Threat Intelligence Report — CVE-2026-65390

Executive Summary

CVE-2026-65390 is an integer overflow vulnerability (CWE-190) in Apple's WebKit rendering engine, consumed across Safari, iOS, iPadOS, macOS, tvOS, visionOS, and watchOS. The flaw is triggered when a user processes maliciously crafted web content, and successful exploitation can result in memory corruption. Apple addressed the issue through improved input validation in the 26.6.1 / 26.6.2 / 27 release train. No public proof-of-concept, Nuclei template, or exploit code is currently available, and the CVE is not listed in CISA KEV.

Technical Analysis

The NVD description states plainly: "An integer overflow was addressed with improved input validation." The impact is characterized as "Processing maliciously crafted web content may lead to memory corruption." This places the vulnerability squarely within the well-understood class of WebKit memory-safety defects that have historically been weaponized in browser exploitation chains.

Vulnerability class. CWE-190 (Integer Overflow or Wraparound) occurs when an arithmetic operation produces a value that exceeds the representable range of the destination integer type. In memory-unsafe C/C++ codebases such as WebKit, an unchecked overflow frequently precedes an undersized heap allocation: a length or size calculation wraps to a small value, the allocator returns a small buffer, and a subsequent copy or write operation overruns the region — resulting in heap buffer overflow and, ultimately, memory corruption. In other configurations, an overflowed index or offset can produce an out-of-bounds read/write relative to a legitimate object.

Attack surface. Because the flaw is reachable via "maliciously crafted web content," the attack surface is the browser/web-content rendering pipeline across every affected platform: WebKit on iOS/iPadOS/macOS, and the WebKit-derived engines on tvOS, visionOS, and watchOS. Any component that parses or processes attacker-influenced numeric input — markup parsing, script engine internals, media decoding, image/font parsing, or layout arithmetic — is a plausible locus. The NVD advisory does not name a specific subcomponent, function, class, or file path; therefore this report does not assert one. Analysts should treat the specific vulnerable routine as unconfirmed pending Apple's advisory text or independent reverse engineering.

User interaction. The CVSS vector includes UI:R, reflecting that a victim must load the crafted content (e.g., navigate to, or be redirected to, a hostile page). No privileges are required (PR:N), the attack is network-delivered (AV:N), and complexity is low (AC:L) — the combination that makes browser-engine bugs high-value for drive-by delivery.

Severity. Base score 8.8 (High). The C:H/I:H/A:H triad indicates total loss of confidentiality, integrity, and availability of the browser process context on successful exploitation; scope is unchanged (S:U).

Patch confidence. 2/4 — the fix is CLAIMED on a single source (NVD). No independent vendor confirmation, no Apple security advisory text, and no patch diff were available at time of writing. EPSS is 0.00168 (6.4th percentile), indicating low predicted exploitation likelihood in the near term.

Exploitation Scenarios

No public PoC, exploit code, or Nuclei template exists for this CVE. The chain below is inferred from the vulnerability class (integer overflow → memory corruption reachable from web content) and is not based on a published exploit.

1. Reconnaissance / target identification. An adversary identifies victims running an unpatched WebKit build. This is passively achievable: browser user-agent strings and feature-detection fingerprinting reveal Safari/WebKit versions. No scanning of the target's network is required — the "target" is a user's browser, reached through normal web browsing, malvertising, compromised sites, or watering-hole placement.

2. Trigger delivery. The attacker hosts or injects maliciously crafted web content — a page, an ad creative, or a compromised third-party resource — and induces the victim to load it (phishing link, redirect chain, or drive-by on a legitimate-but-compromised site). Per the NVD description, no action beyond processing this content is required on the victim's part.

3. Flaw-class trigger. Crafted content supplies numeric input that drives an arithmetic operation in the WebKit pipeline past the bounds of its integer type, wrapping the value. Consistent with CWE-190 behavior, this produces an incorrectly sized allocation or an out-of-range index/offset. The precise parser, API, or memory-structure involved is not specified in the available data and is deliberately not fabricated here. Public reporting on comparable WebKit integer-overflow fixes typically attributes such defects to length/size handling in markup, script, or media processing.

4. Memory corruption. The wrapped value leads to an out-of-bounds write or read, corrupting heap memory in the renderer/web-content process. In a full exploit chain this corruption is groomed into a controlled primitive (e.g., an out-of-bounds read/write), which can then be leveraged toward code execution — typically after escaping the WebKit sandbox via a separate kernel or IPC bug. Note that CVE-2026-65390's advisory describes only the memory-corruption outcome; subsequent stages (sandbox escape, persistence) would require additional, unrelated vulnerabilities that are not part of this CVE.

5. Post-exploitation impact. On success, the CVSS C:H/I:H/A:H subscores describe complete compromise of the affected browser process's data — exfiltration of secrets, tampering, and denial of the process. Because the affected products span iOS, macOS, tvOS, visionOS, and watchOS, a working chain targeting the shared engine would have broad cross-platform reach.

Caveat: Absent a vendor advisory naming the component and absent any published PoC, the specific trigger input and the exact memory-management defect remain unconfirmed. Defenders should treat any WebKit integer-overflow crash near this CVE as potentially related and preserve forensic artifacts.

Impact Assessment

• Business impact. A remotely triggerable, low-complexity memory-corruption bug in the default browser of an entire device ecosystem carries significant risk. Enterprises with large Apple device fleets face exposure on endpoints that lag the patched versions. If weaponized, it could serve as the initial-access vector in a broader intrusion — data theft, ransomware staging, or espionage.
• Affected systems. All platforms listed by Apple: Safari on macOS and iOS/iPadOS, and the WebKit-based browsers/engines bundled with tvOS, visionOS, and watchOS. Because Apple mandates WebKit for third-party iOS browsers, all iOS browsers are exposed on unpatched devices.
• Blast radius. Broad in population terms, but currently constrained in exploitation terms: no KEV listing, no public exploit, and low EPSS (0.00168). The practical blast radius today is governed by patch-adoption speed, not by active exploitation.
• CVSS subscores. AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H → 8.8 High. The only mitigating factor relative to a 9.x+ score is the required user interaction and the unchanged scope.

Detection Guidance

Note on detection content. The NVD advisory provides no function names, URLs, file paths, or parameters, so network/rule signatures below are necessarily generic to the vulnerability class (WebKit memory corruption / integer overflow) rather than to specific exploit bytes. Rules are written to flag high-value contexts (WebKit crash telemetry, anomalous renderer behavior) and should be tuned to your environment.

Sigma Rule

title: Potential WebKit Integer Overflow Exploitation (CVE-2026-65390)
id: 8f4c1a2e-7b9d-4f3a-9c6e-202665390aa1
status: experimental
description: Detects WebKit/Safari renderer crashes or memory-corruption indicators consistent with CVE-2026-65390 integer overflow.
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-65390
author: valtersit.com
date: 2026-01-01
logsource:
  product: macos
  category: process_creation
detection:
  selection_binary:
    Image|endswith:
      - '/com.apple.WebKit.WebContent'
      - '/Safari'
  selection_crash:
    CommandLine|contains:
      - 'SIGSEGV'
      - 'SIGBUS'
      - 'EXC_BAD_ACCESS'
  condition: selection_binary and selection_crash
fields:
  - Image
  - CommandLine
  - ParentImage
falsepositives:
  - Legitimate browser crashes unrelated to exploitation
  - Debugging or fuzzing activity
level: medium
tags:
  - attack.t1203
  - cve.2026-65390

YARA Rule

rule EXPL_WebKit_IntegerOverflow_CVE_2026_65390
{
    meta:
        author      = "valtersit.com"
        description = "Heuristic detection of WebKit integer-overflow exploitation artifacts (CVE-2026-65390)"
        reference   = "https://nvd.nist.gov/vuln/detail/CVE-2026-65390"
        date        = "2026-01-01"
        severity    = "medium"

    strings:
        $webkit1 = "WebKit" ascii wide
        $webkit2 = "WebContent" ascii wide
        $safari  = "Safari" ascii wide

    condition:
        filesize < 5MB and
        any of ($webkit*) and $safari
}

Suricata IDS Rule

alert http any any -> any any (msg:"CVE-2026-65390 Possible WebKit Crafted Content Delivery (Integer Overflow)"; flow:established,to_client; http.user_agent; content:"Safari"; http.header_names; content:"Content-Type"; content:"text/html"; http.response_body; content:"<script"; threshold:type both, track by_src, count 5, seconds 60; classtype:attempted-user; reference:cve,2026-65390; reference:url,nvd.nist.gov/vuln/detail/CVE-2026-65390; sid:2026653901; rev:1;)

Elastic Detection

sequence by process.entity_id with maxspan=30s
  [process where process.name == "com.apple.WebKit.WebContent"]
  [process where event.action == "end" and process.exit_code != 0]

Splunk SPL Query

index=endpoint sourcetype=macos_unified_log OR sourcetype=syslog
(process="WebContent" OR process="Safari" OR process="WebKit")
(("SIGSEGV" OR "SIGBUS" OR "EXC_BAD_ACCESS" OR "integer overflow" OR "heap corruption"))
| stats count by host, process, _time
| where count > 3
| sort - count

Wazuh OSSEC Rule

<group name="apple,webkit,cve-2026-65390,">
  <rule id="202665" level="10">
    <decoded_as>json</decoded_as>
    <field name="process.name">WebContent|Safari|WebKit</field>
    <field name="event.reason">EXC_BAD_ACCESS|SIGSEGV|SIGBUS|heap corruption|integer overflow</field>
    <description>CVE-2026-65390: possible WebKit memory-corruption crash (integer overflow)</description>
    <mitre>
      <id>T1203</id>
    </mitre>
    <group>webkit_memory_corruption,</group>
  </rule>
</group>

Mitigation & Patch Guidance

Patch confidence: 2/4 (CLAIMED — single source claims patch; not independently confirmed). Treat the version numbers below as the NVD-stated fixed release line and verify against Apple's own security advisories before relying on them operationally.

1. Immediate mitigations (apply now, before patch rollout completes).

   • Prioritize updating any internet-exposed or high-value Apple endpoints (executive, finance, engineering) to the fixed release line immediately.
   • Restrict browsing for at-risk populations to hardened configurations until devices are patched; consider blocking untrusted web content via network controls where operationally feasible.
   • Because exploitation of this class requires the victim to process attacker-supplied content, user awareness regarding watering-hole and malvertising delivery remains a relevant stopgap.
   • Enable/enforce endpoint crash telemetry (see Detection Guidance) so WebKit memory-corruption crashes are captured for triage.

2. Patch / upgrade steps.

   • Update to (per NVD): Safari 26.6.1, iOS 26.6.1, iPadOS 26.6.1, macOS Tahoe 26.6.2, tvOS 27, visionOS 27, watchOS 27, or later.
   • Inventory all Apple devices against these versions; track fleet compliance and force updates on managed devices via MDM where possible.
   • Re-verify fixed versions against the authoritative Apple security advisory and CVE entry once vendor text is available, since patch confidence here is only "claimed."

3. Long-term hardening.

   • Maintain an aggressive patch SLA for browser and OS updates on Apple endpoints — the shared-engine architecture means a single WebKit flaw propagates across every platform at once.
   • Deploy exploit-mitigation and crash-monitoring tooling to detect renderer memory corruption in real time.
   • Subscribing to Apple security advisories and monitoring CISA KEV for future escalation of this CVE is recommended. Should weaponization occur, re-prioritize and reassess risk accordingly.

References

1. NVD — CVE-2026-65390: https://nvd.nist.gov/vuln/detail/CVE-2026-65390
2. Apple Security Releases (vendor advisory hub — verify 26.6.1 / 26.6.2 / 27 advisories here): https://support.apple.com/en-us/HT201222
3. MITRE CWE-190 — Integer Overflow or Wraparound: https://cwe.mitre.org/data/definitions/190.html
4. MITRE ATT&CK T1203 — Exploitation for Client Execution: https://attack.mitre.org/techniques/T1203/
5. CISA Known Exploited Vulnerabilities Catalog (CVE-2026-65390 not listed as of report generation): https://www.cisa.gov/known-exploited-vulnerabilities-catalog

Threat actor / exploitation status. No known threat actor campaigns, public exploits, or exploit-in-the-wild evidence for CVE-2026-65390 were identified in the provided intelligence. EPSS 0.00168 (6.4th percentile) and absence from KEV both indicate low observed exploitation activity to date. This report should be revisited if the vendor advisory publishes component-level detail or if exploitation is observed.