CVE-1999-0867: Denial of service in IIS 4.0 via a flood of HTTP requests with malformed headers.

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MEDIUM 5.0WATCHLISTDOWNGRADEDOWNGRADEDOWNGRADE

CVE-1999-0867 is a medium-severity flaw (CVSS 5.0) affecting commercial internet system, internet information server, site server. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

Denial of service in IIS 4.0 via a flood of HTTP requests with malformed headers.

Why this priority

EPSS top 2%: very likely to be exploited
Public exploit reference
On your watchlist

No fix is published yet: follow the downgrade or mitigation steps.

Risk facts

CVSS score	5.0 (Medium)
EPSS exploit chance	21.5% (top 2%)
CISA exploited list	No
Weakness	CWE-20
Published	1999-08-11
Last modified	2026-06-16

Publisher (CNA): MITRE · Vendor: microsoft

Last updated 2026-06-16 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

1999-08-11 04:00 UTCPublished CVE record published
2026-06-16 21:49 UTCUpdated Record last modified

microsoft commercial internet system DOWNGRADE

Affected: 2.0 · 2.5

Fixed in: no fixed release known

Safe to downgrade to: a release before 2.0

1. Check installed version

apt-cache policy commercial-internet-system || rpm -q commercial-internet-system || commercial-internet-system --version

2. No fixed release: DOWNGRADE to a release before 2.0 (Debian / Ubuntu)

apt-cache madison commercial-internet-system sudo apt install commercial-internet-system=<VERSION_BEFORE_2.0> sudo apt-mark hold commercial-internet-system

2. No fixed release: DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list commercial-internet-system sudo dnf downgrade commercial-internet-system-<VERSION_BEFORE_2.0> sudo dnf versionlock add commercial-internet-system

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

microsoft internet information server DOWNGRADE

Affected: 4.0

Fixed in: no fixed release known

Safe to downgrade to: a release before 4.0

1. Check installed version

apt-cache policy internet-information-server || rpm -q internet-information-server || internet-information-server --version

2. No fixed release: DOWNGRADE to a release before 4.0 (Debian / Ubuntu)

apt-cache madison internet-information-server sudo apt install internet-information-server=<VERSION_BEFORE_4.0> sudo apt-mark hold internet-information-server

2. No fixed release: DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list internet-information-server sudo dnf downgrade internet-information-server-<VERSION_BEFORE_4.0> sudo dnf versionlock add internet-information-server

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

microsoft site server DOWNGRADE

Affected: 3.0 unknown

Fixed in: no fixed release known

Safe to downgrade to: a release before 3.0

1. Check installed version

apt-cache policy site-server || rpm -q site-server || site-server --version

2. No fixed release: DOWNGRADE to a release before 3.0 (Debian / Ubuntu)

apt-cache madison site-server sudo apt install site-server=<VERSION_BEFORE_3.0> sudo apt-mark hold site-server

2. No fixed release: DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list site-server sudo dnf downgrade site-server-<VERSION_BEFORE_3.0> sudo dnf versionlock add site-server

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

Public exploits and detection

Public code exists for this CVE. Treat unpatched systems as at risk, and only run PoCs in a lab.

Exploit-DB: Microsoft Commercial Internet System 2.0/2.5 / IIS 4.0 / Site Server Commerce Edition 3.0 alpha/3.0 - Denial of Service

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-1999-0867?

commercial internet system: downgrade or apply the listed mitigation; internet information server: downgrade or apply the listed mitigation; site server: downgrade or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-1999-0867 being exploited?

CVE-1999-0867 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-1999-0867 affect?

CVE-1999-0867 affects commercial internet system, internet information server, site server. Severity is medium (CVSS 5.0). Denial of service in IIS 4.0 via a flood of HTTP requests with malformed headers.

How urgent is CVE-1999-0867?

Priority 52/100. EPSS top 2%: very likely to be exploited; Public exploit reference; On your watchlist

Related CVEs

CVE-2000-0246 MEDIUM 5.0 DOWNGRADE DOWNGRADE IIS 4.0 and 5.0 does not properly perform ISAPI extension processing if a virtual directory is mapped to a UNC
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CVE-1999-0910 MEDIUM 5.0 DOWNGRADE DOWNGRADE Microsoft Site Server and Commercial Internet System (MCIS) do not set an expiration for a cookie, which could
CVE-1999-0861 LOW 2.6 DOWNGRADE DOWNGRADE Race condition in the SSL ISAPI filter in IIS and other servers may leak information in plaintext.
CVE-1999-0777 HIGH 7.5 DOWNGRADE DOWNGRADE IIS FTP servers may allow a remote attacker to read or delete files on the server, even if they have "No Acces

References

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