CVE-2021-21424: Prevent user enumeration in authentication mechanisms

Affected versions

Symfony >=2.8.0, <3.4.48 || >= 4.0.0, <4.4.23 || >= 5.0.0, <5.2.8 versions of
the Symfony Security, Security Guard, Security Core,
and Security HTTP components are affected by this security issue.

The issue has been fixed in Symfony 3.4.48, 4.4.23, 5.2.8, and 5.3.0 beta4. All
other affected minor versions of Symfony won’t be patched as they are not
maintained anymore.


The ability to enumerate users was possible without relevant permissions due to
different exception messages depending on whether the user existed or not. It
was also possible to enumerate users by using a timing attack, by comparing
time elapsed when authenticating an existing user and authenticating a
non-existing user.

We now ensure that 403s are returned whether the user exists or not if the
password is invalid or if the user does not exist.

The patch for this issue is available here
for branch 3.4.


I would like to thank James Isaac and Mathias Brodala for reporting the issue
and Robin Chalas for fixing the issue.

Sponsor the Symfony project.

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