Security readout for executives and security teams
Plain-English summary
Smarty is a PHP template engine used by web applications. In affected versions, someone allowed to write templates could bypass intended restrictions and run static PHP methods that should have been blocked. This can affect confidentiality, integrity, and availability, so exposed applications should be upgraded promptly.
Executive priority
Treat as high priority for web applications with delegated template editing or CMS-style customization. Upgrade during the next urgent maintenance window. If template editing is not exposed beyond trusted developers, prioritize after confirming no bundled vulnerable package is reachable in production.
Technical view
CVE-2021-21408 affects smarty-php/smarty before 3.1.43 and 4.0.0 through 4.0.2. The issue involves dynamic static class access allowing template authors to run restricted static PHP methods. CVSS 3.1 is 8.8 with low complexity and required privileges, no user interaction, and high CIA impact.
Likely exposure
Exposure is most likely in PHP applications using Smarty where admins, CMS users, theme editors, plugin authors, or other semi-trusted users can create or modify templates. Risk is lower when templates are only changed by trusted developers through controlled releases.
Exploitation context
The source bundle does not show CISA KEV listing or cited evidence of active exploitation. The CVSS vector indicates network reachability, low attack complexity, and required privileges. Practical exploitation depends on having template-authoring access in an affected application.
Researcher notes
The advisory and CVE record support affected version ranges and patched releases. The commit is the primary code-change reference. Evidence does not establish active exploitation, public exploit availability, or a standalone workaround beyond upgrading or applying vendor packages.
Mitigation direction
- Upgrade Smarty to 3.1.43 or 4.0.3 or later.
- Apply distribution security updates from Debian, Gentoo, Fedora, or other package vendors.
- Restrict template editing to trusted administrative roles only.
- Review downstream application vendor guidance if Smarty is bundled indirectly.
Validation and detection
- Inventory PHP dependencies and package-manager records for Smarty versions.
- Check containers and deployed hosts, not only source repositories.
- Identify application features that allow template creation or editing.
- Confirm production runs Smarty 3.1.43, 4.0.3, or a vendor-patched package.
- Run regression tests for template rendering after upgrading.
Public sources used
Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.
Conservative CVE-to-ATT&CK context
These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.
ATT&CK lookup starting points
Use these exact CWE pages and searches to review the Glexia ATT&CK library from this CVE's weakness and description context.
CWE-20: Exact CWE lookup
Use the exact CWE identifier as the starting point before reviewing related ATT&CK behavior. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.
Open ATT&CK lookupCVE-2021-21408 mapping review
Open the CVE-to-ATT&CK bridge for reviewed, inferred, or future official mappings tied to this CVE.
Open ATT&CK lookup- Severity
- High
- CVSS
- 8.8 (3.1)
- Known Exploited
- No
- Published
Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
CNA and ADP enrichment extracted from CVE v5
These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.
CVSS vector scores
1 official scoreWe collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H2.85.9Primary CVE scoreVulnerability scoring details
Base CVSS 3.1 score
8.8HighVector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
Source materials
- CVE List V5 sourceCVE List V5
- https://github.com/smarty-php/smarty/security/advisories/GHSA-4h9c-v5vg-5m6mCVE reference
- https://github.com/smarty-php/smarty/commit/19ae410bf56007a5ef24441cdc6414619cfaf664CVE reference
- https://github.com/smarty-php/smarty/releases/tag/v3.1.43CVE reference
- https://github.com/smarty-php/smarty/releases/tag/v4.0.3CVE reference
- [debian-lts-announce] 20220505 [SECURITY] [DLA 2995-1] smarty3 security updateCVE reference · mailing-list
- DSA-5151CVE reference · vendor-advisory
- GLSA-202209-09CVE reference · vendor-advisory
- FEDORA-2022-52154efd61CVE reference · vendor-advisory
- FEDORA-2022-d5fc9dcdd7CVE reference · vendor-advisory
Products and packages named in the record
CWE details
CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.
Improper Input Validation
Improper Input Validation represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.