Home/CVE/Sandbox escape due to incorrect boundary conditions in the Widget: Win32 component. This vulnerability was fixed in Fire
CVE
CVE-2026-8959
Sandbox escape due to incorrect boundary conditions in the Widget: Win32 component. This vulnerability was fixed in Fire
Sandbox escape due to incorrect boundary conditions in the Widget: Win32 component. This vulnerability was fixed in Firefox 151, Firefox ESR 140.11, Thunderbird 151, and Thunderbird 140.11.
CRITICAL · CVSS 9.6
EPSS 0.00164
Schedule remediation
- CVSS base score ≥ 7.0
Sigma rules1
YARA rules0
Look this up elsewhere - one-click external pivots
▸
How to read a CVE - triage first, then detect and patch
This page is every public fact about CVE-2026-8959, cross-linked. Its job is to answer one question fast - does this need my attention now? - and then hand you the two things you do about it. Here is how an analyst reads it.
Triage: should I act now? Four signals, and they are not interchangeable:
CVSSseverity - how bad it is IF exploited, 0-10. A high CVSS alone is not urgency; a flaw can be a perfect 10 and never actually be attacked.
EPSSprobability - a model’s estimate of the chance it is exploited in the next 30 days, 0-1. This is the “will it actually happen” signal.
CISA KEVconfirmed - it is being exploited in the wild right now. The strongest signal on the page; KEV beats any score.
Weaponisedavailability - public exploits / PoCs, and especially Metasploit modules rated Excellent / Great. Reliable, packaged exploit code means low-skill attackers can use it today.
How they combine: KEV, or a dependable Metasploit module, means patch now regardless of CVSS. High CVSS + low EPSS + no exploit is real but not an emergency - schedule it. Low CVSS but KEV-listed still gets patched now. The verdict above already weighed these for you; this is how it got there.
Then what - two workflows:
Detectwhen you cannot patch today, follow this CVE to the ATT&CK techniques it enables, then Build a SIEM detection (the green button) - author a rule, test it in Atomic, deploy it. That buys visibility while the patch waits.
PatchAffected products / packages tell you if you are exposed; Fixed versions by distribution and Vendor advisories give the exact version that closes it.
Reading order for the panels below: verdict + badges, then Public exploits / Metasploit (is it weaponised), then ATT&CK techniques + Sigma / IDS rules (can I detect it), then Affected products / packages + Fixed versions (am I exposed, what patches it), then Threat actors / IOCs (who uses it), then Scoring & timeline / references (the evidence).
◆
ATT&CK techniques
15Techniques this CVE enables - linked via CWECAPECATT&CK. High◆ = named directly in ATT&CK or Nuclei templates.
T1027 · Obfuscated Files or Information T1036.001 · Invalid Code Signature T1040 · Network Sniffing T1083 · File and Directory Discovery T1190 · Exploit Public-Facing Application T1482 · Domain Trust Discovery T1539 · Steal Web Session Cookie T1553.002 · Code Signing T1562.003 · Impair Command History Logging T1565.002 · Transmitted Data Manipulation T1574.005 · Executable Installer File Permissions Weakness T1574.006 · Dynamic Linker Hijacking T1574.007 · Path Interception by PATH Environment Variable T1574.010 · Services File Permissions Weakness T1611 · Escape to Host
▤ Build a SIEM detection for these techniques
▤
CAPEC attack patterns
12Attack patterns this CVE enables - the bridge from weakness to ATT&CK technique.
CAPEC-CAPEC-1 · Accessing Functionality Not Properly Constrained by ACLs CAPEC-CAPEC-10 · Buffer Overflow via Environment Variables CAPEC-CAPEC-100 · Overflow Buffers CAPEC-CAPEC-101 · Server Side Include (SSI) Injection CAPEC-CAPEC-104 · Cross Zone Scripting CAPEC-CAPEC-107 · Cross Site Tracing CAPEC-CAPEC-108 · Command Line Execution through SQL Injection CAPEC-CAPEC-109 · Object Relational Mapping Injection CAPEC-CAPEC-110 · SQL Injection through SOAP Parameter Tampering CAPEC-CAPEC-120 · Double Encoding CAPEC-CAPEC-123 · Buffer Manipulation CAPEC-CAPEC-127 · Directory Indexing
⬡
Weakness Classification
▤
Affected Products & Versions
4mozilla firefox< 140.11.0
mozilla firefox< 151.0.0
mozilla thunderbird< 140.11
mozilla thunderbird< 151.0.0
📦
Fixed versions by distribution
10The package version that resolves this CVE on each Linux distribution, from the vendor’s published security data. fixed in shows a patched version exists; open means the package is listed as affected with no fix yet.
oracle allthunderbird fixed in 0:140.11.0-1.0.1.el8_10
rhel 8thunderbird fixed in 0:140.11.0-1.el8_10
rhel 9thunderbird open
suse sle15MozillaFirefox fixed in 0:140.11.0-150200.152.239.1
suse sle15MozillaFirefox-devel fixed in 0:140.11.0-150200.152.239.1
suse sle15MozillaFirefox-translations-common fixed in 0:140.11.0-150200.152.239.1
suse sle15MozillaFirefox-translations-other fixed in 0:140.11.0-150200.152.239.1
suse sle15MozillaThunderbird fixed in 0:140.11.0-150200.8.274.1
suse sle15MozillaThunderbird-translations-common fixed in 0:140.11.0-150200.8.274.1
suse sle15MozillaThunderbird-translations-other fixed in 0:140.11.0-150200.8.274.1
◈
Sigma Hunt Rules
1Exact rules name this CVE ID. Product rules name an affected product in their title. Related rules cover techniques used by actors who exploited this CVE. Showing the most relevant matches; the complete related set is on the full drill-down.
producthighSQLite Firefox Profile Data DB Access
▣
Scoring & Timeline
9.6
CRITICAL · CVSS v3.1 · [email protected]
Attack Vector
Network
Adjacent
Local
Physical
Attack Complexity
Low
High
Privileges Required
None
Low
High
User Interaction
None
Required
Scope
Unchanged
Changed
Confidentiality
None
Low
High
Integrity
None
Low
High
Availability
None
Low
High
SSVC triage · cisa-vulnrichment
Exploitation
none
Automatable
no
Technical impact
total
SSVC asks the questions that actually drive patch urgency: is it being exploited, can attacks be automated, and how total is the impact.
⚑
Vendor Advisories
12suse-csafSUSE-SU-2026:2271-1
suse-csafSUSE-SU-2026:2109-1
suse-csafopenSUSE-SU-2026:10863-1
suse-csafopenSUSE-SU-2026:10864-1
suse-csafopenSUSE-SU-2026:20789-1
🔗
References & Sources
5Source URLs (vendor pages, mailing lists, write-ups). Exploit/PoC links are in their own section above to avoid duplication.
https://bugzilla.mozilla.org/show_bug.cgi?id=2034754Permissions Required
https://www.mozilla.org/security/advisories/mfsa2026-46/Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2026-48/Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2026-50/Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2026-51/Vendor Advisory