CVE-2026-39972 — mercure
Fix: dunglas/mercure@4964a69CVE-2026-39972 is a CWE-1289 vulnerability in github.com/dunglas/mercure. A fix is available for github.com/dunglas/mercure — see the affected versions and patch details below.
Mercure has a Topic Selector Cache Key Collision
Exploitation Status
No confirmed exploitation observed yet
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-39972.
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
Real-World Exposure
github.com/dunglas/mercureReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects Go packages — download data is not available via public APIs for these ecosystems.
Description
Impact
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.
Patches
The vulnerability is fixed by replacing string-encoded cache keys with typed Go struct keys that are inherently collision-free:
type matchCacheKey struct {
topicSelector string
topic string
}
The internal TopicSelectorStoreCache interface and sharded cache abstraction have also been removed in favor of a single typed otter cache.
Users should upgrade to version 0.22.0 or later.
Workarounds
Disable the topic selector cache by setting topic_selector_cache to -1 in the Caddyfile, or by passing a cache size of 0 when using the library directly. This eliminates the vulnerability at the cost of reduced performance.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐹Go | github.com/dunglas/mercure | all versions | 0.22.0go get github.com/dunglas/mercure@v0.22.0 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/dunglas/mercure, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update github.com/dunglas/mercure to 0.22.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-39972 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2026-39972 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2026-39972. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2026-39972 in your dependencies?
O3 Security finds CVE-2026-39972 across Go dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.