Symphony Denial of Service Via Overlong Usernames

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

7.5 · CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

EPSS

• CVE-2016-4423: 1.86% estimated exploitation activity in the next 30 days; percentile 79.28 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

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.

Published by the source

17 May 2022, 03:54 UTC

Last modified by the source

25 Apr 2024, 22:08 UTC

Source last checked

26 Oct 2026, 14:20 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-whgv-8cg3-7hcm

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# Patch Next advisory brief: CVE-2016-4423

Advisory: Symphony Denial of Service Via Overlong Usernames
IDs: CVE-2016-4423, GHSA-whgv-8cg3-7hcm
Record: https://patchnext.com/advisories/cve-2016-4423 (revision 1, source checked 26 Oct 2026, 14:20 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 7.5)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: symfony/symfony 3.0.6, 2.8.6, 2.7.13, 2.3.41; symfony/security-http 3.0.6, 2.8.6, 2.7.13, 2.3.41; symfony/security 3.0.6, 2.8.6, 2.7.13, 2.3.41. A vendor fix does not mean a deployment is fixed.

## Affected components
- Packagist symfony/symfony: affected >= 2.3.0, < 2.3.41; >= 2.4.0, < 2.7.13; >= 2.8.0, < 2.8.6; >= 3.0.0, < 3.0.6; fixed in 2.8.6, 3.0.6, 2.7.13, 2.3.41
- Packagist symfony/security-http: affected >= 2.3.0, < 2.3.41; >= 2.4.0, < 2.7.13; >= 2.8.0, < 2.8.6; >= 3.0.0, < 3.0.6; fixed in 2.8.6, 3.0.6, 2.7.13, 2.3.41
- Packagist symfony/security: affected >= 2.3.0, < 2.3.41; >= 2.4.0, < 2.7.13; >= 2.8.0, < 2.8.6; >= 3.0.0, < 3.0.6; fixed in 2.8.6, 3.0.6, 2.7.13, 2.3.41

## Suggested inspection (read-only)
- Packagist symfony/symfony: check the installed version with your package manager.
- Packagist symfony/security-http: check the installed version with your package manager.
- Packagist symfony/security: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on symfony/symfony, symfony/security-http, symfony/security.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-whgv-8cg3-7hcm (GitHub-reviewed advisory GHSA-whgv-8cg3-7hcm lists affected ranges: Packagist symfony/security-http >=2.3.0 =2.4.0 =2.8.0 =3.0.0 =2.3.0 =2.4.0 =2.8.0 =3.0.0 =2.3.0 =2.4.0 =2.8.0 =3.0.0 <3.0.6)

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