CVE-2021-4240 is a medium-severity (CVSS 5.3) CWE-331 vulnerability in phpservermon/phpservermon. 1 public exploit or proof-of-concept reference is indexed, so weaponization risk is real. A fix is available for phpservermon/phpservermon — see the affected versions and patch details below.
Insufficient Entropy in PHPServerMon PRNG
Exploitation Status
Proof-of-concept exploit code exists
- CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.
Exploitation and automatability from CISA’s SSVC triage for CVE-2021-4240.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
Where this sits among everything scored
Real-World Exposure
phpservermon/phpservermonReal-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
A vulnerability, which was classified as problematic, was found in phpservermon. This affects the function generatePasswordResetToken of the file src/psm/Service/User.php. The manipulation leads to use of predictable algorithm in random number generator. The exploit has been disclosed to the public and may be used. The name of the patch is 3daa804d5f56c55b3ae13bfac368bb84ec632193. It is recommended to apply a patch to fix this issue. The identifier VDB-213717 was assigned to this vulnerability.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐘Packagist | phpservermon/phpservermon | all versions | 3.6.0composer require phpservermon/phpservermon:^3.6.0 |
Affected Products
php server monitorphpservermonitorResearch use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for phpservermon/phpservermon, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update phpservermon/phpservermon to 3.6.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-4240 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.
Frequently Asked Questions
Is CVE-2021-4240 in your dependencies?
Find it across Packagist, including transitive dependencies.