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CVE-2026-33868— Mastodon has a GET-Based Open Redirect via '/web/%2F<domain>'

Quick assessment

Affected
mastodon mastodon
Exploitation
Public or AI PoC available; prioritize validation
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 4.5.8之前版本、4.4.15之前版本和4.3.21之前版本存在输入验证错误漏洞，该漏洞源于/web/*路由对URL编码路径段处理不当，可能导致未经验证的重定向和钓鱼攻击。

CVSS 4.3 · Medium EPSS 0.55% · P44

Public Exploits 1

Nuclei · 1 CVE-2026-33868.yaml [template]

Possible ATT&CK Techniques 1 AI

T1189 · Drive-by Compromise
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I. Basic Information for CVE-2026-33868

Vulnerability Information
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Vulnerability Title
Mastodon has a GET-Based Open Redirect via '/web/%2F<domain>'
Source: CVE Program / CVE List V5
Vulnerability Description
Mastodon is a free, open-source social network server based on ActivityPub. Prior to versions 4.5.8, 4.4.15, and 4.3.21, an unauthenticated Open Redirect vulnerability (CWE-601) exists in the `/web/*` route due to improper handling of URL-encoded path segments. An attacker can craft a specially encoded URL that causes the application to redirect users to an arbitrary external domain, enabling phishing attacks and potential OAuth credential theft. The issue occurs because URL-encoded slashes (`%2F`) bypass Rails path normalization and are interpreted as host-relative redirects. Versions 4.5.8, 4.4.15, and 4.3.21 patch the issue.
Source: CVE Program / CVE List V5
CVSS Information
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:L/A:N
Source: CVE Program / CVE List V5
Vulnerability Type
指向未可信站点的URL重定向(开放重定向)
Source: CVE Program / CVE List V5
Vulnerability Title
Mastodon 输入验证错误漏洞
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 4.5.8之前版本、4.4.15之前版本和4.3.21之前版本存在输入验证错误漏洞，该漏洞源于/web/*路由对URL编码路径段处理不当，可能导致未经验证的重定向和钓鱼攻击。
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 >= 4.5.0, < 4.5.8 -

II. Public POCs for CVE-2026-33868

# POC Description Source Link Shenlong Link
1 Mastodon version < 4.5.8, < 4.4.15, < 4.3.21 is vulnerable to unauthenticated Open Redirect vulnerability (CWE-601) exists in the /web/* route due to improper handling of URL-encoded path segments. https://github.com/projectdiscovery/nuclei-templates/blob/main/http/cves/2026/CVE-2026-33868.yaml POC Details
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III. Intelligence Information for CVE-2026-33868

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Vendor Advisories for CVE-2026-33868 (1)

IV. Related Vulnerabilities

Same product: mastodon
Same vendor: mastodon
Same weakness: CWE-601

V. Comments for CVE-2026-33868

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