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🛡️ CVE-2026-39972 — mercure

🟠 CVSS 8.0 — High ✅ No Known Exploit CWE-1289 NVD
8.0
CVSS Score
0 Low4 Medium7 High9 Critical10

Description

mercure has Topic Selector Cache Key Collision

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:

```go

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:

```go

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.

How this vulnerability can be exploited

This issue can be reached over the network, attack complexity is low, an attacker needs low-level privileges on the target. No user interaction is required. Rated impact: confidentiality high, integrity none, availability low.

CVSS metrics in full

The score comes from this vector: 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

  • Attack vector: Network — reachable from anywhere that can route to the service.
  • Attack complexity: Low — the attack works reliably, with no preparation.
  • Attack requirements: None — no deployment-specific condition has to hold.
  • Privileges required: Low — an ordinary user account is enough.
  • User interaction: None — nobody has to be tricked into anything.
  • Confidentiality impact: High — total loss, or loss the attacker controls.
  • Integrity impact: None.
  • Availability impact: Low — limited, and the attacker does not choose what is affected.

Weakness class

CVE-2026-39972 is classified as CWE-1289: Improper Validation of Unsafe Equivalence in Input. 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.

Affected software

CVE-2026-39972 is recorded against 2 packages.

  • github.com/dunglas/mercure
  • unknown

Timeline and source

Published on 25 June 2026. No public exploit is currently recorded for this entry. A vendor advisory or fix has been published. Record sourced from NVD.

References

github.com (Advisory)
nvd.nist.gov (Advisory)
github.com (Fix)
github.com (Web)

Same weakness in other software

These advisories are the same class of weakness (CWE-1289: Improper Validation of Unsafe Equivalence in Input) in other software:

Details

Severity HIGH
CVSS Score 8.0
CVSS Vector 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
CWE CWE-1289
Public Exploit ✅ No
Source NVD
Published 2026-06-25
Updated 2026-08-20
Modified 2026-06-25

Affected Packages

Software From version Fixed in
github.com/dunglas/mercure — —
unknown — —

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