# 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.

- **Severity:** High 7.1 (`CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:N/VC:H/VI:N/VA:L/SC:N/SI:N/SA:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 9 Apr 2026
- **Updated:** 9 Apr 2026
- **Also known as:** GHSA-hwr4-mq23-wcv5, GO-2026-5444

## 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 (Improper Validation of Unsafe Equivalence in Input)

## Which versions are affected by CVE-2026-39972?

3 open source packages ship an affected version, including github.com/dunglas/mercure.


| Package | Ecosystem | 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.

## Is CVE-2026-39972 being exploited?

There is no known exploitation of CVE-2026-39972 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-39972 and fix it if it is affected.

What it is: 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.
Exploitation: There is no known exploitation of CVE-2026-39972 in the wild and no public exploit code.

Affected packages and fixed versions:
- github.com/dunglas/mercure (Go): affected all versions; fixed in 0.22.0
- github.com/dunglas/mercure (Go): affected all versions; fixed in 0.22.0
- github.com/dunglas/mercure (Go): affected all versions; fixed in 0.22.0

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-39972/ (Markdown: https://insights.sbomify.com/CVE-2026-39972/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-39972
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-39972
- OSV: https://osv.dev/vulnerability/CVE-2026-39972

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-39972/
Summaries are generated and may contain mistakes; the official description above is the record.