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CVE-2026-25540— Mastodon's signature-dependent ActivityPub collection responses cached under signature-independent keys (Web Cache Poisoning via `Rails.cache`)

Quick assessment

Affected
mastodon mastodon
Exploitation
No confirmed in-the-wild exploitation; assess based on exposure
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

Mastodon是Mastodon开源的一款基于ActivityPub的开源社交网络服务器。 Mastodon 4.3.19之前版本、4.4.13之前版本和4.5.6之前版本存在安全漏洞，该漏洞源于Web缓存投毒，可能导致缓存内容被错误复用。

CVSS 6.5 · Medium EPSS 0.46% · P38

Possible ATT&CK Techniques 1 AI

T1498 · Network Denial of Service
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I. Basic Information for CVE-2026-25540

Vulnerability Information
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Vulnerability Title
Mastodon's signature-dependent ActivityPub collection responses cached under signature-independent keys (Web Cache Poisoning via `Rails.cache`)
Source: CVE Program / CVE List V5
Vulnerability 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.
Source: CVE Program / CVE List V5
CVSS Information
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:L
Source: CVE Program / CVE List V5
Vulnerability Type
通过缓存导致的信息暴露
Source: CVE Program / CVE List V5
Vulnerability Title
Mastodon 安全漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
Mastodon是Mastodon开源的一款基于ActivityPub的开源社交网络服务器。 Mastodon 4.3.19之前版本、4.4.13之前版本和4.5.6之前版本存在安全漏洞，该漏洞源于Web缓存投毒，可能导致缓存内容被错误复用。
Source: CNNVD (China National Vulnerability Database)
CVSS Information
N/A
Source: CNNVD (China National Vulnerability Database)
Vulnerability Type
N/A
Source: CNNVD (China National Vulnerability Database)

Affected Products

Vendor Product Affected Versions CPE Subscribe
mastodon mastodon < 4.3.19 -

II. Public POCs for CVE-2026-25540

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III. Intelligence Information for CVE-2026-25540

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Vendor Advisories for CVE-2026-25540 (1)

IV. Related Vulnerabilities

Same product: mastodon
Same vendor: mastodon
Same weakness: CWE-524

V. Comments for CVE-2026-25540

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