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