# CVE-2026-6409: Google Protobuf DoS via negative varints or deep recursion in PHP

Source: https://junglewise.ai/threats/cve-2026-6409-google-protobuf-dos-via-negative-varints-or-deep-recursion-in-php  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-04-16. Updated: 2026-05-11  
License: CC BY 4.0 with a link back

CVE-2026-6409 · Severity: high · CVSS 4 · Published 2026-04-16

A denial-of-service vulnerability exists in the Google Protobuf PHP library, which is used for serializing and structured data exchange. An attacker can send specially crafted messages to crash applications using this library, leading to service outages and unavailability. This impact can disrupt business operations that rely on automated data processing or communication between services.

Technologies: [Google Protobuf](https://junglewise.ai/threats/technologies/protobuf), [google/protobuf (Packagist)](https://junglewise.ai/threats/technologies/google-protobuf). Vendors: [Google](https://junglewise.ai/threats/vendors/google), [Packagist](https://junglewise.ai/threats/vendors/packagist).

## Quick answers

**Is CVE-2026-6409 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-6409 being exploited in the wild.

**How severe is CVE-2026-6409?** High severity, with a CVSS score of 4, published 16 April 2026.

**Which products and versions are affected?** Google Protobuf < 4.33.6; google/protobuf (Packagist) < 4.33.6, ECOSYSTEM:introduced=0,fixed=4.33.6.

**Is there a fix for CVE-2026-6409?** Yes. google/protobuf (Packagist): upgrade to 4.33.6 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-6409 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-6409?** Check whether you run Google Protobuf or google/protobuf (Packagist) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- google/protobuf (Packagist): upgrade to 4.33.6 or later

  ```sh
  composer require google/protobuf:^4.33.6
  ```


## Technical details

A Denial of Service (DoS) vulnerability exists in the Google Protobuf PHP library (google/protobuf) due to improper handling of resource consumption during the parsing of untrusted input. Specifically, the library fails to safely process maliciously structured messages containing negative varints or deeply nested recursive structures. An unauthenticated remote attacker can exploit this by sending a crafted payload over the network, causing the PHP application to crash or consume excessive resources. This issue is tracked as CWE-400 (Uncontrolled Resource Consumption). Patches have been released in versions 4.33.6 and 5.34.0-RC1.

## Affected products

- google protobuf < 4.33.6

## Identifiers

- CVE-2026-6409
- GHSA-p2gh-cfq4-4wjc

## Timeline

- 2026-03-23: disclosed: Vulnerability reported to protocolbuffers/protobuf repository.
- 2026-03-25: advisory: GitHub Advisory GHSA-p2gh-cfq4-4wjc published.
- 2026-03-25: patched: Fixes released in version 4.33.6.

## References

- [https://api.github.com/users/34selen](https://api.github.com/users/34selen)
- [https://github.com/34selen](https://github.com/34selen)
- [https://api.github.com/users/34selen/gists%7B/gist_id%7D](https://api.github.com/users/34selen/gists%7B/gist_id%7D)
- [https://api.github.com/users/34selen/repos](https://api.github.com/users/34selen/repos)
- [https://avatars.githubusercontent.com/u/73600142?v=4](https://avatars.githubusercontent.com/u/73600142?v=4)
- [https://api.github.com/users/34selen/events%7B/privacy%7D](https://api.github.com/users/34selen/events%7B/privacy%7D)

## Related threats

- [Google Protobuf PHP denial of service via malicious messages](https://junglewise.ai/threats/google-protobuf-php-denial-of-service-via-malicious-messages-485224ad)
- [Google Protobuf denial of service in Python json_format.ParseDict](https://junglewise.ai/threats/cve-2026-0994-google-protobuf-denial-of-service-in-python-json-format-parsedict)
- [Google Protobuf heap buffer overflow](https://junglewise.ai/threats/cve-2015-5237-google-protobuf-heap-buffer-overflow)
- [Withdrawn Advisory: NULL Pointer Dereference in Protocol Buffers](https://junglewise.ai/threats/cve-2021-22570-withdrawn-advisory-null-pointer-dereference-in-protocol-buffers)