CVE-2026-6409: Protobuf Denial of Service Vulnerability

HIGH
Advisory Identifier: GHSA-p2gh-cfq4-4wjc
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CWE-400
Published
Advisory Summary

CVE-2026-6409 is a HIGH denial of service vulnerability (CWE-400: Uncontrolled Resource Consumption) affecting Protobuf libraries in the Packagist ecosystem. The issue involves denial of service issue through malicious messages containing negative varints or deep recursion. Remediation is available by upgrading affected packages to version 4.33.6.

CVE-2026-6409 at a Glance

CVE Identifier CVE-2026-6409
GHSA Identifier GHSA-p2gh-cfq4-4wjc
Severity HIGH
CVSS v3.1 Base Score N/A
CWE Classification CWE-400: Uncontrolled Resource Consumption
Affected Product Protobuf
Package Ecosystem Packagist
Published Date
Last Updated

How to Fix CVE-2026-6409

To remediate this vulnerability, upgrade all affected Protobuf dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.

google/protobuf fixed: 4.33.6 
Upgrade google/protobuf to 4.33.6
Verify Direct and Transitive Dependencies

Run the following command in your project directory to verify that all resolved dependencies use the patched release: 

inspect dependencies for google/protobuf

How to Check if You Are Affected

To determine whether your application is affected by CVE-2026-6409, inspect your project dependencies for Protobuf:

inspect dependencies for google/protobuf

Analyze the terminal output to locate any version of Protobuf in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 4.33.6), your application is affected.

Note: Verify declared build manifests and deployed runtime container environments.

Technical Details

Vulnerability Mechanism & Technical Analysis

This vulnerability is classified under CWE-400: Uncontrolled Resource Consumption, where the product does not properly control the allocation and maintenance of a limited resource. In Protobuf, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.

Source Advisory Statement

### 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.

CVE-2026-6409 Affected Protobuf Versions

1 component(s)
Fixed Versions:
4.33.6
Product / Package Ecosystem Affected Versions Fixed Version
google/protobuf
 Packagist
< 4.33.6
4.33.6

References

GitHub Advisory / Wiki ADVISORY
https://github.com/protocolbuffers/protobuf/security/advisories/GHSA-p2gh-cfq4-4wjc
NVD — CVE-2026-6409 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-6409
Protobuf Source Repository REPOSITORY
https://github.com/protocolbuffers/protobuf/issues/24159
Protobuf Source Repository REPOSITORY
https://github.com/protocolbuffers/protobuf/issues/25067
Security Fix Commit FIX COMMIT
https://github.com/protocolbuffers/protobuf/commit/60e93d2d104f2af9cd345b1c6f3891d91430244a
Security Fix Commit FIX COMMIT
https://github.com/protocolbuffers/protobuf/commit/c8e9b27d95c6ab2d0668b5889e7dac2c477b7038
Protobuf Source Repository REPOSITORY
https://github.com/protocolbuffers/protobuf

Frequently Asked Questions

What is CVE-2026-6409?
CVE-2026-6409 is a security vulnerability in Protobuf involving Denial of Service issue through malicious messages containing negative varints or deep recursion under CWE-400: Uncontrolled Resource Consumption.
Which Protobuf versions are affected?
Affected versions include releases prior to 4.33.6 across affected packages: google/protobuf.
What version fixes CVE-2026-6409?
Upgrading to 4.33.6 (or the latest patched release for your respective branch) resolves the vulnerability.
Is google/protobuf affected?
Yes, google/protobuf is affected in releases prior to 4.33.6. Upgrade your dependency to 4.33.6 or later.
What CWE is associated with CVE-2026-6409?
CVE-2026-6409 is classified under CWE-400: Uncontrolled Resource Consumption (The product does not properly control the allocation and maintenance of a limited resource.).
How can I check Packagist dependencies for CVE-2026-6409?
Run the dependency tree command: 'inspect dependencies for google/protobuf'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.
Machine-readable OSV JSON OSV v1.5 Schema
Normalized schema output
View Raw JSON 
{
  "schema_version" : "1.4.0",
  "id" : "GHSA-p2gh-cfq4-4wjc",
  "modified" : "2026-04-16T22:59:37Z",
  "published" : "2026-03-25T21:02:08Z",
  "aliases" : [ "CVE-2026-6409" ],
  "summary" : "Protobuf: Denial of Service issue through malicious messages containing negative varints or deep recursion",
  "details" : "### Impact\nA 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.\n\n### Patches\nPatches have been released to 5.34.0-RC1 and 4.33.6.",
  "severity" : [ {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Packagist",
      "name" : "google/protobuf"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "4.33.6"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/protocolbuffers/protobuf/security/advisories/GHSA-p2gh-cfq4-4wjc"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-6409"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/protocolbuffers/protobuf/issues/24159"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/protocolbuffers/protobuf/issues/25067"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/protocolbuffers/protobuf/commit/60e93d2d104f2af9cd345b1c6f3891d91430244a"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/protocolbuffers/protobuf/commit/c8e9b27d95c6ab2d0668b5889e7dac2c477b7038"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/protocolbuffers/protobuf"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-400" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-03-25T21:02:08Z",
    "nvd_published_at" : null
  }
}