CVE-2021-21408: Smarty is a template engine for PHP, facilitating the separation of presentation (HTML/CSS) from application l

This month

HIGH 8.8WATCHLISTUPGRADEMITIGATEMITIGATE

CVE-2021-21408 is a high-severity flaw (CVSS 8.8) affecting smarty, debian linux, fedora. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

Smarty is a template engine for PHP, facilitating the separation of presentation (HTML/CSS) from application logic. Prior to versions 3.1.43 and 4.0.3, template authors could run restricted static php methods. Users should upgrade to version 3.1.43 or 4.0.3 to receive a patch.

Why this priority

High severity
On your watchlist

A fixed release or distro patch exists.

Risk facts

CVSS score	8.8 (High)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	Low
User interaction	None
EPSS exploit chance	2.1% (top 19%)
CISA exploited list	No
Weakness	CWE-20
Published	2022-01-10
Last modified	2026-06-17

Publisher (CNA): GitHub, Inc. · Vendor: debian · fedoraproject · smarty

Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2022-01-10 20:15 UTCPublished CVE record published
2026-06-17 03:35 UTCUpdated Record last modified

smarty smarty UPGRADE

Affected: < 3.1.43 · >= 4.0.0 and < 4.0.3

Fixed in: 3.1.43, 4.0.3

Safe to downgrade to: a release before 4.0.0

1. Check installed version

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

2. UPGRADE to 3.1.43 (Debian / Ubuntu)

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

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade smarty

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 4.0.0 (Debian / Ubuntu)

apt-cache madison smarty sudo apt install smarty=<VERSION_BEFORE_4.0.0> sudo apt-mark hold smarty

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

dnf --showduplicates list smarty sudo dnf downgrade smarty-<VERSION_BEFORE_4.0.0> sudo dnf versionlock add smarty

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.

debian debian linux MITIGATE

Affected: 9.0 · 10.0 · 11.0

Fixed in: no fixed release known

Safe to downgrade to: a release before 9.0

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.

fedoraproject fedora MITIGATE

Affected: 36 · 37

Fixed in: no fixed release known

Safe to downgrade to: a release before 36

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-2021-21408?

smarty: upgrade to 3.1.43, 4.0.3; debian linux: mitigate or apply the listed mitigation; fedora: mitigate or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2021-21408 being exploited?

CVE-2021-21408 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-2021-21408 affect?

CVE-2021-21408 affects smarty, debian linux, fedora. Severity is high (CVSS 8.8). Smarty is a template engine for PHP, facilitating the separation of presentation (HTML/CSS) from application logic. Prior to versions 3.1.43 and 4.0.3, template authors could run restricted static php methods. Users should upgrade to versio

How urgent is CVE-2021-21408?

Priority 56/100. High severity; On your watchlist

Software pages

PHP vulnerabilities · Debian vulnerabilities

Related CVEs

CVE-2023-41661 MEDIUM 5.9 UPGRADE Auth
CVE-2023-28447 HIGH 7.1 UPGRADE MITIGATE Smarty is a template engine for PHP
CVE-2022-29221 HIGH 8.8 UPGRADE MITIGATE Smarty is a template engine for PHP, facilitating the separation of presentation (HTML/CSS) from application l
CVE-2021-29454 HIGH 8.1 UPGRADE MITIGATE Smarty is a template engine for PHP, facilitating the separation of presentation (HTML/CSS) from application l
CVE-2021-26120 CRITICAL 9.8 UPGRADE MITIGATE Smarty before 3.1.39 allows code injection via an unexpected function name after a {function name= substring.

References

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