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CVE-2026-27477— Mastodon has SSRF via unvalidated FASP Provider base_url

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 4.4.0版本至4.4.13版本和4.5.0版本至4.5.6版本存在代码问题漏洞，该漏洞源于未经验证的攻击者可以注册一个base_url指向本地内部地址的FASP，可能导致Mastodon服务器向内部系统发起请求。

AI Predicted 5.3 Difficulty: Easy EPSS 0.36% · P27
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I. Basic Information for CVE-2026-27477

Vulnerability Information
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Vulnerability Title
Mastodon has SSRF via unvalidated FASP Provider base_url
Source: CVE Program / CVE List V5
Vulnerability Description
Mastodon is a free, open-source social network server based on ActivityPub. FASP registration requires manual approval by an administrator. In versions 4.4.0 through 4.4.13 and 4.5.0 through 4.5.6, an unauthenticated attacker can register a FASP with an attacker-chosen `base_url` that includes or resolves to a local / internal address, leading to the Mastodon server making requests to that address. This only affects Mastodon servers that have opted in to testing the experimental FASP feature by setting the environment variable `EXPERIMENTAL_FEATURES` to a value including `fasp`. An attacker can force the Mastodon server to make http(s) requests to internal systems. While they cannot control the full URL that is being requested (only the prefix) and cannot see the result of those requests, vulnerabilities or other undesired behavior could be triggered in those systems. The fix is included in the 4.4.14 and 4.5.7 releases. Admins that are actively testing the experimental "fasp" feature should update their systems. Servers not using the experimental feature flag `fasp` are not affected.
Source: CVE Program / CVE List V5
CVSS Information
CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N/E:U
Source: CVE Program / CVE List V5
Vulnerability Type
服务端请求伪造(SSRF)
Source: CVE Program / CVE List V5
Vulnerability Title
Mastodon 代码问题漏洞
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 4.4.0版本至4.4.13版本和4.5.0版本至4.5.6版本存在代码问题漏洞，该漏洞源于未经验证的攻击者可以注册一个base_url指向本地内部地址的FASP，可能导致Mastodon服务器向内部系统发起请求。
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 >= 4.4.0, < 4.4.14 -

II. Public POCs for CVE-2026-27477

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III. Intelligence Information for CVE-2026-27477

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Patches & Fixes for CVE-2026-27477 (1)

IV. Related Vulnerabilities

Same product: mastodon
Same vendor: mastodon
Same weakness: CWE-918

V. Comments for CVE-2026-27477

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