CVE-2021-21424
MEDIUMCVE-2021-21424 is a medium-severity (CVSS 5.3) Information Exposure vulnerability in symfony/security. O3 Security confirms whether CVE-2021-21424 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
Prevent user enumeration using Guard or the new Authenticator-based Security
Blast Radius
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 | 5.2.8 |
| 🐘Packagist | symfony/security-guard | ≥ 2.8.0&&< 3.4.48 | 3.4.48 |
| 🐘Packagist | symfony/security-guard | ≥ 4.0.0&&< 4.4.23 | 4.4.23 |
| 🐘Packagist | symfony/security-guard | ≥ 5.0.0&&< 5.2.8 | 5.2.8 |
| 🐘Packagist | symfony/security-core | ≥ 2.8.0&&< 3.4.48 | 3.4.48 |
| 🐘Packagist | symfony/security-core | ≥ 4.0.0&&< 4.4.23 | 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. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
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 pinpoints whether CVE-2021-21424 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
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.
Frequently Asked Questions
Is CVE-2021-21424 in your dependencies?
O3 detects CVE-2021-21424 across Packagist dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.