---
id: CVE-2016-9063
title: An integer overflow during the parsing of XML using the Expat library
summary: >-
  An integer overflow during the parsing of XML using the Expat library. This
  vulnerability affects Firefox < 50.
severity: critical
cvss: 9.8
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H'
cwe:
  - CWE-190
vendor: mozilla
product: firefox
affected:
  - firefox < 50
  - debian_linux = 8.0
  - debian_linux = 9.0
  - debian_linux = 10.0
  - 'python >= 2.7.0, < 2.7.15'
  - 'python >= 3.3.0, < 3.3.7'
  - 'python >= 3.4.0, < 3.4.7'
  - 'python >= 3.5.0, < 3.5.4'
  - 'python >= 3.6.0, < 3.6.2'
patched:
  - firefox 50
  - python 3.6.2
published: '2018-06-11'
updated: '2026-10-08'
sourceUpdated: '2026-10-08T21:17:05.440'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2016-9063'
references:
  - url: 'http://www.securityfocus.com/bid/94337'
    label: security@mozilla.org
  - url: 'http://www.securitytracker.com/id/1037298'
    label: security@mozilla.org
  - url: 'http://www.securitytracker.com/id/1039427'
    label: security@mozilla.org
  - url: 'https://bugzilla.mozilla.org/show_bug.cgi?id=1274777'
    label: security@mozilla.org
  - url: 'https://www.debian.org/security/2017/dsa-3898'
    label: security@mozilla.org
  - url: 'https://www.mozilla.org/security/advisories/mfsa2016-89/'
    label: security@mozilla.org
  - url: 'http://www.securityfocus.com/bid/94337'
    label: af854a3a-2127-422b-91ae-364da2661108
  - url: 'http://www.securitytracker.com/id/1037298'
    label: af854a3a-2127-422b-91ae-364da2661108
  - url: 'http://www.securitytracker.com/id/1039427'
    label: af854a3a-2127-422b-91ae-364da2661108
  - url: 'https://bugzilla.mozilla.org/show_bug.cgi?id=1274777'
    label: af854a3a-2127-422b-91ae-364da2661108
  - url: 'https://www.debian.org/security/2017/dsa-3898'
    label: af854a3a-2127-422b-91ae-364da2661108
  - url: 'https://www.mozilla.org/security/advisories/mfsa2016-89/'
    label: af854a3a-2127-422b-91ae-364da2661108
tags:
  - nvd
  - cve.org
epss: 0.05472
epssPercentile: 0.9259
ingestedAt: '2026-10-08T22:11:53.693Z'
ssvc:
  exploitation: none
  automatable: 'yes'
  technicalImpact: total
  timestamp: '2026-10-08T20:58:11.571362Z'
---

## Overview

An integer overflow during the parsing of XML using the Expat library. This vulnerability affects Firefox < 50.

## Affected

- `firefox < 50`
- `debian_linux = 8.0`
- `debian_linux = 9.0`
- `debian_linux = 10.0`
- `python >= 2.7.0, < 2.7.15`
- `python >= 3.3.0, < 3.3.7`
- `python >= 3.4.0, < 3.4.7`
- `python >= 3.5.0, < 3.5.4`
- `python >= 3.6.0, < 3.6.2`

## Remediation

Upgrade past the affected range:

- `firefox 50`
- `python 3.6.2`