How severe is CVE-2026-6409?

No CVSS score has been assigned to CVE-2026-6409 yet. Review the advisory details and affected package list to assess your exposure.

Which packages are affected by CVE-2026-6409?

CVE-2026-6409 affects the following packages: google/protobuf (Packagist). Ecosystems affected: Packagist.

How do I fix CVE-2026-6409?

Update google/protobuf to 4.33.6 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-6409 is resolved across your whole dependency graph.

How do I detect CVE-2026-6409 in my Packagist dependencies?

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.

How do I mitigate CVE-2026-6409 if there is no patch (or I can't update yet)?

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.

Is CVE-2026-6409 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-6409 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2026-6409?

CVE-2026-6409 has an EPSS (Exploit Prediction Scoring System) score of 0.4%, placing it in the 30th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2026-6409?

CVE-2026-6409 is classified as Improper Input Validation (CWE-20). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2026-6409 published, and has it been updated?

CVE-2026-6409 was published on April 16, 2026 and was last updated on October 28, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🐘 Packagist

Not in CISA KEV

CVE-2026-6409 — protobuf

Fix: protocolbuffers/protobuf@60e93d2

CVE-2026-6409 is a Improper Input Validation vulnerability in google/protobuf. A fix is available for google/protobuf — see the affected versions and patch details below.

Denial of Service (DoS) vulnerability exists in the Protobuf PHP library during the parsing of untrusted input

Also known asGHSA-p2gh-cfq4-4wjc

Published

Apr 16, 2026

Updated

Oct 28, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Nov 8, 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 CVE-2026-6409.

EPSS Exploitation Probability

via FIRST.org ↗

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

Probability of exploitation in the next 30 days, from 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 varints 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

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.
Fix
Update google/protobuf to 4.33.6 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-6409 is resolved across your whole dependency graph.
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.

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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