dunglas mercure up to 0.21.x TopicSelectorStore improper validation of unsafe equivalence in input

Summaryinfo

A vulnerability labeled as critical has been found in dunglas mercure up to 0.21.x. Affected by this issue is some unknown functionality of the component TopicSelectorStore. Executing a manipulation can lead to improper validation of unsafe equivalence in input. This vulnerability is tracked as CVE-2026-39972. The attack can be launched remotely. No exploit exists. The affected component should be upgraded.

Detailsinfo

A vulnerability was found in dunglas mercure up to 0.21.x. It has been declared as critical. This vulnerability affects an unknown part of the component TopicSelectorStore. The manipulation with an unknown input leads to a improper validation of unsafe equivalence in input vulnerability. The CWE definition for the vulnerability is CWE-1289. The product receives an input value that is used as a resource identifier or other type of reference, but it does not validate or incorrectly validates that the input is equivalent to a potentially-unsafe value. As an impact it is known to affect confidentiality, integrity, and availability. CVE summarizes:

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.

The advisory is shared for download at github.com. This vulnerability was named CVE-2026-39972 since 04/08/2026. The exploitation appears to be easy. The attack can be initiated remotely. There are neither technical details nor an exploit publicly available.

Upgrading to version 0.22.0 eliminates this vulnerability. Applying the patch 4964a69be904fd61e35b5f1e691271663b6fdd64 is able to eliminate this problem. The bugfix is ready for download at github.com. The best possible mitigation is suggested to be upgrading to the latest version.

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Productinfo

Vendor

• dunglas

Name

• mercure

Version

• 0.1
• 0.2
• 0.3
• 0.4
• 0.5
• 0.6
• 0.7
• 0.8
• 0.9
• 0.10
• 0.11
• 0.12
• 0.13
• 0.14
• 0.15
• 0.16
• 0.17
• 0.18
• 0.19
• 0.20
• 0.21

License

• open-source

Website

• Product: https://github.com/dunglas/mercure/

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