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Exploited. On CISA's list since 2021-11-03.
CVE-2021-20090 · Arcadyan Buffalo Firmware · Published 2021-04-29
9.8 out of 10
CVSS rates it critical: about as damaging as flaws get.
over 99%
Estimated chance of exploitation in the next 30 days, among the top 1% of all scored flaws (EPSS, 2026-10-29).
Yes
CISA has seen it exploited in real attacks. For US government agencies the fix is deadline passed 1808 days ago (2021-11-17).
Apply updates per vendor instructions.
Arcadyan Buffalo firmware contains a path traversal vulnerability that could allow unauthenticated, remote attackers to bypass authentication and access sensitive information. This vulnerability affects multiple routers across several different vendors.
A path traversal vulnerability in the web interfaces of Buffalo WSR-2533DHPL2 firmware version <= 1.02 and WSR-2533DHP3 firmware version <= 1.24 could allow unauthenticated remote attackers to bypass authentication.
From the NVD.
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A supplied file path escapes the directory the application intended to allow.
CWE-22 · CISA KEV; NVD record: 134c704f-9b21-4f2e-91b3-4a467353bcc0; NVD record: nvd@nist.gov
Reading and practice labs · Free account for labs. Basic HTTP knowledge; some labs use Burp Suite.
CWE is a catalogue of weakness types. Labels come from the sources shown and can differ or change. Resource matches reviewed 16 October 2026.
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Exploited flaws from CISA's KEV catalogue. Chance of exploitation from EPSS by FIRST. Descriptions and scores from the NVD. This product uses the NVD API but is not endorsed or certified by the NVD.