Avast Secure Browser 76.0.1659.101 Local Privilege Escalation

Credit: Silton Santos
Risk: Medium
Local: Yes
Remote: No

CVSS Base Score: 7.2/10
Impact Subscore: 10/10
Exploitability Subscore: 3.9/10
Exploit range: Local
Attack complexity: Low
Authentication: No required
Confidentiality impact: Complete
Integrity impact: Complete
Availability impact: Complete

=====[ Tempest Security Intelligence - ADV-01/2020 ]========================== Avast Secure Browser 76.0.1659.101 Author: Silton Santos Tempest Security Intelligence - Recife, Pernambuco - Brazil =====[ Table of Contents]===================================================== * Overview * Detailed description * Timeline of disclosure * Thanks & Acknowledgements * References =====[ Vulnerability Information]============================================= * Class: Improper Access Control[CWE-284][1] * CVE-2019-17190[2] =====[Overview]====================================================================== * System affected : Avast Secure Browser [3] * Software Version : 76.0.1659.101 * Impact : An unprivileged user could obtain SYSTEM privileges. =====[ Detailed description]========================================================== A Local Privilege Escalation issue was discovered in Avast Secure Browser 76.0.1659.101. The vulnerability is due to an insecure ACL set by the AvastBrowserUpdate.exe (which is running as NT AUTHORITY\SYSTEM) when AvastSecureBrowser.exe checks for new updates. When the update check is triggered, the elevated process cleans the ACL of the Update.ini file in %PROGRAMDATA%\Avast Software\Browser\Update\ and sets all privileges to group Everyone. Because any low-privileged user can create, delete, or modify the Update.ini file stored in this location, an attacker with low privileges can create a hard link named Update.ini in this folder, and make it point to a file writable by NT AUTHORITY\SYSTEM. Once AvastBrowserUpdate.exe is triggered by the update check functionality, the DACL is set to a misconfigured value on the crafted Update.ini and, consequently, to the target file that was previously not writable by the low-privileged attacker. More Details: https://sidechannel.tempestsi.com/vulnerability-in-avast-secure-browser-enables-escalation-of-privileges-on-windows-eb770d196c45 =====[ Timeline of disclosure]======================================================= * 23/Aug/2019 — Responsible disclosure is started with Avast; * 26/Aug/2019 — Vulnerability analysis is started; * 15/Sep/2019 — Vulnerability is confirmed by Avast which initiates correction; * 20/Dec/2019 — Avast informs that it is performing the final checks and that the patch is scheduled for 20/Jan/2020; * 20/Dec/2019 — Avast thanks all the support provided and asks for a name to carry out a public thank you; * 20/Jan/2020 — Avast communicates that there is a public release with the fixed vulnerability; * 21/Jan/2020 — Avast releases a thank you note for all the given support =====[ Thanks & Acknowledgements]==================================================== - Tempest Security Intelligence [4] =====[ References ]=========================================================== [1] https://cwe.mitre.org/data/definitions/284.html [2] https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2019-17190 [3] https://www.avast.com/pt-br/index#pc [4] http://www.tempest.com.br =====[ EOF ]====================================================================

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