CVE-2001-0669: Various Intrusion Detection Systems (IDS) including (1) Cisco Secure Intrusion Detection System, (2) Cisco Cat

This month

HIGH 7.5MITIGATEMITIGATEDOWNGRADE

CVE-2001-0669 is a high-severity flaw (CVSS 7.5) affecting catalyst 6000 intrusion detection system module, secure intrusion detection system, realsecure network sensor. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

Various Intrusion Detection Systems (IDS) including (1) Cisco Secure Intrusion Detection System, (2) Cisco Catalyst 6000 Intrusion Detection System Module, (3) Dragon Sensor 4.x, (4) Snort before 1.8.1, (5) ISS RealSecure Network Sensor 5.x and 6.x before XPU 3.2, and (6) ISS RealSecure Server Sensor 5.5 and 6.0 for Windows, allow remote attackers to evade detection of HTTP attacks via non-standard "%u" Unicode encoding of ASCII characters in the

Why this priority

High severity
EPSS top 9%: very likely to be exploited
Public exploit reference

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

Risk facts

CVSS score	7.5 (High)
EPSS exploit chance	4.4% (top 9%)
CISA exploited list	No
Published	2001-10-30
Last modified	2026-06-16

Publisher (CNA): MITRE · Vendor: cisco · enterasys · iss · snort

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

Am I affected?

Timeline

2001-10-30 05:00 UTCPublished CVE record published
2026-06-16 21:54 UTCUpdated Record last modified

cisco catalyst 6000 intrusion detection system module MITIGATE

Affected: all versions

Fixed in: no fixed release known

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy catalyst-6000-intrusion-detection-system-module || rpm -q catalyst-6000-intrusion-detection-system-module || catalyst-6000-intrusion-detection-system-module --version

No fixed or unaffected release known

Apply the vendor mitigation from the advisory, disable the affected feature, or restrict network access.

cisco secure intrusion detection system MITIGATE

Affected: all versions

Fixed in: no fixed release known

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy secure-intrusion-detection-system || rpm -q secure-intrusion-detection-system || secure-intrusion-detection-system --version

No fixed or unaffected release known

Apply the vendor mitigation from the advisory, disable the affected feature, or restrict network access.

iss realsecure network sensor DOWNGRADE

Affected: 5.x · 6.x

Fixed in: no fixed release known

Safe to downgrade to: a release before 5.x

1. Check installed version

apt-cache policy realsecure-network-sensor || rpm -q realsecure-network-sensor || realsecure-network-sensor --version

2. No fixed release: DOWNGRADE to a release before 5.x (Debian / Ubuntu)

apt-cache madison realsecure-network-sensor sudo apt install realsecure-network-sensor=<VERSION_BEFORE_5.x> sudo apt-mark hold realsecure-network-sensor

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

dnf --showduplicates list realsecure-network-sensor sudo dnf downgrade realsecure-network-sensor-<VERSION_BEFORE_5.x> sudo dnf versionlock add realsecure-network-sensor

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.

iss realsecure server sensor DOWNGRADE

Affected: 5.5 · 6.0

Fixed in: no fixed release known

Safe to downgrade to: a release before 5.5

1. Check installed version

apt-cache policy realsecure-server-sensor || rpm -q realsecure-server-sensor || realsecure-server-sensor --version

2. No fixed release: DOWNGRADE to a release before 5.5 (Debian / Ubuntu)

apt-cache madison realsecure-server-sensor sudo apt install realsecure-server-sensor=<VERSION_BEFORE_5.5> sudo apt-mark hold realsecure-server-sensor

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

dnf --showduplicates list realsecure-server-sensor sudo dnf downgrade realsecure-server-sensor-<VERSION_BEFORE_5.5> sudo dnf versionlock add realsecure-server-sensor

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.

snort snort DOWNGRADE

Affected: 1.8.1

Fixed in: no fixed release known

Safe to downgrade to: a release before 1.8.1

1. Check installed version

apt-cache policy snort || rpm -q snort || snort --version

2. No fixed release: DOWNGRADE to a release before 1.8.1 (Debian / Ubuntu)

apt-cache madison snort sudo apt install snort=<VERSION_BEFORE_1.8.1> sudo apt-mark hold snort

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

dnf --showduplicates list snort sudo dnf downgrade snort-<VERSION_BEFORE_1.8.1> sudo dnf versionlock add snort

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.

enterasys dragon MITIGATE

Affected: 4.x

Fixed in: no fixed release known

Safe to downgrade to: a release before 4.x

Firmware

Install the latest firmware from the vendor advisory; if none, apply the vendor workaround.

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: Cisco Secure IDS 2.0/3.0 / Snort 1.x / ISS RealSecure 5/6 / NFR 5.0 - Encoded IIS Detection Evasion

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-2001-0669?

catalyst 6000 intrusion detection system module: mitigate or apply the listed mitigation; secure intrusion detection system: mitigate or apply the listed mitigation; realsecure network sensor: downgrade or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2001-0669 being exploited?

CVE-2001-0669 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-2001-0669 affect?

CVE-2001-0669 affects catalyst 6000 intrusion detection system module, secure intrusion detection system, realsecure network sensor. Severity is high (CVSS 7.5). Various Intrusion Detection Systems (IDS) including (1) Cisco Secure Intrusion Detection System, (2) Cisco Catalyst 6000 Intrusion Detection System Module, (3) Dragon Sensor 4.x, (4) Snort before 1.8.1, (5) ISS RealSecure Network Sensor 5.x

How urgent is CVE-2001-0669?

Priority 54/100. High severity; EPSS top 9%: very likely to be exploited; Public exploit reference

Software pages

Windows vulnerabilities · Cisco vulnerabilities · Suricata and Snort vulnerabilities

References

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