Suddenly started using another logged in user's session

[systems-diplomatic]

Preconditions (*)

1. Magento 2.2.6 (but we have also seen it on 2.2.4)
2. FPC enabled or disabled
3. SID NOT included in URL
4. At least one item (doesn't matter which one) needs to be in the shopping cart in two user's sessions

Steps to reproduce (*)

1. Click on link to view cart (not minicart) or go to checkout

Expected result (*)

1. The user must only see their own details, none from another user.

Actual result (*)

1. You may see data from another user's session including their cart contents.
2. From this point the user will see all the information from the other user's session including account information, order history, etc.

[magento-engcom-team]

Hi @systems-diplomatic. Thank you for your report.
To help us process this issue please make sure that you provided the following information:

• Summary of the issue
• Information on your environment
• Steps to reproduce
• Expected and actual results

Please make sure that the issue is reproducible on the vanilla Magento instance following Steps to reproduce. To deploy vanilla Magento instance on our environment, please, add a comment to the issue:

@magento-engcom-team give me $VERSION instance

where $VERSION is version tags (starting from 2.2.0+) or develop branches (for example: 2.3-develop).
For more details, please, review the Magento Contributor Assistant documentation.

@systems-diplomatic do you confirm that you was able to reproduce the issue on vanilla Magento instance following steps to reproduce?

• yes
• no

[systems-diplomatic]

@magento-engcom-team give me 2.2.6 instance

[magento-engcom-team]

Hi @systems-diplomatic. Thank you for your request. I'm working on Magento 2.2.6 instance for you

[magento-engcom-team]

Hi @systems-diplomatic, here is your Magento instance.
Admin access: http://34.228.235.121/i-19211-2-2-6//admin
Login: admin Password: 123123q
Instance will be terminated in up to 3 hours.

[magento-engcom-team]

Hi @engcom-backlog-nazar. Thank you for working on this issue.
In order to make sure that issue has enough information and ready for development, please read and check the following instruction: 👇

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[ghost]

Hi @systems-diplomatic thank you for you report, i'm not able to reproduce following steps you described, i'm really don't know in which cases you have the actual result. this may bge related to your custom code.

[ghost]

@systems-diplomatic, also take a look this commit, #17608

[Jilco]

@systems-diplomatic, wat was the root cause and how did you solve this? I'm facing the same issue

[systems-diplomatic]

@Jilco, we installed redis to handle the sessions and finally the problem went away, however we never really found the root cause.

[Jilco]

I think i found it, it is in engintron NGINX. The micro cache is the problem