CVE-2026-59876
ADVISORY - githubSummary
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.
Common Weakness Enumeration (CWE)
Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution')
GitHub
2.2
CVSS SCORE
4.8medium| Package | Type | OS Name | OS Version | Affected Ranges | Fix Versions |
|---|---|---|---|---|---|
| protobufjs | npm | - | - | >=8.2.0,<=8.6.4 | 8.6.5 |
CVSS:3 Severity and metrics
The CVSS metrics represent different qualitative aspects of a vulnerability that impact the overall score, as defined by the CVSS Specification.
The vulnerable component is bound to the network stack, but the attack is limited at the protocol level to a logically adjacent topology. This can mean an attack must be launched from the same shared physical (e.g., Bluetooth or IEEE 802.11) or logical (e.g., local IP subnet) network, or from within a secure or otherwise limited administrative domain (e.g., MPLS, secure VPN to an administrative network zone). One example of an Adjacent attack would be an ARP (IPv4) or neighbor discovery (IPv6) flood leading to a denial of service on the local LAN segment (e.g., CVE-2013-6014).
A successful attack depends on conditions beyond the attacker's control, requiring investing a measurable amount of effort in research, preparation, or execution against the vulnerable component before a successful attack.
The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files of the vulnerable system to carry out an attack.
The vulnerable system can be exploited without interaction from any user.
An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.
There is some loss of confidentiality. Access to some restricted information is obtained, but the attacker does not have control over what information is obtained, or the amount or kind of loss is limited. The information disclosure does not cause a direct, serious loss to the impacted component.
Modification of data is possible, but the attacker does not have control over the consequence of a modification, or the amount of modification is limited. The data modification does not have a direct, serious impact on the impacted component.
There is no impact to availability within the impacted component.