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Cisco Webex Universal Links Redirect

Medium

Synopsis

Cisco WebEx for macOS, in particular the Cisco Start WebEx component, does not properly validate the URLs provided with the "webexstart://" (and potentially others, such as "wbx://") protocol handlers. It is possible, though unlikely, that iOS applications are also affected in some manner, but we were unable to test these components to confirm.

A typical protocol handler used for launching WebEx can be found in meeting invite/launch pages and looks similar to the following:

webexstart: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?t=99999999999999&t1=%URLProtocolLaunchTime%&sip=[email protected]&p=eyJ1dWlkIjoiNGVjYjdlNTJhODI3NGYzN2JlNDFhZWY1NTMxZDg3MmMiLCJjdiI6IjQxLjYuNC44IiwiY3dzdiI6IjExLDQxLDA2MDQsMjEwNjA4LDAiLCJzdCI6Ik1DIiwibXRpZCI6Im02NjkyMGNlNzJkMzYwMGEyNDZiMWUxMGE4YWY5MmJkNyIsInB2IjoiVDMzXzY0VU1DIiwiY24iOiJBVENPTkZVSS5CVU5ETEUiLCJmbGFnIjozMzU1NDQzMiwiZWpmIjoiMiIsImNwcCI6ImV3b2dJQ0FnSW1OdmJXMXZiaUk2SUhzS0lDQWdJQ0FnSUNBaVJHVnNZWGxTWldScGNtVmpkQ0k2SUNKMGNuVmxJZ29nSUNBZ2ZTd0tJQ0FnSUNKM1pXSmxlQ0k2SUhzS0lDQWdJQ0FnSUNBaVNtOXBia1pwY25OMFFteGhZMnRNYVhOMElqb2dXd29nSUNBZ0lDQWdJQ0FnSUNBZ0lDQWdJalF4TGpRaUxBb2dJQ0FnSUNBZ0lDQWdJQ0FnSUNBZ0lqUXhMalVpQ2lBZ0lDQWdJQ0FnWFFvZ0lDQWdmU3dLSUNBZ0lDSmxkbVZ1ZENJNklIc0tDaUFnSUNCOUxBb2odJQ0FnSW5SeVlXbHVhVzVuSWpvZ2V3b0tJQ0FnSUgwc0NpQWdJQ0FpYzNWd2NHOXlkQ0k2SUhzS0lDQWdJQ0FnSUNBaVIzQmpRMjl0Y0c5dVpXNTBUbUZ0WlNJNklDSkRhWE5qYnlCWFpXSmxlQ0JUZFhCd2IzSjBMbUZ3Y0NJS0lDQWdJSDBLZlFvPSIsInVsaW5rIjoiYUhSMGNITTZMeTl0WldWME1URXpMbmRsWW1WNExtTnZiUzkzWW5odGFuTXZhbTlwYm5ObGNuWnBZMlV2YzJsMFpYTXZiV1ZsZERFeE15OXRaV1YwYVc1bkwzTmxkSFZ3ZFc1cGRtVnljMkZzYkdsdWEzTS9jMmwwWlhWeWJEMXRaV1YwTVRFekptMWxaWFJwYm1kclpYazlNVGd5TWpnMk5qTTBOeVpqYjI1MFpYaDBTVVE5YzJWMGRYQjFibWwyWlhKellXeHNhVzVyWHpBek16azFZamN3WmpjMU1UUmpPR1U0TTJJek5qZ3lNV1V4T1dZd05UVXlYekUyTWpNNU5UQTBNVGMzTURZbWRHOXJaVzQ5VTBSS1ZGTjNRVUZCUVZoWVlqVkVMVTFtTUZKZlVXcHFka3BTWkdacmJFRmFZVzkxY1Voa1RYbHVjSFppWHpCS1IyeFJhVEYzTWlac1lXNW5kV0ZuWlQxbGJsOVZVdz09IiwidXRvZ2dsZSI6IjEiLCJtZSI6IjEiLCJqZnYiOiIxIiwidGlmIjoiUEQ5NGJXd2dkbVZ5YzJsdmJqMGlNUzR3SWlCbGJtTnZaR2x1WnowaVZWUkdMVGdpUHo0S1BGUmxiR1ZOWlhSeWVVbHVabTgrUEUxbGRISnBZM05GYm1GaWJHVStNVHd2VFdWMGNtbGpjMFZ1WVdKc1pUNDhUV1YwY21samMxVlNURDVvZEhSd2N6b3ZMM1J6WVRNdWQyVmlaWGd1WTI5dEwyMWxkSEpwWXk5Mk1Ud3ZUV1YwY21samMxVlNURDQ4VFdWMGNtbGpjMUJoY21GdFpYUmxjbk0rUEUxbGRISnBZM05VYVdOclpYUStVbnBJTHk5M1FVRkJRVmhqUkhCSlFTOVFja0ZWSzJGeWFXTnliVEF3TlRjMVpubFZUM0EwVFc4d1NrTnpWVXh0V2pKR1IyTkJQVDA4TDAxbGRISnBZM05VYVdOclpYUStQRU52Ym1aSlJENHhPVGN4T1RnME5UYzBNakkzTnpJek5EYzhMME52Ym1aSlJENDhVMmwwWlVsRVBqRTBNakkyTXpZeVBDOVRhWFJsU1VRK1BGUnBiV1ZUZEdGdGNENHhOakl6T0RZME1ERTNOekEzUEM5VWFXMWxVM1JoYlhBK1BFRlFVRTVoYldVK1UyVnpjMmx2Ymt0bGVUd3ZRVkJRVG1GdFpUNDhMMDFsZEhKcFkzTlFZWEpoYldWMFpYSnpQanhOWlhSeWFXTnpSVzVoWW14bFRXVmthV0ZSZFdGc2FYUjVSWFpsYm5RK01Ud3ZUV1YwY21samMwVnVZV0pzWlUxbFpHbGhVWFZoYkdsMGVVVjJaVzUwUGp3dlZHVnNaVTFsZEhKNVNXNW1iejQ9In0=

While there are several components to this URL, we'll focus on the last one - 'p'. 'p' is a base64 encoded string that contains settings information such as support app information, telementry configurations, and the information required to setup Universal Links for macOS. When decoding the above, we can see that 'p' decodes to:

{"uuid":"8e18fa93cd10432a907c94fb9d3a63e6","cv":"41.6.4.8","cwsv":"11,41,0604,210608,0","st":"MC","pv":"T33_64UMC","cn":"ATCONFUI.BUNDLE","flag":33554432,"ejf":"2","cpp":"ewogICAgImNvbW1vbiI6IHsKICAgICAgICAiRGVsYXlSZWRpcmVjdCI6ICJ0cnVlIgogICAgfSwKICAgICJ3ZWJleCI6IHsKICAgICAgICAiSm9pbkZpcnN0QmxhY2tMaXN0IjogWwogICAgICAgICAgICAgICAgIjQxLjQiLAogICAgICAgICAgICAgICAgIjQxLjUiCiAgICAgICAgXQogICAgfSwKICAgICJldmVudCI6IHsKCiAgICB9LAogICAgInRyYWluaW5nIjogewoKICAgIH0sCiAgICAic3VwcG9ydCI6IHsKICAgICAgICAiR3BjQ29tcG9uZW50TmFtZSI6ICJDaXNjbyBXZWJleCBTdXBwb3J0LmFwcCIKICAgIH0KfQo=","ulink":"aHR0cHM6Ly9tZWV0MTEzLndlYmV4LmNvbS93YnhtanMvam9pbnNlcnZpY2Uvc2l0ZXMvbWVldDExMy9tZWV0aW5nL3NldHVwdW5pdmVyc2FsbGlua3M/c2l0ZXVybD1tZWV0MTEzJm1lZXRpbmdrZXk9MTgyMDIxMDYwOCZjb250ZXh0SUQ9c2V0dXB1bml2ZXJzYWxsaW5rXzNlNjNjZDFlODcyMzRlOTE4OWU2OWM2NjI2MDcxMzBiXzE2MjQwMjA4ODUwNTImdG9rZW49U0RKVFN3QUFBQVd4c0pGelhzSW1Da2l3aHQya2t4TE1WWFdJVFZpTTh4OWVnUWJlejVUaWhBMiZsYW5ndWFnZT1lbl9VUw==","utoggle":"1","me":"1","jfv":"1","tif":"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"}

Further decoding the 'ulink' parameter gets us:

https://meet113.webex.com/wbxmjs/joinservice/sites/meet113/meeting/setupuniversallinks?siteurl=meet113&meetingkey=1820210608&contextID=setupuniversallink_3e63cd1e87234e9189e69c662607130b_1624020885052&token=SDJTSwAAAAWxsJFzXsImCkiwht2kkxLMVXWITViM8x9egQbez5TihA2&language=en_US

When the protocol handler URL is invoked, particularly from a non-Apple browser like Google Chrome, macOS will pass the URL to the configured protocol handler - Cisco WebEx Start. Cisco WebEx Start will parse the URL and launch WebEx Meetings or perform some other valid action. When the meeting is closed, however, WebEx Meetings will open Safari as a means to configure Universal Links for macOS. The kicker here is that it opens Safari at the link provided in the 'ulink' parameter. By modifying this link and executing the protocol handler, we can see that we can force Safari to open at a link of our choosing, which could potentially be malicious.

For example, the following URL will open WebEx, and upon closing the application, Safari will be opened to https://tenable.com:

webexstart: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?t=99999999999999&t1=%URLProtocolLaunchTime%&sip=[email protected]&p=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

A gif has been attached demonstrating this functionality.

It may also be possible for a specially crafted URL to contain modified domains used for telemetry data, debug information, or other configurable options, which could lead to possible information disclosures. Our suggestion is to have Cisco WebEx Start, or another component, properly validate the URLs provided within the protocol handler.

We want to emphasize that the flaw described above is relatively complex as it requires user interaction and is of relatively low impact. For starters, this attack requires an attacker to trick a user into visiting a malicious link (providing a fake meeting invite via a custom domain for example) and then allowing WebEx to launch from their browser. In this case, we already have an attacker getting someone to visit a possibly malicious link. In general, we wouldn't report this sort of issue due to no security boundary being crossed. In this case, however, there is security boundary being crossed in that we are able to force the victim to open a malcious link with a specific browser (Safari), which would allow an attacker to specially craft payloads for that target browser.

Our recommendation for a fix would be to have WebEx components properly validate URLs supplied via the URL protocol handler links.

Solution

Apply vendor supplied update.

Disclosure Timeline

June 23, 2021 - Tenable discloses to vendor. Cisco acknowledges
June 25, 2021 - Cisco provides information of dedicated contact.
July 19, 2021 - Tenable requests status update. Cisco states that they've requested the information internally.
August 5, 2021 - Cisco provides status update.

All information within TRA advisories is provided “as is”, without warranty of any kind, including the implied warranties of merchantability and fitness for a particular purpose, and with no guarantee of completeness, accuracy, or timeliness. Individuals and organizations are responsible for assessing the impact of any actual or potential security vulnerability.

Tenable takes product security very seriously. If you believe you have found a vulnerability in one of our products, we ask that you please work with us to quickly resolve it in order to protect customers. Tenable believes in responding quickly to such reports, maintaining communication with researchers, and providing a solution in short order.

For more details on submitting vulnerability information, please see our Vulnerability Reporting Guidelines page.

If you have questions or corrections about this advisory, please email [email protected]

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