CVE-2026-59876: protobufjs compiles protobuf definitions into JavaScript (JS) functions

Q: How do I fix CVE-2026-59876?

protobufjs: upgrade to 8.6.5. Exact commands for each OS are on this page.

Q: Is CVE-2026-59876 being exploited?

CVE-2026-59876 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

Q: What does CVE-2026-59876 affect?

CVE-2026-59876 affects protobufjs. Severity is medium (CVSS 4.8). protobufjs compiles protobuf definitions into JavaScript (JS) functions. From 8.2.0 until 8.6.5, the protobufjs Text Format extension parsed string-keyed map entries using ordinary property assignment, allowing a map entry with key __proto_

Q: How urgent is CVE-2026-59876?

Priority 29/100. Remotely exploitable, no login, no user action

Routine

MEDIUM 4.8REMOTEUPGRADE

CVE-2026-59876 is a medium-severity flaw (CVSS 4.8) affecting protobufjs. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

protobufjs compiles protobuf definitions into JavaScript (JS) functions. From 8.2.0 until 8.6.5, the protobufjs Text Format extension parsed string-keyed map entries using ordinary property assignment, allowing a map entry with key __proto__ to change the prototype of the returned map object instead of creating an own map entry in protobufjs/ext/textformat. This issue is fixed in version 8.6.5.

Why this priority

Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	4.8 (Medium)
Attack vector	Network (remote)
Attack complexity	High
Privileges needed	None
User interaction	None
EPSS exploit chance	0.3% (top 72%)
CISA exploited list	No
Weakness	CWE-1321
Published	2026-07-08
Last modified	2026-10-25

Publisher (CNA): GitHub, Inc. · Vendor: protobufjs project

Last updated 2026-10-25 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-07-08 16:16 UTCPublished CVE record published
2026-10-25 17:10 UTCUpdated Record last modified

protobufjs project protobufjs UPGRADE

Affected: >= 8.2.0 and < 8.6.5

Fixed in: 8.6.5

Safe to downgrade to: a release before 8.2.0

1. Check installed version

npm ls protobufjs

2. UPGRADE to 8.6.5

npm install protobufjs@">=8.6.5"

3. If no update is available yet, DOWNGRADE to a release before 8.2.0

npm view protobufjs versions npm install protobufjs@"<8.2.0"

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-59876?

protobufjs: upgrade to 8.6.5. Exact commands for each OS are on this page.

Is CVE-2026-59876 being exploited?

CVE-2026-59876 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-59876 affect?

CVE-2026-59876 affects protobufjs. Severity is medium (CVSS 4.8). protobufjs compiles protobuf definitions into JavaScript (JS) functions. From 8.2.0 until 8.6.5, the protobufjs Text Format extension parsed string-keyed map entries using ordinary property assignment, allowing a map entry with key __proto_

How urgent is CVE-2026-59876?

Priority 29/100. Remotely exploitable, no login, no user action

Related CVEs

CVE-2026-59877 MEDIUM 5.3 UPGRADE protobufjs compiles protobuf definitions into JavaScript (JS) functions

References

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