CVE-2026-59876 is a medium-severity (CVSS 4.8) CWE-1321 vulnerability in protobufjs. A fix is available for protobufjs — see the affected versions and patch details below.
protobufjs: Text Format string map parsing can mutate returned map object prototype
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-2026-59876.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
CVE-2026-59876 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.
Where this sits among everything scored
Of 394,803 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.
Real-World Exposure
How broadly this vulnerability is actually deployed: weekly install volume shows current usage, and reverse-dependency count shows how many other packages break if it stays unpatched.
protobufjsnpmDescription
Summary
The protobuf.js text format extension parsed string-keyed map entries using ordinary property assignment. A text-format map entry with key __proto__ could therefore change the prototype of the returned map object instead of creating an own map entry.
This affected the optional Text Format extension. Other protobufjs input paths, including binary decode, fromObject, and ProtoJSON conversion, are not affected.
Impact
An attacker who can provide protobuf text-format input parsed by an application using protobufjs/ext/textformat may be able to create message objects whose string-keyed map fields have attacker-controlled prototypes.
This is per-object prototype mutation, not global Object.prototype pollution. Impact depends on downstream application logic treating inherited properties as meaningful, for example by using in, truthiness checks, or direct property access on parsed map objects instead of own-property checks.
Applications that do not parse untrusted Text Format input, or that do not use inherited properties from parsed map objects in security-relevant logic, are not directly affected.
Preconditions
- The application must parse attacker-controlled protobuf Text Format input with
protobufjs/ext/textformat. - The target schema must contain a string-keyed map field.
- The crafted input must provide a map entry with key
__proto__. - Downstream application logic must treat inherited properties on the returned map object as meaningful for impact beyond malformed output.
Workarounds
Upgrade to protobufjs 8.6.5 or newer.
If immediate upgrade is not possible, do not parse untrusted protobuf Text Format input with affected versions. Applications can also reject string map keys named __proto__ before or during Text Format parsing, and should use own-property checks such as Object.hasOwnProperty.call(map, key) when consuming parsed map objects.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | protobufjs | ≥ 8.2.0&&< 8.6.5 | 8.6.5npm install protobufjs@8.6.5 |
Affected Products
protobufjsprotobufjs_projectDetection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for protobufjs, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update protobufjs to 8.6.5 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-59876 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
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.
This Moderate-impact flaw in protobufjs allows an attacker to perform prototype pollution by providing specially crafted input to the Text Format extension. This could lead to information disclosure or other impacts by altering the behavior of JavaScript objects within applications utilizing protobufjs to process…
Mitigation for this issue is either not available or the currently available options do not meet the Red Hat Product Security criteria comprising ease of use and deployment, applicability to widespread installation base, or stability.Source: Red Hat security advisory for CVE-2026-59876 (CC BY 4.0)
Frequently Asked Questions
Is CVE-2026-59876 in your dependencies?
Find it across npm, including transitive dependencies.