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Fencer Vulns
medium 5.3

CVE-2021-21424 json

Prevent user enumeration using Guard or the new Authenticator-based Security

published 2021-05-13 5 y ago modified 2026-09-10 1 mo ago also BIT-symfony-2021-21424 , EUVD-2021-0960 , GHSA-5pv8-ppvj-4h68

cvss
5.3
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
1.6%
EPSS 1.6% chance of exploitation in 30 days, percentile 76%
public exploits
0
none known

Summary

Symfony is a PHP framework for web and console applications and a set of reusable PHP components. 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 for branch 3.4.

Patch / fix

  • packagist symfony/security 3.4.49, 4.4.24, 5.2.8
  • packagist symfony/security-guard 3.4.48, 4.4.23, 5.2.8
  • packagist symfony/security-core 3.4.48, 4.4.23, 5.2.8
  • packagist lexik/jwt-authentication-bundle 2.10.7, 2.11.3
  • packagist symfony/maker-bundle 1.29.2, 1.31.1
  • packagist symfony/security-http 5.2.8
  • packagist symfony/symfony 3.4.49, 4.4.24, 5.2.9
  • bitnami symfony 3.4.48, 4.4.23, 5.2.8
  • sensiolabs symfony 3.4.48, 4.4.23, 5.2.8
  • symfony Debian 14, Debian 13, Debian 12 4.4.19+dfsg-2

debian advisories DEBIAN-CVE-2021-21424

  • symfony Ubuntu Pro 18.04 LTS 3.4.6+dfsg-1ubuntu0.1+esm2
  • symfony Ubuntu Pro 20.04 LTS 4.3.8+dfsg-1ubuntu1+esm1

ubuntu advisories UBUNTU-CVE-2021-21424

Ubuntu security data © Canonical Ltd., from https://github.com/canonical/ubuntu-security-notices via OSV; changed: normalised to fixed package versions per release. CC BY-SA 4.0

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2021-21424 updated cvss 5.3

php-symfony: user enumeration in authentication mechanisms

  • Red Hat All currently supported Red Hat products
    not affected: vulnerable code not present

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

Exploit prediction

1.64% chance of exploitation in the next 30 days, higher than 76% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by version score vector
CNA 3.1 5.3 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N
GitHub 3.1 5.3 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N
NVD 3.1 5.3 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N
NVD 2.0 5.0 AV:N/AC:L/Au:N/C:P/I:N/A:N
OSV 3.1 5.3 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N

Affected packages

ecosystem package affected fixed in
packagist symfony/security
>=5.0.0, <5.2.8
>=2.8.0, <3.4.49
>=4.0.0, <4.4.24
5.2.8
3.4.49
4.4.24
packagist symfony/security-guard
>=2.8.0, <3.4.48
>=4.0.0, <4.4.23
>=5.0.0, <5.2.8
3.4.48
4.4.23
5.2.8
packagist symfony/security-core
>=2.8.0, <3.4.48
>=4.0.0, <4.4.23
>=5.0.0, <5.2.8
3.4.48
4.4.23
5.2.8
packagist lexik/jwt-authentication-bundle
>=2.0.0, <2.10.7
>=2.11.0, <2.11.3
2.10.7
2.11.3
packagist symfony/maker-bundle
>=1.27.0, <1.29.2
>=1.30.0, <1.31.1
1.29.2
1.31.1
packagist symfony/security-http
>=5.1.0, <5.2.8
5.2.8
packagist symfony/symfony
>=2.8.0, <3.4.49
>=4.0.0, <4.4.24
>=5.0.0, <5.2.9
3.4.49
4.4.24
5.2.9
bitnami symfony
>=2.8.0, <3.4.48
>=4.0.0, <4.4.23
>=5.0.0, <5.2.8
3.4.48
4.4.23
5.2.8

Affected products

  • symfony symfony

Classification

Weakness
  • CWE-200 Exposure of Sensitive Information to an Unauthorized Actor
  • CWE-203 Observable Discrepancy

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.