GHSA-hwr4-mq23-wcv5
Fix: dunglas/mercure@4964a69GHSA-hwr4-mq23-wcv5 is a CWE-1289 vulnerability in github.com/dunglas/mercure. O3 Security confirms whether GHSA-hwr4-mq23-wcv5 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
mercure has Topic Selector Cache Key Collision
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 | 0.22.0 |
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. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
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 GHSA-hwr4-mq23-wcv5 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 pinpoints whether GHSA-hwr4-mq23-wcv5 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to GHSA-hwr4-mq23-wcv5. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is GHSA-hwr4-mq23-wcv5 in your dependencies?
O3 detects GHSA-hwr4-mq23-wcv5 across Go dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.