Access-control bypass via actor-agnostic ActivityPub cache keys
Published Feb 4, 2026 · Updated Feb 4, 2026
Web cache poisoning in Mastodon before 4.3.19, 4.4.13, and 4.5.6 allows remote attackers to bypass ActivityPub block controls. Pinned-post and featured-hashtag endpoints cache signer-dependent responses under keys that omit the signing actor, so later actors receive the earlier result. AUTHORIZED_FETCH must be enabled and the target must block the actor or its server; exploitation exposes collections or suppresses them for legitimate actors.
Summary
What happened
Web cache poisoning in Mastodon before 4.3.19, 4.4.13, and 4.5.6 allows remote attackers to bypass ActivityPub block controls. Pinned-post and featured-hashtag endpoints cache signer-dependent responses under keys that omit the signing actor, so later actors receive the earlier result. AUTHORIZED_FETCH must be enabled and the target must block the actor or its server; exploitation exposes collections or suppresses them for legitimate actors.
The record
- CVE
- CVE-2026-25540
- Published
- Feb 4, 2026
- Updated
- Feb 4, 2026
- Vendor
- Mastodon
- Product
- Mastodon
- Classifications
- CWE-524, T1190
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Feb 4, 2026CVE publishedPublication date reported by the CVE source.
- Feb 4, 2026Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
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Is a vulnerable build present?
Compare these published version ranges with your installed build and any vendor patches.
- Affected versionversion=< 4.3.19
- Affected versionversion=< 4.4.13
- Affected versionversion=< 4.5.6
What conditions does exploitation require?
What is affected?
Published CVSS scores
CVSS describes severity. EPSS estimates exploitation probability.
Attacks
What attackers are doing with it
Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.
Weakness, pattern, technique
Public exploit references
No public exploit references are available in this record.
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Technologies
Your stack
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Your stack
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