Security readout for executives and security teams
Plain-English summary
This PHP flaw can affect applications that validate floating-point input with FILTER_VALIDATE_FLOAT and min/max limits. On vulnerable PHP releases, failed validation can trigger memory use after free, causing crashes and potentially memory corruption or remote code execution. Impact is high where affected code is reachable from web input.
Executive priority
Prioritize remediation for internet-facing PHP applications on affected versions. Business urgency is high if customer input reaches this validation pattern, because sources describe possible service crashes and potential RCE, although active exploitation is not established in the provided evidence.
Technical view
CVE-2021-21708 is a CWE-416 use-after-free in php_filter_float(). It affects PHP 7.4.x before 7.4.28, 8.0.x before 8.0.16, and 8.1.x before 8.1.3 when FILTER_VALIDATE_FLOAT is used with min/max limits and validation fails. CVSS 3.1 is 8.2 with network, unauthenticated, no-user-interaction characteristics.
Likely exposure
Exposure is limited to systems running vulnerable PHP versions and application code that uses FILTER_VALIDATE_FLOAT with min/max limits, especially on request parameters or other untrusted input. PHP installations without that code path are less directly exposed based on the provided sources.
Exploitation context
The source bundle does not show CISA KEV listing or confirmed active exploitation. The CVE description states crashes are possible and memory overwrite or RCE may be possible. Treat reachable web-facing validation paths as higher risk, but do not assume exploitation without environment-specific evidence.
Researcher notes
The vulnerable condition is specific: php_filter_float() handling failed FILTER_VALIDATE_FLOAT checks with min/max limits. Focus analysis on reachable validation failures rather than all PHP float parsing. Public sources identify affected version ranges and impact class but provide limited evidence on exploit reliability or in-the-wild activity.
Mitigation direction
- Upgrade PHP to 7.4.28, 8.0.16, 8.1.3, or later supported releases.
- Review vendor advisories for platform-specific backports or package names.
- Inventory applications using FILTER_VALIDATE_FLOAT with min/max validation limits.
- Prioritize exposed applications processing untrusted numeric input.
- Retire unsupported PHP branches where upgrade paths are unavailable.
Validation and detection
- Check runtime PHP versions across servers, containers, and CI images.
- Search application code for FILTER_VALIDATE_FLOAT with min_range or max_range options.
- Confirm vulnerable code paths are reachable from user-controlled input.
- Verify package advisories from OS, appliance, or hosting vendors.
- After patching, retest affected validation paths for crashes or regressions.
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-416: 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 lookupExecution behavior lookup
The CVE wording references code or command execution, so execution technique review may help defensive triage. This is a Glexia inferred lookup path, not an official MITRE, ATT&CK, or CVE Program mapping.
Open ATT&CK lookupCVE-2021-21708 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
- 8.2 (3.1)
- Known Exploited
- No
- Published
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:L
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:N/UI:N/S:U/C:N/I:H/A:L3.94.2Primary CVE scoreVulnerability scoring details
Base CVSS 3.1 score
8.2HighVector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:L
Source materials
- CVE List V5 sourceCVE List V5
- https://bugs.php.net/bug.php?id=81708CVE reference · x_refsource_MISC
- https://security.netapp.com/advisory/ntap-20220325-0004/CVE reference · x_refsource_CONFIRM
- GLSA-202209-20CVE reference · vendor-advisory, x_refsource_GENTOO
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.
Use After Free
Use After Free represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.