CVE-2026-74976: Mozilla Firefox SpiderMonkey JIT Miscompilation & Type Confusion

HERMES

HERMES THREAT SCORE & JIT EXPLOITATION PROFILE

Target: Mozilla Firefox SpiderMonkey JavaScript JIT Engine (WarpMonkey / IonMonkey)

Confidence: 97%

86 / 100

HIGH

Measures real-world operational relevance, exploit weaponization, and active threat posture.

Dimension Breakdown

Exploitability 18 / 20

Threat Activity 15 / 20

Weaponization 17 / 20

Exposure 18 / 20

Prevalence 20 / 20

Impact 18 / 20

Exploit Maturity 16 / 20

Attack Chain Potential 19 / 20

⚖️ Divergence & Operational Rationale

Hermes evaluates CVE-2026-74976 at HTS 86 (High). JavaScript JIT engine vulnerabilities form the standard primary vector for browser tab compromise, allowing hostile scripts to create arbitrary read/write primitives.

🕸️ Connected Knowledge Graph & Provenance

CVE-2026-74976: Mozilla Firefox SpiderMonkey JIT Miscompilation & Type ConfusionVULNERABILITY

Connected Nodes: 0

JIT Miscompilation & Type Confusion

CVSS v3.1: 8.8 High (CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H) / CWE-843: Type Confusion.
Disclosed in Mozilla security updates. An optimization flaw in the JIT compiler misinterprets object types during array index optimizations, enabling an attacker who convinces a user to visit a malicious website to achieve arbitrary code execution within the browser tab process. Addressed in Firefox 154.

Executive Summary

Mozilla resolved CVE-2026-74976, a type confusion vulnerability in the SpiderMonkey JIT engine.

When compiling speculative loops, the JIT tier incorrectly removed array boundary checks, assuming object types remained invariant. By modifying an object prototype mid-execution, a hostile script forced the compiler to read memory beyond array bounds, laying the groundwork for an arbitrary read/write primitive.

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1. Metadata & Attack Surface

Metric	Technical Specification	Operational Impact
CVE Identifier	CVE-2026-74976	Standardized vulnerability identifier
Affected Engine	SpiderMonkey JavaScript JIT	Core execution runtime
Affected Releases	Firefox < 154	Standard desktop & mobile builds
Fixed Release	Firefox 154	Official security release
Associated CWE	CWE-843: Type Confusion	Memory layout corruption
Delivery Vector	Web Browsing (AV:N/UI:R)	Malicious web page visit

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2. Remediation & Hardening

1. Update Firefox: Upgrade to version 154 or later.
2. Harden JIT Configuration (Optional for High-Risk Environments): In sensitive environments, JIT can be toggled via javascript.options.ion = false in about:config.