Cyber Resilience

CVE-2026-33868

Medium

Published: 27 March 2026

Published
27 March 2026
Modified
30 March 2026
KEV Added
—
Patch
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CVSS Score v3.1 4.3 CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:L/A:N
EPSS Score 0.0140 80.8th percentile
Risk Priority 9 60% EPSS · 20% KEV · 20% CVSS

Summary

CVE-2026-33868 is a medium-severity Open Redirect (CWE-601) vulnerability in Joinmastodon Mastodon. Its CVSS base score is 4.3 (Medium).

Operationally, exploitation aligns with the MITRE ATT&CK technique Spearphishing Link (T1566.002); ranked in the top 19.2% of CVEs by exploit likelihood; it is not currently listed in the CISA KEV catalog.

EU & UK References

Vulnerability details

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…

more

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.

CWE(s)
CWE-601

Related Threats

MITRE ATT&CK Enterprise TechniquesAI

T1566.002 Spearphishing Link Initial Access
Adversaries may send spearphishing emails with a malicious link in an attempt to gain access to victim systems.
attack.mitre.org →
T1528 Steal Application Access Token Credential Access
Adversaries can steal application access tokens as a means of acquiring credentials to access remote systems and resources.
attack.mitre.org →
Why these techniques?

Open redirect directly enables crafted phishing links (T1566.002) that can steal OAuth tokens (T1528).

Confidence: MEDIUM · MITRE ATT&CK Enterprise v18.1

Affected Assets

joinmastodon
mastodon
≤ 4.3.21 · 4.4.0 — 4.4.15 · 4.5.0 — 4.5.8

Mitigating Controls

Likely Mitigating Controls AI

Per-CVE control mapping for this CVE has not run yet; the list below is derived from the weakness types (CWEs) cited in the NVD entry.

AT-2 Literacy Training and Awareness
addresses: CWE-601

Security awareness includes verifying URLs and avoiding untrusted redirects that lead to malicious sites.

SI-10 Information Input Validation
addresses: CWE-601

Validates redirect targets and URLs to ensure they conform to allowed destinations.

References