Allowed UNC hostnames for attachments interpreted as a regular expression (CVE-2026-84642) | HOL Guard CVE

Unknown severity · Source severity not reportedCVE-2026-84642

Allowed UNC hostnames for attachments interpreted as a regular expression

Answer in brief

CVE-2026-84642 records a Unknown severity vulnerability in Allowed UNC hostnames for attachments interpreted as a regular expression. The current sources do not mark it as known exploited. Affected software not mapped in the current feed; confirm the named product and version against your inventory.

Analysis pending evidence review

HOL Guard separates source facts from reviewed analysis. See the methodology.

Published Sep 1, 2026Updated Sep 1, 2026Source checked Sep 1, 2026First seen by HOL Sep 1, 2026Material review Sep 1, 2026

Upstream Advisory

Key facts

Risk: Unknown severitySource severity not reported
Exploitation: Not marked as known exploited
Affected software: Not mapped
Fix availability: Not reported

Why this deserves its current priority

A CVSS score is not reported in the current record. The current sources do not mark it as known exploited. Treat this as a source-backed prioritization signal, not a statement about your environment.

Analysis status

Analysis pending evidence review

Factual feed record only; HOL analysis is not approved for indexing. Read the methodology.

Affected scope and exposure questions

Affected software not mapped in the current feed; confirm the named product and version against your inventory.

Recommended response

Confirm exposure. Identify whether the named product or interface exists in your inventory, then review the linked upstream references for affected versions and vendor guidance.

Evidence timeline and material changes

Published upstream
Sep 1, 2026
Evidence: source:cvelist:source_dates:source-dates:record
Source modified
Sep 1, 2026
Evidence: source:cvelist:source_dates:source-dates:record
First seen by HOL
Sep 1, 2026

Sources and claim methodology

Upstream source description

The values of the mail.allowed_attachment_hostnames advanced config setting were used in a regular expression without escaping. For some possible valid hostnames, this could allow certain unintended hostnames to also match and serve remote attachments. This vulnerability was fixed in Thunderbird 155 and Thunderbird 153.2.

Quoted source text, attributed separately from HOL analysis.