Proxy auto-config (PAC) files have access to full HTTPS URLs

ID VU:877625
Type cert
Reporter CERT
Modified 2017-07-11T00:00:00



Web proxy auto-config (PAC) files are passed the full HTTPS URL in GET requests which may expose sensitive data.


CWE-212: Improper Cross-boundary Removal of Sensitive Data - CVE-2016-5134 (Google), CVE-2016-1801 (Apple)

Web proxy auto-configuration files (proxy.pac) have access to the full URL including the path and parameters in HTTPS GET requests, which may expose sensitive data intended to be protected by HTTPS. This information is passed to the FindProxyForURL() function in the proxy.pac. The PAC file is often retrieved by the browser automatically using the WPAD protocol. An attacker in the position to conduct man-in-the-middle attacks may provide a malicious PAC file capable of exploiting the FindProxyForURL() function to exfiltrate sensitive data.


An attacker who can provide a specially crafted PAC file can read URLs, including the path and query string, which may contain sensitive information intended to be protected by HTTPS.


Apply an update.

Apply the latest updates to your browser, see Vendor Information section below.

Users who are unable to or do not wish to update their browsers should consider the following workaround.

Disable WPAD.

If proxy auto-configuration is not necessary, consider disabling WPAD functionality for your browser.

Vendor Information

This vendors listed below are suspected to be affected by the vulnerability. Other browser vendors not listed may be affected as well. The CERT/CC has no further evidence that any particular vendor is impacted unless marked Affected; vendors are encouraged to reach out to us to clarify their status.

Vendor| Status| Date Notified| Date Updated
Apple| | 27 Jul 2016| 04 Aug 2016
Google| | 27 Jul 2016| 04 Aug 2016
Mozilla| | 27 Jul 2016| 04 Aug 2016
Opera| | 27 Jul 2016| 04 Aug 2016
Microsoft Corporation| | 27 Jul 2016| 11 Jul 2017
If you are a vendor and your product is affected, let us know.

CVSS Metrics

Group | Score | Vector
Base | 2.9 | AV:A/AC:M/Au:N/C:P/I:N/A:N
Temporal | 2.3 | E:POC/RL:OF/RC:C
Environmental | 1.7 | CDP:ND/TD:M/CR:ND/IR:ND/AR:ND


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Thanks to Bas Venis for reporting this vulnerability. We also would like to thank Itzik Kotler and Amit Klein for their presentation at Black Hat 2016, and Alex Chapman and Paul Stone for their presentation at DEF CON 24

This document was written by Trent Novelly.

Other Information

  • CVE IDs: CVE-2016-5134 CVE-2016-1801
  • Date Public: 04 Aug 2016
  • Date First Published: 04 Aug 2016
  • Date Last Updated: 11 Jul 2017
  • Document Revision: 24