CVE-2026-39972: mercure has Topic Selector Cache Key Collision

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:

k = "m_" + topicSelector + "_" + topic

Because both topic selectors and topics can contain underscores, two distinct pairs can produce the same key:

selector="foo_bar"  topic="baz"     → key: "m_foo_bar_baz"
selector="foo"      topic="bar_baz" → key: "m_foo_bar_baz"

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.