# CVE-2026-6409: Denial of Service (DoS) vulnerability exists in the Protobuf PHP library during the parsing of untrusted input

CVE-2026-6409 is fixed in protobuf on 1 platform, first by DEBIAN-CVE-2026-6409, released April 16, 2026. It is not reported as exploited. Patch Radar priority: High.

- Web page: https://patchblox.com/security/cve/CVE-2026-6409
- Priority: High
- CVSS: n/a (Important)
- EPSS: 0.0036
- CISA KEV: not listed
- Exploited (vendor report): no
- Publicly disclosed: no
- Published: Apr 16, 2026
- Weakness: Improper Input Validation
- Vendor advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-6409

## Description

A Denial of Service (DoS) vulnerability exists in the Protobuf PHP library during the parsing of untrusted input. Maliciously structured messages—specifically those containing negative varints or deep recursion—can be used to crash the application, impacting service availability.

## Where it's fixed

| Platform | Product | Fixed in | Fix | Released |
|---|---|---|---|---|
| Linux | protobuf Debian 12.15 (bookworm) | 3.21.12-3+deb12u1 | [DEBIAN-CVE-2026-6409](https://patchblox.com/security/data/fix/linux/debian-cve-2026-6409.md) | Apr 16, 2026 |
| Linux | protobuf Debian 13.5 (trixie) | 3.21.12-11+deb13u1 | [DEBIAN-CVE-2026-6409](https://patchblox.com/security/data/fix/linux/debian-cve-2026-6409.md) | Apr 16, 2026 |
| Linux | protobuf Debian 14 | 3.21.12-16 | [DEBIAN-CVE-2026-6409](https://patchblox.com/security/data/fix/linux/debian-cve-2026-6409.md) | Apr 16, 2026 |

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Data as of 2026-11-08T00:53:09Z UTC. Source: Patchblox Patch Radar (https://patchblox.com/security/patch-radar). Patchblox finds out-of-date software on Windows, macOS and Linux and updates it through release rings: https://patchblox.com/demo

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