CVE-2026-25540
Joinmastodon Mastodon ≤ 4.3.19
Raw vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:LCVSS and EPSS are reproduced from their sources (NVD, FIRST EPSS). Risk Priority is our own derived reading, not an NVD score.
Summary
CVE-2026-25540 is a medium-severity Use of Cache Containing Sensitive Information (CWE-524) vulnerability in Joinmastodon Mastodon. Its CVSS base score is 6.5 (Medium).
Operationally, exploitation aligns with the MITRE ATT&CK technique Unsecured Credentials (T1552); ranked at the 32th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog.
The strongest mitigations our analysis identified map to AC-3 (Access Enforcement) and SC-4 (Information in Shared System Resources) — see the control section below for these in your framework.
EU & UK References
- 🇪🇺 ENISA EUVD: EUVD-2026-5329
Vulnerability Data
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…
more
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(s)
Related Threats
MITRE ATT&CK Enterprise Techniques
CVEs Like This One
Affected Assets
Mitigating Controls
Control response
—
—
—
V14.3.2
Mitigating Controls (NIST 800-53 r5) AI
Enforces access authorizations so that cached sensitive data cannot be read by actors outside the intended sphere.
Directly prevents unauthorized transfer of information through shared resources such as caches.
Controls information flows to keep sensitive data from leaving its authorized control sphere via caches.
Requires cryptographic or other protection of sensitive information at rest, which covers cached data.
Mitigating Controls (NIST CSF 2.0) AI
Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→CSF cross-walk (authority under review) — links open the control.
Protecting confidentiality of data-at-rest covers caches that persist sensitive information.
Enforcing least-privilege access permissions directly limits who can read cached sensitive data.
Protecting data-in-use can mitigate exposure when caches hold transient sensitive values.
Logical access controls on environments reduce the chance an outsider can reach the cache.
Hardened configuration baselines can disable or secure caching of sensitive data.
Mitigating Controls (ISO/IEC 27001:2022 Annex A) AI
Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→ISO cross-walk (authority under review) — links open the control.
Data leakage prevention can block unauthorized access to cached sensitive information.
Information access restriction directly limits who can read cached sensitive data.
Information deletion ensures sensitive data is removed from caches when no longer needed.
Cryptography can protect cached data at rest, but does not address access control scope.
Secure coding practices prevent caching of sensitive data outside intended boundaries.