What’s Vulnerable › Vulnerabilities

CVE-2026-39972: Mercure has a Topic Selector Cache Key Collision

• Severity: high (CVSS 7.1 v4.0)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.3% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-04-09 · updated 2026-10-26
• Weakness: CWE-1289

Description

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 products

• dunglas mercure: before 0.22.0

References

• https://github.com/dunglas/mercure/commit/4964a69be904fd61e35b5f1e691271663b6fdd64
• https://github.com/dunglas/mercure/security/advisories/GHSA-hwr4-mq23-wcv5

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