CVE-2015-0817

What it actually does

The asm.js implementation fails to correctly determine safe bounds-checking elision cases during JIT compilation, permitting crafted JavaScript to perform out-of-bounds reads and writes into process memory and achieve arbitrary code execution.

Why it matters to the business

Attackers can silently compromise user systems through malicious web content, leading to data theft, credential capture, or malware installation; the vulnerability is over a decade old with stable low exploitation probability and no known ransomware usage.

How an attack unfolds

An attacker hosts a web page containing specially crafted asm.js code that triggers the bounds-checking flaw during JIT compilation; when a user visits the page with an affected browser, the script reads or writes arbitrary memory locations to execute attacker-controlled code.

How to fix it

Upgrade Mozilla Firefox to version 36.0.3 or later, Firefox ESR 31.x to 31.5.2 or later, and SeaMonkey to 2.33.1 or later; disable JavaScript where feasible or enforce strict content security policies as a temporary mitigation.

How to confirm the fix worked

Confirm browser version numbers match or exceed the fixed releases listed in the remediation guidance; re-test affected asm.js test cases in a lab environment to verify bounds checks are no longer elided incorrectly.

Typical attack path

1. Attacker crafts malicious JavaScript using asm.js features that bypass intended bounds checks
2. Victim is lured to a web page serving the exploit (via phishing, compromised site, or ad network)
3. Browser JIT compiler processes the asm.js code and skips required bounds validation
4. Script performs out-of-bounds memory access to corrupt data structures or inject shellcode
5. Arbitrary code executes with the privileges of the browser process

How to detect it

Review web proxy, browser telemetry, and endpoint logs for visits to unexpected domains followed by anomalous process creation or memory allocation patterns originating from browser processes; because the issue is historical and unexploited in KEV, focus on retrospective hunting for post-2015 artifacts in preserved logs.

What your security team should do

• Prioritize inventory of all Mozilla browser deployments and schedule immediate upgrades to the fixed versions
• Monitor EPSS and threat intelligence feeds for any future increase in exploitation activity despite current low scores
• Apply network-level controls to block known malicious domains until patching is complete