CVE-2026-25540: 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…

PriorityP338medium6.5CVSS 3.1

AVNACLPRNUINSUCLINAL

EPSS

0.39%

31.2th percentile

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.

Affected

6 ranges

Vendor	Product	Version range	Fixed in
joinmastodon	mastodon	< 4.3.19	4.3.19
joinmastodon	mastodon	>= 4.4.0 < 4.4.13	4.4.13
joinmastodon	mastodon	>= 4.5.0 < 4.5.6	4.5.6
mastodon	mastodon	< 4.3.19	4.3.19
mastodon	mastodon	< 4.4.13	4.4.13
mastodon	mastodon	< 4.5.6	4.5.6

Stop checking back — get the weekly exploitation signal.

Every Monday: what got weaponized or added to CISA KEV in the last seven days — each CVE cross-linked to its PoC, Nuclei template, and detection rule. Free, one email a week, unsubscribe in one click.