CVE-2019-18886: Prevent user enumeration using switch user functionality
Symfony 4.2.0 to 4.2.11 and 4.3.0 to 4.3.7 versions of the Symfony Security/Http component are affected by this security issue.
The issue has been fixed in Symfony 4.2.12 and 4.3.8.
Note that no fixes are provided for Symfony 4.1 as they are not maintained anymore.
The ability to enumerate users was possible without relevant permissions due to different handling depending on whether the user existed or not when attempting to use the switch users functionality.
We now ensure that 403s are returned whether the user exists or not if a user cannot switch to a user or if the user does not exist.
The patch for this issue is available here for branch 4.2.
I would like to thank Matt Daum for reporting & Nicolas Grekas for fixing the issue.
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