---
title: "CVE-2017-7820\n    | Ubuntu"
description: Ubuntu is an open source software operating system that runs from the
  desktop, to the cloud, to all your internet connected things.
url: https://ubuntu.com/security/CVE-2017-7820?format=md
keywords: index, follow
---

# CVE-2017-7820

Publication date 2 October 2017

Last updated 25 August 2025

---

Ubuntu priority

**Low**

[Why this priority?](https://ubuntu.com/security/cves/about#priority )

## Cvss 3 Severity Score

**5.3 · Medium**

[Score breakdown](https://ubuntu.com/security/CVE-2017-7820?format=md#impact-score)

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## Description

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.

## Status

Show unmaintained releases

| Package | Ubuntu Release | Status |
| --- | --- | --- |
| firefox | 18.04 LTS bionic | Fixed 56.0+build6-0ubuntu1 |
| 17.10 artful | Fixed 56.0+build6-0ubuntu1 |
| 17.04 zesty | Fixed 56.0+build6-0ubuntu0.17.04.1 |
| 16.04 LTS xenial | Fixed 56.0+build6-0ubuntu0.16.04.1 |
| 14.04 LTS trusty | Fixed 56.0+build6-0ubuntu0.14.04.1 |

---

* [How can I get the fixes?](https://ubuntu.com/security/cves/about#security)
* [What do statuses mean?](https://ubuntu.com/security/cves/about#statuses)

## Severity score breakdown

CVSS version:
CVSS v3.0

**Base score**

5.3 · Medium

* Base metrics

  | Parameter | Value |
  | --- | --- |
  | Attack vector | Network |
  | Attack complexity | Low |
  | Privileges required | None |
  | User interaction | None |
  | Scope | Unchanged |
  | Confidentiality impact | None |
  | Integrity impact | Low |
  | Availability impact | None |
* Scores

  | Parameter | Value |
  | --- | --- |
  | Base score | 5.3 · Medium |
  | Exploitability score | - |
  | Impact score | - |

**Vector:** CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N

## References

* [MITRE](https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2017-7820)
* [NVD](https://nvd.nist.gov/vuln/detail/CVE-2017-7820)
* [Launchpad](https://launchpad.net/bugs/cve/CVE-2017-7820)
* [Debian](https://security-tracker.debian.org/tracker/CVE-2017-7820)

### Related Ubuntu Security Notices (USN)

+ [USN-3435-1](https://usn.ubuntu.com/USN-3435-1)
+ Firefox vulnerabilities
+ 2 October 2017

### Other references

* <https://www.mozilla.org/en-US/security/advisories/mfsa2017-21/>
* <https://www.cve.org/CVERecord?id=CVE-2017-7820>