CVE-2021-1495: Multiple Cisco products are affected by a vulnerability in the Snort detection engine that could allow an unau

Q: How do I fix CVE-2021-1495?

secure firewall threat defense: upgrade to 6.4.0.12, 6.6.4, 6.7.0.2; ios xe: upgrade to 16.12.5; snort: upgrade to 2.9.17.1. Exact commands for each OS are on this page.

Q: Is CVE-2021-1495 being exploited?

CVE-2021-1495 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

Q: What does CVE-2021-1495 affect?

CVE-2021-1495 affects secure firewall threat defense, ios xe, snort. Severity is medium (CVSS 5.8). Multiple Cisco products are affected by a vulnerability in the Snort detection engine that could allow an unauthenticated, remote attacker to bypass a configured file policy for HTTP. The vulnerability is due to incorrect handling of specif

Q: How urgent is CVE-2021-1495?

Priority 45/100. Remotely exploitable, no login, no user action

This month

MEDIUM 5.8REMOTEUPGRADEUPGRADEUPGRADE

CVE-2021-1495 is a medium-severity flaw (CVSS 5.8) affecting secure firewall threat defense, ios xe, snort. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

Multiple Cisco products are affected by a vulnerability in the Snort detection engine that could allow an unauthenticated, remote attacker to bypass a configured file policy for HTTP. The vulnerability is due to incorrect handling of specific HTTP header parameters. An attacker could exploit this vulnerability by sending crafted HTTP packets through an affected device. A successful exploit could allow the attacker to bypass a configured file poli

Why this priority

Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	5.8 (Medium)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	1.7% (top 22%)
CISA exploited list	No
Weakness	CWE-755
Published	2021-04-29
Last modified	2026-08-11

Publisher (CNA): Cisco Systems, Inc. · Vendor: cisco · snort

Last updated 2026-08-11 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2021-04-29 18:15 UTCPublished CVE record published
2026-08-11 19:37 UTCUpdated Record last modified

cisco secure firewall threat defense UPGRADE

Affected: < 6.4.0.12 · >= 6.5.0 and < 6.6.4 · >= 6.7.0 and < 6.7.0.2

Fixed in: 6.4.0.12, 6.6.4, 6.7.0.2

Safe to downgrade to: a release before 6.5.0

1. Check installed version

apt-cache policy secure-firewall-threat-defense || rpm -q secure-firewall-threat-defense || secure-firewall-threat-defense --version

2. UPGRADE to 6.4.0.12 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade secure-firewall-threat-defense

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade secure-firewall-threat-defense

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

3. If no update is available yet, DOWNGRADE to a release before 6.5.0 (Debian / Ubuntu)

apt-cache madison secure-firewall-threat-defense sudo apt install secure-firewall-threat-defense=<VERSION_BEFORE_6.5.0> sudo apt-mark hold secure-firewall-threat-defense

3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list secure-firewall-threat-defense sudo dnf downgrade secure-firewall-threat-defense-<VERSION_BEFORE_6.5.0> sudo dnf versionlock add secure-firewall-threat-defense

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.

cisco ios xe UPGRADE

Affected: >= 16.12 and < 16.12.5

Fixed in: 16.12.5

Safe to downgrade to: a release before 16.12

1. Check installed version

apt-cache policy ios-xe || rpm -q ios-xe || ios-xe --version

2. UPGRADE to 16.12.5 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade ios-xe

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade ios-xe

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

3. If no update is available yet, DOWNGRADE to a release before 16.12 (Debian / Ubuntu)

apt-cache madison ios-xe sudo apt install ios-xe=<VERSION_BEFORE_16.12> sudo apt-mark hold ios-xe

3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list ios-xe sudo dnf downgrade ios-xe-<VERSION_BEFORE_16.12> sudo dnf versionlock add ios-xe

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 UPGRADE

Affected: < 2.9.17.1

Fixed in: 2.9.17.1

Safe to downgrade to: no unaffected older release

1. Check installed version

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

2. UPGRADE to 2.9.17.1 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade snort

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade snort

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

cisco ios xe UPGRADE

Affected: >= 17.1 and < 17.3.3 · >= 17.4 and < 17.4.1

Fixed in: 17.3.3, 17.4.1

Safe to downgrade to: a release before 17.1

Check your OS release

cat /etc/os-release; uname -r

Debian / Ubuntu

sudo apt update && sudo apt full-upgrade

RHEL / Amazon Linux / Fedora

sudo dnf upgrade --refresh

Windows

Install the latest cumulative update from Windows Update

If no patch exists yet

OS downgrades are not safe. Apply the vendor mitigation from the advisory and restrict exposure.

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-2021-1495?

secure firewall threat defense: upgrade to 6.4.0.12, 6.6.4, 6.7.0.2; ios xe: upgrade to 16.12.5; snort: upgrade to 2.9.17.1. Exact commands for each OS are on this page.

Is CVE-2021-1495 being exploited?

CVE-2021-1495 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-2021-1495 affect?

CVE-2021-1495 affects secure firewall threat defense, ios xe, snort. Severity is medium (CVSS 5.8). Multiple Cisco products are affected by a vulnerability in the Snort detection engine that could allow an unauthenticated, remote attacker to bypass a configured file policy for HTTP. The vulnerability is due to incorrect handling of specif

How urgent is CVE-2021-1495?

Priority 45/100. Remotely exploitable, no login, no user action

Software pages

iOS vulnerabilities · Cisco IOS XE vulnerabilities · Cisco vulnerabilities · Suricata and Snort vulnerabilities

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References

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