Is CVE-2026-25540 in the CISA KEV catalog?

No. CVE-2026-25540 is not currently on the CISA Known Exploited Vulnerabilities list.

What is the CVSS severity of CVE-2026-25540?

CVE-2026-25540 has a CVSS v3 base score of 6.5 (MEDIUM).

What is the EPSS score for CVE-2026-25540?

CVE-2026-25540 has an EPSS score of 0.39% (31th percentile), estimating the probability of exploitation in the next 30 days.

How do I fix CVE-2026-25540?

Upgrade mastodon to 4.5.6 or later.

CVE & CISA-KEV Catalog

CVE-2026-25540

MEDIUM

6.5

CVSS v3

NVD

Description

Mastodon is a free, open-source social network server based on ActivityPub. Prior to versions 4.3.19, 4.4.13, 4.5.6, Mastodon is vulnerable to web cache poisoning via `Rails.cache. When AUTHORIZED_FETCH is enabled, the ActivityPub endpoints for pinned posts and featured hashtags have contents that depend on the account that signed the HTTP request. However, these contents are stored in an internal cache and reused with no regards to the signing actor. As a result, an empty response generated for a blocked user account may be served to requests from legitimate non-blocked actors, or conversely, content intended for non-blocked actors may be returned to blocked actors. This issue has been patched in versions 4.3.19, 4.4.13, 4.5.6.

CWE-524

How to fix

Remediation Available

mastodonNVD

Affected:>= 4.5.0, < 4.5.6Fixed in:4.5.6CVE-2026-25540derived from NVD

Remediation is compiled from vendor and distribution security advisories. Always confirm against the linked source for your exact version and platform.

CVSS v3 Vector

Exploitability

Attack VectorNetwork

Attack ComplexityLow

Privileges RequiredNone

User InteractionNone

ScopeUnchanged

Impact

ConfidentialityLow

IntegrityNone

AvailabilityLow

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:L

Exploit Intelligence

0.39%probability of exploitation in 30 days

31stpercentile

Low risk: more likely to be exploited than 31% of all known CVEs.

References

Vendor Advisory1

github.com

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This product uses NVD data but is not endorsed or certified by the NVD. EPSS scores courtesy of FIRST.org (https://www.first.org/epss). Source: CISA KEV Catalog. Data as of 2026-02-20.