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CVE-2026-39972
Published:April 09, 2026
Updated:April 26, 2026
Mercure is a protocol for pushing data updates to web browsers and other HTTP clients in a battery-efficient way. Prior to 0.22.0, a cache key collision vulnerability in TopicSelectorStore allows an attacker to poison the match result cache, potentially causing private updates to be delivered to unauthorized subscribers or blocking delivery to authorized ones. The cache key was constructed by concatenating the topic selector and topic with an underscore separator. Because both topic selectors and topics can contain underscores, two distinct pairs can produce the same key. An attacker who can subscribe to the hub or publish updates with crafted topic names can exploit this to bypass authorization checks on private updates. This vulnerability is fixed in 0.22.0.
Affected Packages
https://github.com/dunglas/mercure.git (GITHUB):
Affected version(s) >=v0.1.0 <v0.22.0
Fix Suggestion:
Update to version v0.22.0
github.com/dunglas/mercure (GO):
Affected version(s) >=v0.0.0-20201125161040-506fa7a2eb4d <v0.22.0
Fix Suggestion:
Update to version v0.22.0
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CVSS v4
Base Score:
7.1
Attack Vector
NETWORK
Attack Complexity
LOW
Attack Requirements
NONE
Privileges Required
LOW
User Interaction
NONE
Vulnerable System Confidentiality
HIGH
Vulnerable System Integrity
NONE
Vulnerable System Availability
LOW
Subsequent System Confidentiality
NONE
Subsequent System Integrity
NONE
Subsequent System Availability
NONE
CVSS v3
Base Score:
7.1
Attack Vector
NETWORK
Attack Complexity
LOW
Privileges Required
LOW
User Interaction
NONE
Scope
UNCHANGED
Confidentiality
HIGH
Integrity
NONE
Availability
LOW
Weakness Type (CWE)
Improper Validation of Unsafe Equivalence in Input
EPSS
Base Score:
0.09