LiveActive security incident?Get immediate response
CVE Record

CVE-2021-23995: When Responsive Design Mode was enabled, it used references to objects that were previously freed.

When Responsive Design Mode was enabled, it used references to objects that were previously freed. We presume that with enough effort this could have been exploited to run arbitrary code. This vulnerability affects Firefox ESR < 78.10, Thunderbird < 78.10, and Firefox < 88.

UnknownCVSS not scoredNot KEV-listedUpdated
Discuss exposureBack to CVEs
Glexia's TakeAutomated analysishigh

Security readout for executives and security teams

Plain-English summary

This Mozilla flaw is a memory safety issue in Responsive Design Mode. Mozilla said it may have been exploitable, with enough effort, to run arbitrary code. The business risk is highest on machines still running older Firefox, Firefox ESR, or Thunderbird versions.

Executive priority

Treat as a prompt patching and inventory issue, especially for unmanaged workstations. The likely arbitrary-code impact justifies high priority, but lack of cited exploitation evidence keeps it below emergency response unless affected versions remain widespread.

Technical view

CVE-2021-23995 is a use-after-free condition involving references to previously freed objects when Responsive Design Mode was enabled. The source bundle lists Firefox before 88, Firefox ESR before 78.10, and Thunderbird before 78.10 as affected.

Likely exposure

Exposure is limited to systems running the affected Mozilla versions. Developer and QA workstations may be more relevant because Responsive Design Mode is a browser developer feature. The sources do not define all required user actions or remote trigger conditions.

Exploitation context

The provided sources do not show active exploitation, and KEV status is false. Mozilla stated arbitrary code execution was presumed possible with enough effort, but the bundle does not provide exploit maturity, public exploit evidence, or weaponized details.

Researcher notes

The bundle identifies a use-after-free in Responsive Design Mode but provides no CVSS, CWE, detailed trigger path, or confirmed exploit status. Focus validation on affected-version presence and Mozilla advisory alignment, not exploit reproduction.

Mitigation direction

  • Upgrade Firefox to version 88 or later.
  • Upgrade Firefox ESR to version 78.10 or later.
  • Upgrade Thunderbird to version 78.10 or later.
  • Check Mozilla advisories for any environment-specific guidance.
  • Remove unsupported affected versions from managed endpoints.

Validation and detection

  • Inventory Firefox, Firefox ESR, and Thunderbird versions across endpoints.
  • Confirm no Firefox installations remain below version 88.
  • Confirm no Firefox ESR or Thunderbird installations remain below 78.10.
  • Review developer and QA machines for affected browser versions.
  • Verify endpoint management enforces current Mozilla updates.
Prepared
Confidence
medium
Sources
6

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.

description · low confidence lookup

Execution 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 lookup
cve · low confidence lookup

CVE-2021-23995 mapping review

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

Open ATT&CK lookup
Vulnerability profileCVE Program record
Severity
Unknown
CVSS
Not scored
Known Exploited
No
Published
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.

0CVSS vectors
0Timeline events
0ADP providers
5Source links

CVSS and timeline data

No CVSS vectors or timeline events were available in the normalized CVE source material.

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
MozillaFirefox ESRunspecifiedListed
MozillaThunderbirdunspecifiedListed
MozillaFirefoxunspecifiedListed
Weakness

CWE details

No CWE listed

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