CVE-2026-39972 Mercure has a Topic Selector Cache Key Collision
An attacker can poison the cache to bypass authorization checks, delivering private updates to unauthorized subscribers or blocking authorized ones.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-39972?
CVE-2026-39972 is a high-severity authentication bypass vulnerability in dunglas mercure, published on 9 Apr 2026. Any signed-in user can exploit it over the network, with no action from the victim.
Official 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.
Weakness: CWE-1289
Which versions are affected by CVE-2026-39972?
3 open source packages ship an affected version, including github.com/dunglas/mercure.
| Package | Affected | Fixed in |
|---|---|---|
github.com/dunglas/mercure Go | All versions | 0.22.0 |
github.com/dunglas/mercure Go | All versions | 0.22.0 |
github.com/dunglas/mercure Go | All versions | 0.22.0 |
How do I fix CVE-2026-39972?
Upgrade github.com/dunglas/mercure to 0.22.0 or later.
Upgrade, and rotate credentials and sessions that may have been exposed. More about Authentication bypass.
Is CVE-2026-39972 being exploited?
There is no known exploitation of CVE-2026-39972 in the wild and no public exploit code.
Sources
Also known as GHSA-hwr4-mq23-wcv5, GO-2026-5444.