Microsoft Internet Information Services (IIS) server 4.0 SP4, without certain hotfixes released for SP4, does not require authentication credentials under certain conditions, which allows remote attackers to bypass authentication requirements, as demonstrated by connecting via Microsoft Visual InterDev 6.0.
Conclusion & alert: CVE-1999-1591 is rated Moderate Risk (62.1/100): CVSS High severity, with high exploitation likelihood (EPSS 11.27%, 95th percentile). Core evidence: EPSS ranks this CVE among the most likely to be exploited in the near term. Mandatory action: High exploitation likelihood—assess exposure and prioritize remediation.
Risk is dynamic; we continuously reassess and refresh what is shown on this page as upstream context changes.
EPSS lead: Daily EPSS estimates relative likelihood of exploitation; percentile ranks this CVE among scored vulnerabilities (higher = more severe relative rank).
| # | Date | Old EPSS score | New EPSS score | Delta (New - Old) |
|---|---|---|---|---|
| 1 | 2026-06-15 | 12.55% | 11.27% | -1.28% |
| 2 | 2026-04-02 | 10.47% | 12.55% | +2.08% |
| 3 | 2026-03-18 | — | 10.47% | — |
Full EPSS history (25 records total)
CVSS metrics for this CVE.
| Base score | Version | Severity | Vector | Exploitability | Impact | Score source |
|---|---|---|---|---|---|---|
| 7.5 | 2.0 | HIGH |
|
10.0 | 6.4 | [email protected] |
| Vendor | Product | Version | Raw CPE |
|---|---|---|---|
| microsoft | internet_information_server | 4.0 | cpe:2.3:a:microsoft:internet_information_server:4.0:sp4:*:*:*:*:*:* |
| microsoft | visual_interdev | 6.0 | cpe:2.3:a:microsoft:visual_interdev:6.0:*:*:*:*:*:*:* |