors or timeline events were available in the normalized CVE source material.

Source materials

CVE List V5 sourceCVE List V5
https://bugzilla.mozilla.org/show_bug.cgi?id=1378207CVE reference · x_refsource_CONFIRM
https://www.mozilla.org/security/advisories/mfsa2017-21/CVE reference · x_refsource_CONFIRM

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus

MozillaFirefoxunspecifiedListed

Weakness

CWE details

No CWE listed

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