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Instacart SMS Link Spoofing Vulnerability

Medium

Synopsis

When purchasing an item from Instacart via the web interface, post-checkout, users that do not have the mobile app are given the option to have a download link sent to them once order tracking becomes available. When doing so, a user is able to control the link that is sent to them. The message generated by this endpoint to be sent to a user via SMS comes from the same phone numbers and services that all other legitimate Instacart SMS messages come from -- such as order updates, delivery notices, etc.

 

The affected domain is: https://www.instacart.com/

The affected endpoint is: "/welcome/request_invite?source=web&warehouse_id=<warehouse id>&zone_id=<zone id>"

 

Submitting a POST request to this endpoint with valid cookies, visitor ID, and order numbers with the following form-encoded parameters allows a malicious actor to send spoofed messages to any phone number with a link of their choice as Instacart.

 

phone=5551234567&link=https%3a%2f%2fattacker.com

 

There do appear to be some precautions in place that inadvertently mitigate some of the impact of this flaw, but they are relatively easy to bypass. For instance, this endpoint is not easily accessible as it requires a purchase to have been made to appear (we did, however, confirm arbitrary usage by placing an order to obtain the link and then cancelling the order). Additionally, after a short period of time, this endpoint does stop sending messages and requires a new session to be created, which limits the amount of messages that an attacker can send at once.

Solution

No action is required by end users.

Disclosure Timeline

April 24, 2020 - Issue discovered
April 24, 2020 - Tenable requests security contact via [email protected] and [email protected]
April 28, 2020 - Tenable requests security contact via [email protected]
April 28, 2020 - Instacart provides contact details
April 29, 2020 - Tenable discloses issue to vendor
May 1, 2020 - Instacart acknowledges flaw and states that the issue has been fixed.

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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