# Protobuf: Denial of Service issue through malicious…

🐘 Packagist

Not in CISA KEV

# Protobuf: Denial of Service issue through malicious messages containing negative varints or deep recursionGHSA-p2gh-cfq4-4wjc

[Fix: protocolbuffers/protobuf@60e93d2](https://github.com/protocolbuffers/protobuf/commit/60e93d2d104f2af9cd345b1c6f3891d91430244a)

GHSA-p2gh-cfq4-4wjc is a Improper Input Validation vulnerability in google/protobuf. A fix is available for google/protobuf — see the affected versions and patch details below.

Also known as[CVE-2026-6409](/vulnerability/CVE-2026-6409)

Published

Mar 25, 2026

Updated

Apr 16, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Nov 10, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

## Exploitation Status

### No confirmed exploitation observed yet

-   CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.

Exploitation and automatability from CISA’s SSVC triage for GHSA-p2gh-cfq4-4wjc.

## EPSS Exploitation Probability

[via FIRST.org ↗](https://www.first.org/epss/)

0.4%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs30th percentile — riskier than 30% of all scored CVEsHighest risk

0.00%0.31%0.62%0.93%0.1%0.4%May 26Sep 26Nov 26

Probability of exploitation in the next 30 days, from [FIRST.org EPSS](https://www.first.org/epss/).

## Real-World Exposure

1 pkg affected

🐘`google/protobuf`

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Packagist packages — download data is not available via public APIs for these ecosystems.

## Description

#### Impact

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 `varint`s or deep recursion—can be used to crash the application, impacting service availability.

#### Patches

Patches have been released to 5.34.0-RC1 and 4.33.6.

## Affected Packages

1 total 1 fixed

Ecosystem

Package

Vulnerable range

Fix

🐘Packagist

`google/protobuf` 

all versions

4.33.6`composer require google/protobuf:^4.33.6`

## Detection & mitigation playbook

Open-source dependency

1.  ### Detect
    
    Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for google/protobuf, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
    
2.  ### Fix
    
    Update google/protobuf to 4.33.6 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-p2gh-cfq4-4wjc is resolved across your whole dependency graph.
    
3.  ### Workarounds
    
    Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.
    

## Fixing This On Your OS

If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.

Ubuntu

[USN-8943-1Ubuntu](https://ubuntu.com/security/notices/USN-8943-1)

## Frequently Asked Questions

\### Impact 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 \`varint\`s or deep recursion—can be used to crash the application, impacting service availability. ### Patches Patches have been released to 5.34.0-RC1 and 4.33.6.

O3 Security · Impact-Aware SCA

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