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CVE-2026-84642— Allowed UNC hostnames for attachments interpreted as a regular expression

Quick assessment

Affected
Mozilla Thunderbird
Exploitation
No confirmed in-the-wild exploitation; assess based on exposure
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

Mozilla Thunderbird是Mozilla基金会开源的一款跨平台电子邮件和新闻客户端。 Mozilla Thunderbird 155之前版本和Thunderbird 153.2之前版本存在安全漏洞，该漏洞源于mail.allowed_attachment_hostnames高级配置设置的值在正则表达式中未转义，可能导致某些意外主机名匹配并服务远程附件。

AI Predicted 5.3 Difficulty: Moderate EPSS 0.25% · P17

Possible ATT&CK Techniques 1 AI

T1566.001 · Spearphishing Attachment

Affected Version Matrix 2

VendorProduct Version RangeStatus
Mozilla Thunderbird 153.2≤ 153.* unaffected
155≤ * unaffected
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I. Basic Information for CVE-2026-84642

Vulnerability Information
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Vulnerability Title
Allowed UNC hostnames for attachments interpreted as a regular expression
Source: CVE Program / CVE List V5
Vulnerability 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.
Source: CVE Program / CVE List V5
CVSS Information
N/A
Source: CVE Program / CVE List V5
Vulnerability Type
N/A
Source: CVE Program / CVE List V5
Vulnerability Title
Mozilla Thunderbird 安全漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
Mozilla Thunderbird是Mozilla基金会开源的一款跨平台电子邮件和新闻客户端。 Mozilla Thunderbird 155之前版本和Thunderbird 153.2之前版本存在安全漏洞，该漏洞源于mail.allowed_attachment_hostnames高级配置设置的值在正则表达式中未转义，可能导致某些意外主机名匹配并服务远程附件。
Source: CNNVD (China National Vulnerability Database)
CVSS Information
N/A
Source: CNNVD (China National Vulnerability Database)
Vulnerability Type
N/A
Source: CNNVD (China National Vulnerability Database)

Affected Products

Vendor Product Affected Versions CPE Subscribe
Mozilla Thunderbird 153.2 ~ 153.* -

II. Public POCs for CVE-2026-84642

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III. Intelligence Information for CVE-2026-84642

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Vendor Advisories for CVE-2026-84642 (2)

Same Patch Batch · Mozilla · 2026-09-01 · 34 CVEs total

CVE-2026-84122 Use-after-free in the Audio/Video component
CVE-2026-84130 Information disclosure in the Graphics: WebGPU component
CVE-2026-84129 Site isolation issue in the DOM: Navigation component
CVE-2026-84128 Privilege escalation in the WebDriver BiDi component
CVE-2026-84127 Information disclosure in the WebExtensions component in Firefox for Android
CVE-2026-84126 Incorrect boundary conditions in the Layout: Grid component
CVE-2026-84125 Use-after-free in the DOM: Core & HTML component
CVE-2026-84124 Use-after-free in the DOM: Core & HTML component
CVE-2026-84123 Privilege escalation due to use-after-free in the Graphics: WebGPU component
CVE-2026-84132 Information disclosure in the Networking: HTTP component
CVE-2026-84118 Use-after-free in the JavaScript: GC component
CVE-2026-84117 Privilege escalation in Firefox for Android
CVE-2026-84145 Internally found bugs fixed in Thunderbird 155, Thunderbird ESR 153.2 and Thunderbird ESR
CVE-2026-84131 Privilege escalation due to invalid pointer in the Graphics component
CVE-2026-84121 Sandbox escape due to use-after-free in the DOM: Security component
CVE-2026-84120 Use-after-free in the Audio/Video component
CVE-2026-84119 Sandbox escape due to use-after-free in the DOM: Navigation component
CVE-2026-84640 One byte overflow read in mail parser
CVE-2026-84133 Site isolation issue in the DOM: Push Subscriptions component
CVE-2026-84134 Other issue in the Profile Backup component

Showing top 20 of 34 CVEs. View all on vendor page → →

IV. Related Vulnerabilities

Same product: Thunderbird
Same vendor: Mozilla

V. Comments for CVE-2026-84642

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