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CVE Record

CVE-2021-32764: YouTube Onebox susceptible to XSS

Discourse is an open-source discussion platform. In Discourse versions 2.7.5 and prior, parsing and rendering of YouTube Oneboxes can be susceptible to XSS attacks. This vulnerability only affects sites which have modified or disabled Discourse's default Content Security Policy. The issue is patched in `stable` version 2.7.6, `beta` version 2.8.0.beta3, and `tests-passed` version 2.8.0.beta3. As a workaround, ensure that the Content Security Policy is enabled, and has not been modified in a way which would make it more vulnerable to XSS attacks.

HighCVSS 8.1Not KEV-listedUpdated
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Glexia's TakeAutomated analysishigh

Security readout for executives and security teams

Plain-English summary

This issue could let a logged-in attacker trigger cross-site scripting through YouTube preview rendering in vulnerable Discourse forums. The advisory narrows exposure: it affects sites running Discourse 2.7.5 or earlier only when the default Content Security Policy was disabled or weakened.

Executive priority

Treat this as a high-priority patch for exposed Discourse forums with user-generated content, especially if CSP was customized. If CSP remained default and the platform is patched, urgency decreases but version validation is still needed.

Technical view

CVE-2021-32764 is a CWE-79 XSS flaw in Discourse YouTube Onebox parsing and rendering. The CVSS 3.1 score is 8.1 with network access, low complexity, low privileges, and no user interaction. Reported impact is high confidentiality and integrity impact, with no availability impact.

Likely exposure

Likely exposure is internet-facing Discourse deployments on version 2.7.5 or earlier that changed or disabled Discourse's default CSP. Patched release tracks are stable 2.7.6, beta 2.8.0.beta3, and tests-passed 2.8.0.beta3.

Exploitation context

The source bundle does not show CISA KEV listing or any cited active exploitation. The vulnerability requires low privileges, so risk is higher on communities where untrusted users can create posts that generate YouTube Oneboxes.

Researcher notes

Evidence is limited to the CVE data and Discourse advisory. Do not assume broader Onebox providers, default-CSP deployments, or active exploitation are affected without new sources. Focus validation on version, release track, CSP posture, and user posting permissions.

Mitigation direction

  • Upgrade Discourse stable deployments to 2.7.6 or later.
  • Upgrade beta or tests-passed deployments to 2.8.0.beta3 or later.
  • Enable Discourse's default Content Security Policy.
  • Remove CSP customizations that make script execution more permissive.
  • Check the Discourse advisory for any environment-specific guidance.

Validation and detection

  • Inventory all Discourse instances and release tracks.
  • Confirm each instance is above the patched version for its track.
  • Review whether default CSP is enabled and not weakened.
  • Identify forums where untrusted users can create YouTube Oneboxes.
  • Review security logs for suspicious forum content around embedded previews.
Prepared
Confidence
high
Sources
3

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.

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cve · low confidence lookup

CVE-2021-32764 mapping review

Open the CVE-to-ATT&CK bridge for reviewed, inferred, or future official mappings tied to this CVE.

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Vulnerability profileCVE Program record
Severity
High
CVSS
8.1 (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: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
2Source 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
8.1CVSS 3.1HighCVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N2.85.2Primary CVE score

Vulnerability scoring details

Base CVSS 3.1 score

8.1High
CVSS 3.1 vector shape for CVE-2021-32764Attack VectorAttack ComplexityPrivileges RequiredUser InteractionScopeConfidentiality ImpactIntegrity ImpactAvailability Impact

Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/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
discoursediscourse<= 2.7.5Listed
Weakness

CWE details

CWE-79

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