CVE-2021-21424 is a medium-severity (CVSS 5.3) Information Exposure vulnerability in symfony/security. A fix is available for symfony/security — see the affected versions and patch details below.
Prevent user enumeration using Guard or the new Authenticator-based Security
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
CVE-2021-21424 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 377,238 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
Real-World Exposure
symfony/security🐘symfony/security-guard🐘symfony/security-guard🐘symfony/security-guard🐘symfony/security-core🐘symfony/security-core🐘symfony/security-core🐘lexik/jwt-authentication-bundle+9 moreReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects Packagist packages — download data is not available via public APIs for these ecosystems.
Description
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.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐘Packagist | symfony/security | ≥ 5.0.0&&< 5.2.8 | 5.2.8composer require symfony/security:^5.2.8 |
| 🐘Packagist | symfony/security-guard | ≥ 2.8.0&&< 3.4.48 | 3.4.48composer require symfony/security-guard:^3.4.48 |
| 🐘Packagist | symfony/security-guard | ≥ 4.0.0&&< 4.4.23 | 4.4.23composer require symfony/security-guard:^4.4.23 |
| 🐘Packagist | symfony/security-guard | ≥ 5.0.0&&< 5.2.8 | 5.2.8composer require symfony/security-guard:^5.2.8 |
| 🐘Packagist | symfony/security-core | ≥ 2.8.0&&< 3.4.48 | 3.4.48composer require symfony/security-core:^3.4.48 |
| 🐘Packagist | symfony/security-core | ≥ 4.0.0&&< 4.4.23 | 4.4.23composer require symfony/security-core:^4.4.23 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for symfony/security, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update symfony/security to 5.2.8 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-21424 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2021-21424 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2021-21424. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Fixing This On Your OS
If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.
Frequently Asked Questions
Is CVE-2021-21424 in your dependencies?
O3 Security finds CVE-2021-21424 across Packagist dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.