CVE-2020-9818 is an out-of-bounds write vulnerability in Apple iOS, iPadOS, and watchOS. CISA added it to the Known Exploited Vulnerabilities catalogue on 3 November 2021, which means attackers have used it in real attacks. Apple iOS, iPadOS, and watchOS Mail contains an out-of-bounds write vulnerability which may allow memory modification or application termination when processing a maliciously crafted mail message.
What this vulnerability means
An out-of-bounds write is a memory-safety flaw, not just a crash condition. In a client application such as Mail, malformed input can push execution into unintended memory regions, which is why these bugs are often treated as high-value exploitation paths rather than ordinary stability issues.
For CVE-2020-9818, the dangerous part is the processing of a maliciously crafted mail message inside Apple iOS, iPadOS, and watchOS Mail. That means the attack surface is the message parser and its handling of untrusted content, where a single parsing mistake can lead to memory modification or application termination.
Why this kind of flaw is dangerous in Mail clients
Mail clients are exposed to untrusted content by design, so the vulnerability sits on a trust boundary that attackers can reach remotely. A memory write outside the intended buffer can corrupt state, trigger a crash, or sometimes be chained into deeper compromise depending on the surrounding mitigations and exploit conditions.
Out-of-bounds write weaknesses are especially concerning in mobile and wearable software because the impacted process often has access to user data, accounts, and message history. The CWE Top 25 Guide is a useful reference for understanding why bounds-checking failures remain such persistent and serious software weaknesses.
How organisations check exposure and prioritise response
The first step is confirming whether the vulnerable Apple platforms and Mail application are present in the device fleet, then checking whether those devices are internet-reachable, user-facing, or handling sensitive correspondence. If the issue is in scope, vendor advisories and the NIST National Vulnerability Database help establish the authoritative vulnerability record, while CISA KEV status signals that exploitation has already been observed.
Security teams should treat that combination, a public mail-facing flaw plus KEV inclusion, as a prioritisation trigger. For broader intake and triage discipline, the Vulnerability Management Guide is a practical companion for asset scoping, exception handling, verification, and remediation tracking.
Remediation, detection, and post-exploitation review
The correct response is to apply Apple’s fixed software as soon as operationally possible, and to use compensating controls only when patching is delayed. Until then, reduce exposure by limiting mail handling on high-value devices, tightening phishing and attachment controls, and monitoring for abnormal Mail behaviour, repeated crashes, or other signs of parser abuse.
After remediation, review the affected devices for evidence of compromise rather than assuming the patch alone is enough. Because exploitation can affect memory and process integrity, a post-patch review should look for unexpected configuration changes, suspicious mail content, and any downstream account or session abuse that could have followed initial exploitation. The CVE Program remains the canonical starting point for the vulnerability identifier itself, while vendor guidance should provide the authoritative patch detail.
Risk and Threat Considerations
This flaw matters because attackers can deliver the trigger remotely through ordinary-looking email content, which makes it a practical entry point for exploitation against widely used client software. Once a parser bug exists in a default communications app, the defender is relying on input validation and memory safety to hold under hostile content, and that assumption is exactly what the attacker tries to break.
Failure mechanism: A crafted message reaches the vulnerable mail parser, the out-of-bounds write corrupts memory, and the process may crash or behave unpredictably, with possible chaining into deeper compromise depending on exploitability.
Impact: Users can lose application availability immediately, and the affected device may need post-incident review for persistence, data exposure, or account abuse if exploitation went beyond a simple crash.
Standards & Framework Alignment
This section maps relevant standards and security frameworks to the operational risks and controls described in this guidance.
CIS Controls v8, NIST SP 800-53 Rev 5 and OWASP ASVS set the governance and control requirements practitioners need to meet.
| Framework | Control / Reference | Relevance |
|---|---|---|
| CIS Controls v8 | CIS-7 — Continuous Vulnerability Management | CVE-2020-9818 is a known exploited vulnerability that needs prioritized vulnerability tracking. |
| Recommendation — Prioritise this CVE in continuous vulnerability management and verify remediation on exposed devices. | ||
| NIST SP 800-53 Rev 5 | SI-2 — Flaw Remediation | A memory-safety vulnerability in Apple Mail requires timely flaw remediation and verification. |
| SI-3 — Malicious Code Protection | Mail is an external content entry point, so protective controls help limit exploit delivery. | |
| AU-6 — Audit Record Review, Analysis, and Reporting | Post-exploitation review depends on analyzing logs and device events after suspected abuse. | |
| Recommendation — Apply flaw remediation processes to patch affected Apple devices and confirm the fix. Use malicious content filtering and endpoint protections to reduce exploit delivery through email. Review logs and device telemetry for signs of exploitation and follow-on abuse. | ||
| OWASP ASVS | V16 — Security Logging and Error Handling | This flaw highlights the need to avoid crash-prone error handling and to preserve useful security telemetry. |
| Recommendation — Validate logging and error handling so parser failures are observable without exposing sensitive detail. | ||
Practitioner Guidance
Why practitioners should care: Mail clients are high-frequency attack surfaces, so a known exploited memory-safety bug deserves faster handling than a low-traffic application flaw. Prioritise it based on device exposure, user privilege, and whether the affected endpoint handles sensitive or regulated communications.
What to watch for: Repeated Mail crashes, abnormal message rendering, and unexplained user complaints around opening specific emails are all worth investigating. If a device was exposed before patching, assume the possibility of a broader compromise review rather than treating the issue as resolved by installation alone.