CVE-2026-100793 Overview
CVE-2026-100793 is a Just-In-Time (JIT) miscompilation vulnerability in the JavaScript engine component shared by Mozilla Firefox and Mozilla Thunderbird. The flaw resides in SpiderMonkey's JIT compiler, which generates optimized native code from JavaScript at runtime. A miscompilation produces incorrect machine code, breaking assumptions the runtime depends on for memory safety. Mozilla addressed the issue in Firefox 157 and Thunderbird 157 and documented the fix in advisories MFSA-2026-97 and MFSA-2026-101.
Critical Impact
JIT miscompilation flaws in browser JavaScript engines historically enable type confusion and memory corruption reachable from any web page a user visits.
Affected Products
- Mozilla Firefox versions prior to 157
- Mozilla Thunderbird versions prior to 157
- Products embedding vulnerable versions of the Mozilla SpiderMonkey JavaScript engine
Discovery Timeline
- 2026-09-29 - CVE-2026-100793 published to the National Vulnerability Database
- 2026-09-30 - Last updated in NVD database
Technical Details for CVE-2026-100793
Vulnerability Analysis
SpiderMonkey uses a tiered JIT compilation strategy to translate frequently executed JavaScript into native machine code. During compilation, the optimizer applies assumptions about variable types, object shapes, and control flow to remove runtime checks. A miscompilation occurs when the JIT emits native code that violates one of those assumptions.
The resulting native code can operate on values with the wrong type or access memory outside expected bounds. In browser JavaScript engines, this class of bug typically leads to type confusion, out-of-bounds read or write, or use-after-free conditions inside the content process. Mozilla has not published full technical details of the miscompilation. Refer to Mozilla Bug 2073268 for the tracking record.
Root Cause
The root cause is incorrect code generation within the SpiderMonkey JIT compiler. The optimizer produces machine code whose behavior does not match the semantics of the source JavaScript. This breaks the type and memory-safety invariants the engine relies on.
Attack Vector
An attacker delivers crafted JavaScript through a web page rendered by Firefox or through HTML email content processed by Thunderbird. When the JIT compiles the malicious script, the miscompiled code executes and corrupts engine state. No authentication is required, and user interaction is limited to loading attacker-controlled content.
No public proof-of-concept exploit has been released. Detailed exploitation code is not available in the source data.
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Stop Exploit AttemptsDetection Methods for CVE-2026-100793
Indicators of Compromise
- Firefox or Thunderbird content processes crashing with signals consistent with memory corruption while rendering untrusted web content
- Browser telemetry showing unexpected process termination during JavaScript execution on specific pages
- Endpoint alerts on child processes spawned by firefox.exe or thunderbird.exe that do not match normal browser behavior
Detection Strategies
- Inventory installed Firefox and Thunderbird versions across the estate and flag any build below version 157
- Correlate browser crash reports with visits to newly registered or low-reputation domains
- Monitor for post-exploitation behaviors such as browser processes writing executables to disk, loading unusual DLLs, or initiating outbound connections to non-standard destinations
Monitoring Recommendations
- Ingest browser and mail client process telemetry into a centralized analytics pipeline for behavioral baselining
- Track outbound network connections initiated by browser child processes and alert on anomalies
- Review web proxy logs for user navigation to domains serving heavily obfuscated JavaScript immediately preceding browser crashes
How to Mitigate CVE-2026-100793
Immediate Actions Required
- Upgrade Mozilla Firefox to version 157 or later on all managed endpoints
- Upgrade Mozilla Thunderbird to version 157 or later on all managed endpoints
- Restart the applications after patching to ensure the vulnerable JIT code paths are replaced in memory
- Verify enterprise deployment channels (ESR, MSI, package repositories) have received the fixed builds
Patch Information
Mozilla fixed CVE-2026-100793 in Firefox 157 and Thunderbird 157. Refer to Mozilla Security Advisory MFSA-2026-97 and Mozilla Security Advisory MFSA-2026-101 for the complete list of fixed issues and download links.
Workarounds
- Disable JavaScript in Thunderbird for message display, which the client does by default, and confirm the setting has not been overridden
- Restrict browsing to trusted sites through enterprise web filtering until patches are deployed
- Use browser isolation or remote browser technologies for high-risk user populations that cannot be patched immediately
# Verify installed Firefox version on Linux
firefox --version
# Verify installed Thunderbird version on Linux
thunderbird --version
# Windows: query installed version via registry
reg query "HKLM\SOFTWARE\Mozilla\Mozilla Firefox" /v CurrentVersion
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
