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CVE Record

CVE-2021-46871: tag.ex in Phoenix Phoenix.HTML (aka phoenix_html) before 3.0.4 allows XSS in HEEx class attributes.

tag.ex in Phoenix Phoenix.HTML (aka phoenix_html) before 3.0.4 allows XSS in HEEx class attributes.

MediumCVSS 6.1Not KEV-listedUpdated
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Glexia's TakeAutomated analysismoderate

Security readout for executives and security teams

Plain-English summary

Phoenix.HTML before 3.0.4 had an XSS flaw in HEEx class attribute handling. A malicious value could reach a rendered page and run script in a user's browser, but user interaction is required. This is not listed as CISA KEV in the bundle.

Executive priority

Treat as a routine but real web application security update. Prioritize internet-facing Phoenix applications, customer portals, and admin interfaces where untrusted content may be rendered. No emergency exploitation signal is provided in the bundle.

Technical view

CVE-2021-46871 is CWE-79 in tag.ex for phoenix_html before 3.0.4. The issue affects HEEx class attributes and is scored CVSS 3.1 6.1, with network attack vector, low complexity, no privileges, required user interaction, and changed scope.

Likely exposure

Exposure is likely limited to Phoenix applications using phoenix_html before 3.0.4 and rendering HEEx class attributes from unsafe or user-influenced data. The bundle does not provide CPEs or a complete affected-product matrix.

Exploitation context

The source bundle does not cite active exploitation, public weaponization, or KEV listing. Practical impact depends on whether attacker-controlled values can influence vulnerable HEEx class attribute rendering and whether a user views the affected page.

Researcher notes

Evidence is concise: advisory, CVE metadata, and fix commit. The provided affected section lacks vendor/product CPE detail, so validation should rely on dependency versions and template data-flow review rather than CPE matching alone.

Mitigation direction

  • Identify Phoenix applications using phoenix_html before 3.0.4.
  • Upgrade phoenix_html to 3.0.4 or later where applicable.
  • Review vendor advisory and commit notes before deployment.
  • Prioritize templates where class attributes include user-influenced data.
  • Apply normal XSS defenses, including output encoding and input validation.

Validation and detection

  • Check dependency manifests and lockfiles for phoenix_html versions below 3.0.4.
  • Inventory HEEx templates using dynamic class attributes.
  • Trace whether user-controlled data reaches those class attributes.
  • Confirm upgraded builds include the referenced tag.ex fix.
  • Run safe regression tests for escaped class attribute rendering.
Prepared
Confidence
high
Sources
4

Public sources used

Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.

Potential ATT&CK relevance

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 · medium confidence lookup

CWE-79: User-session and phishing behavior lookup

Client-side and session-facing weaknesses should be reviewed alongside initial-access and user-execution behaviors. 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 lookup
cve · low confidence lookup

CVE-2021-46871 mapping review

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Vulnerability profileCVE Program record
Severity
Medium
CVSS
6.1 (3.1)
Known Exploited
No
Published

Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N

Official CVE source material

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.

1CVSS vectors
0Timeline events
0ADP providers
3Source links

CVSS vector scores

1 official score

We 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.

ScoreVersionSeverityVectorExploitImpactSource
6.1CVSS 3.1MediumCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N2.82.7Primary CVE score

Vulnerability scoring details

Base CVSS 3.1 score

6.1Medium
CVSS 3.1 vector shape for CVE-2021-46871Attack VectorAttack ComplexityPrivileges RequiredUser InteractionScopeConfidentiality ImpactIntegrity ImpactAvailability Impact

Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N

Attack Vector
NetworkAdjacentLocalPhysical
Attack Complexity
LowHigh
Privileges Required
NoneLowHigh
User Interaction
NoneRequired
Scope
ChangedUnchanged
Confidentiality Impact
HighLowNone
Integrity Impact
HighLowNone
Availability Impact
HighLowNone

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
n/an/an/aListed
Weakness

CWE details

CWE-79

CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.

CWE-79 · source CWE mapping

Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')

Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting') represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.

Open CWE intelligenceSame-CWE CVEs