Medium severity · published Jul 8, 2026
CVE-2026-59876
protobufjs: Text Format string map parsing can mutate returned map object prototype
4.8
CVSS
0.3%
EPSS
No
On KEV
Description
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.
CVSS vector
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N
Attack vector
Network
Complexity
High
Privileges
None
User interaction
None
Confidentiality
Low
Integrity
Low
Availability
None
Scope
Unchanged
Exploitation
Is it being used?
Exploit prediction (EPSS)0.3% · higher than 28% of CVEs
Exploited in the wildNo evidence yet
CISA KEVNot listed
Ransomware campaign useNot known
Affected products
Which versions
PR
protobuf.js · protobufjs
>= 8.2.0, < 8.6.5 · source: vendor
PR
Protobufjs · Protobufjs Project
8.2.0 before 8.6.5 · source: NVD
References
NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-59876
CVE.org
https://www.cve.org/CVERecord?id=CVE-2026-59876
Patch
https://github.com/protobufjs/protobuf.js/commit/9f97fe413072d3beb52c74e62d88ea8adc9444d8
Issue Tracking
https://github.com/protobufjs/protobuf.js/pull/2335
Product
https://github.com/protobufjs/protobuf.js/releases/tag/protobufjs-v8.6.5
Mitigation
https://github.com/protobufjs/protobuf.js/security/advisories/GHSA-jfj6-75fj-8934
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