CVE-2016-4423 is a high-severity (CVSS 7.5) CWE-399 vulnerability in symfony/security-http. A fix is available for symfony/security-http — see the affected versions and patch details below.
Symphony Denial of Service Via Overlong Usernames
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-2016-4423 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-http🐘symfony/security-http🐘symfony/security-http🐘symfony/security-http🐘symfony/security🐘symfony/security🐘symfony/security🐘symfony/security+4 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
The attemptAuthentication function in Component/Security/Http/Firewall/UsernamePasswordFormAuthenticationListener.php in Symfony before 2.3.41, 2.7.x before 2.7.13, 2.8.x before 2.8.6, and 3.0.x before 3.0.6 does not limit the length of a username stored in a session, which allows remote attackers to cause a denial of service (session storage consumption) via a series of authentication attempts with long, non-existent usernames.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐘Packagist | symfony/security-http | ≥ 2.3.0&&< 2.3.41 | 2.3.41composer require symfony/security-http:^2.3.41 |
| 🐘Packagist | symfony/security-http | ≥ 2.4.0&&< 2.7.13 | 2.7.13composer require symfony/security-http:^2.7.13 |
| 🐘Packagist | symfony/security-http | ≥ 2.8.0&&< 2.8.6 | 2.8.6composer require symfony/security-http:^2.8.6 |
| 🐘Packagist | symfony/security-http | ≥ 3.0.0&&< 3.0.6 | 3.0.6composer require symfony/security-http:^3.0.6 |
| 🐘Packagist | symfony/security | ≥ 2.3.0&&< 2.3.41 | 2.3.41composer require symfony/security:^2.3.41 |
| 🐘Packagist | symfony/security | ≥ 2.4.0&&< 2.7.13 | 2.7.13composer require symfony/security:^2.7.13 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for symfony/security-http, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update symfony/security-http to 2.3.41 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2016-4423 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-2016-4423 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2016-4423. 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-2016-4423 in your dependencies?
O3 Security finds CVE-2016-4423 across Packagist dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.