CriticalCVE-2021-20090

    Arcadyan Buffalo Firmware Path Traversal Vulnerability (CVE-2021-20090)

    Published In CISA KEV since Sources: CVE.org

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    What is affected, what to do

    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.

    The vulnerability is being actively exploited; treat it as a priority. The available sources name no fixed version. Check the vendor guidance in the references below. Immediate actions, mitigation and detection rules are in the runbook below.

    Affected
    • Buffalo WSR-2533DHPL2, Buffalo WSR-2533DHP3 eq WSR-2533DHPL2 <=1.02, WSR-2533DHP3 <= 1.24..WSR-2533DHPL2 <=1.02, WSR-2533DHP3 <= 1.24

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    Metrics

    9.8

    Priority

    XIEM Effective CVSS: highest available CVSS rating from NVD and vendor data.

    Source: NVD, CVSS v3

    9.8

    CVSS v3

    Severity Critical on the scale from 0 to 10.

    Source: CVE.org

    99.98 %

    EPSS score

    High — model estimates ≥ 50% chance of real-world exploitation within 30 days.

    Source: FIRST EPSS, percentile 99.98

    Show all metrics
    Severity
    Critical
    Priority (0–325)

    The priority score is a consolidated metric that condenses several individual signals into a single, action-guiding ranking: technical severity (CVSS), exploitation likelihood (EPSS), known-exploited status (KEV) and the context of the affected asset. The aim is to answer the only truly important question in vulnerability management: what first?

    Read in glossary
    228.97 Methodology
    Exploit maturity

    An exploit is code or a technique that specifically uses a concrete vulnerability to force unintended behaviour — such as executing code or escalating privileges. It is the tool that turns a theoretical flaw into a practical attack.

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    actively exploited
    XIEM Effective CVSS

    XIEM® is the tiered managed-security operating model from NEOSEC and a registered trademark of NEOSEC GmbH. It extends classic SIEM — the central collection, normalisation and correlation of security-relevant events — with endpoint forensics, network visibility and deception technology, and consolidates them into three building tiers: Sentry, Orchestrate and Command.

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    9.8 Methodology
    CVSS v3

    CVSS is the globally established, open standard for assessing the technical severity of a vulnerability on a scale from 0 to 10. It describes how easily a flaw can be exploited and what damage it can cause — but not how likely its actual exploitation is. CVSS and the complementary EPSS deliberately answer different questions.

    Read in glossary
    9.8
    EPSS score

    The EPSS score is the absolute model output for an individual vulnerability: the estimated probability of real-world exploitation within the next 30 days, expressed as a value between 0 and 1 (i.e. 0 % to 100 %). A score of 0.42 means „the model estimates a 42 % chance of real-world exploitation within 30 days“.

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    99.98 %
    High — model estimates ≥ 50% chance of real-world exploitation within 30 days.
    EPSS percentile

    The EPSS percentile is the relative position of a vulnerability within all EPSS-scored CVEs, expressed as a value between 0 and 1. A percentile of 0.90 means „this CVE has a higher EPSS score than 90 % of all other scored CVEs“. Unlike the EPSS score, it is not a probability — but a positioning within a ranking.

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    99.98
    Critical — this CVE ranks in the top fifth of all CVEs scored today (rank ≥ 80%).
    KEV status

    The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.

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    Actively exploited
    Published
    In CISA KEV since

    The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.

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    CISA assessment (SSVC)
    Exploitation: active · automatable · Technical impact: total
    Source: CISA Vulnrichment, as of 6 February 2025
    CWE

    CWE is a catalogue of typical weakness types in software and hardware — such as flawed input validation, unsafe memory handling or poor authentication. Where a CVE names a concrete vulnerability in a concrete product, a CWE describes the underlying class of defect.

    Read in glossary
    CWE-22

    Response and mitigation

    Why act now?

    Prioritisation rationale

    This CVE sits on KEV because it is network-reachable, requires no authentication and no user interaction, and has been scanned and auto-weaponised by Mirai derivatives (Moobot) since August 2021 — exploitation likelihood ranks among the very highest in the entire CVE corpus. The impact extends well beyond Buffalo: the Arcadyan reference codebase lives in roughly twenty OEM router families, which dramatically widens the attack surface across home-office, branch and remote-worker deployments. For NIS2-scope organisations the critical exposure is not the data centre but remote sites, teleworker routers and industrial gateways carrying Arcadyan SDK; there the CVE belongs on P1 as soon as WAN exposure or remote management is confirmed. Without external exposure and with proven segmentation it can be downgraded to P2/P3, but only after a documented review.

    1. Runbook step 1

      Immediate response (0-24 h)

      • Flash firmware: Buffalo WSR-2533DHPL2 to a version above 1.02, WSR-2533DHP3 to a version above 1.24. Pull the exact build from the Buffalo vendor advisory. Extend the same check to OEM siblings built on the Arcadyan SDK (ASUS, Beeline, BT, Deutsche Telekom, O2/Telefónica, Orange, Verizon, Vodafone branded units) — the bug sits in the Arcadyan codebase, not in Buffalo-specific code.
      • Remove the web management interface (TCP/80 and TCP/443) from the WAN immediately; if remote administration is enabled, disable it or restrict it via ACL to defined admin source ranges.
      • Treat any device that ever exposed port 80/443 on the WAN as potentially compromised: rotate the admin password, re-key the WLAN PSK, verify DNS settings against the known-good baseline (attackers favour rogue-DNS persistence).
      • Shodan/Censys sweep of your external IP ranges for exposed Buffalo/Arcadyan web interfaces; feed every hit into the emergency change process and patch with priority.
      • Export the configuration before reset (forensics), then factory-reset and reprovision on patched firmware — a plain update is insufficient if the device was already tampered with (persistence lives in NVRAM, cron, modified dnsmasq.conf).

    2. Runbook step 2

      Mitigation layers

      • Network segmentation: isolate SOHO and branch routers from the KRITIS perimeter, no management reachability from production LAN, management VLAN only through a hardened jump host.
      • WAF or reverse proxy in front of the admin UI: block requests containing „../", „%2e%2e%2f", „images/..", „bypass_check.html" or hits against /apply_abstract.cgi and /cgi-bin/-style paths carrying traversal patterns. Moobot/Mirai variants reuse exactly these signatures.
      • Perimeter egress filtering: deny and alert on outbound router connections to known Mirai C2 ranges and to non-NTP/non-DNS destinations on TCP/23, TCP/2323, TCP/37215, TCP/60001.
      • Vendor hygiene: schedule end-of-support Arcadyan-based devices (SDK generations from 2017/2018 are dead in many OEM branches) for replacement with actively maintained hardware; anchor the lifecycle in the CMDB.
      • Configuration monitoring: periodically hash the running configuration against a known-good baseline, especially DNS servers, port-forwarding rules, remote-admin flag and firmware version.
      • Default-credential audit: enforce credential rotation after every firmware flash, since the reset path in Arcadyan builds has historically restored vendor defaults.

    3. Runbook step 3

      Detection rules

      • Webserver/reverse-proxy logs: match GET .*images/..%2f or images/../(cgi-bin|apply_abstract\.cgi|info\.html) — Sigma shape: selection: cs-uri-stem|contains: ['images/../', 'images/..%2f']. This URL form is the Juniper Threat Labs exploit fingerprint.
      • Netflow/Zeek: unexpected TCP/23 and TCP/37215 scans originating from your own edge or branch network — strong indicator that a router has been enrolled into Mirai after CVE-2021-20090 was chained.
      • Suricata/Snort: enable community rules referencing CVE-2021-20090 (ET WEB_SERVER „Arcadyan Buffalo Path Traversal"). Alerts whose source IP is your own router WAN address are critical.
      • DNS telemetry: clients resolving against unauthorised recursors — indicator of rogue-DNS pushed through the router. KQL sketch: DnsEvents | where ServerIP !in (approved_resolvers) | summarize count() by ClientIP, ServerIP.
      • Configuration-drift alarm: SNMP- or TR-069-based comparison of active firmware version against the baseline — any deviation or sudden downgrade warrants escalation.

    Detection rules

    No NEOSEC detection rule is available for this vulnerability yet.

    Classification under NIS2 and CRA

    Rule-based classification from exploitation status and severity. It does not replace a case-by-case assessment.

    NIS2

    NIS2 is EU Directive (EU) 2022/2555 to raise the level of cybersecurity across the Union. It substantially broadens obligations compared with the first NIS directive: more sectors, mandatory risk management, staged reporting duties and personal accountability of senior management.

    Read in glossary
    Actively exploited: treat the vulnerability as a priority in vulnerability handling under Art. 21(2)(e) NIS2. If it is exploited in your environment, check the reporting obligation under Art. 23 (early warning within 24 hours).
    CRA

    The CRA is EU Regulation (EU) 2024/2847 setting horizontal cybersecurity requirements for „products with digital elements“ — from industrial controllers and software to connected consumer devices. For the first time manufacturers must ensure security across the entire product lifecycle, handle vulnerabilities and provide security updates.

    Read in glossary
    Since 11.09.2026, manufacturers of affected products with digital elements must report actively exploited vulnerabilities under Art. 14 CRA, with an early warning within 24 hours via ENISA's single reporting platform.
    ISO/IEC 27001

    ISO/IEC 27001 is the internationally leading standard for an information security management system (ISMS). It describes a risk-based, process-oriented framework to systematically plan, implement, review and improve information security.

    Read in glossary
    Control A.8.8, management of technical vulnerabilities: assess relevance to your systems, plan remediation and keep evidence.

    Description

    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.

    Source: NVD (NIST), cvelistv5, CISA KEV Catalog

    Affected products

    The following versions are affected. Older releases of the same product line are also vulnerable unless stated otherwise.

    Affected products
    Affected versionsFixed in
    —Buffalo WSR-2533DHPL2, Buffalo WSR-2533DHP3
    • WSR-2533DHPL2 <=1.02, WSR-2533DHP3 <= 1.24
    • —

    Vendors and products

    Affected operating systems

    Affected operating systems
    Otherbuffalowsr-2533dhp3-bk_firmware—
    Otherbuffalowsr-2533dhpl2-bk_firmware—

    Weakness classes (CWE)

    • CWE-22Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')

      Base

      The product uses external input to construct a pathname that is intended to identify a file or directory that is located underneath a restricted parent directory, but the product does not properly neutralize special elements within the pathname that can cause the pathname to resolve to a location that is outside of the restricted directory.

      Entry at MITRE CWE-22

    Public exploit references

    Publicly reported proof-of-concepts, exploit modules and detection templates for this vulnerability. NEOSEC links to the original sources only and does not provide any exploit code.

    Public exploit references
    CISA KEV CatalogCISA Known Exploited Vulnerability (opens in a new tab)cisa.govactively exploited

    References & sources

    • tenable.comsecurity/research/tra-2021-13Exploit, Third-party advisory
    • kb.cert.orgVU#914124Third-party advisory, Third-party advisory, US government resource
    • secpod.comblog/arcadyan-based-routers-and-modems-under-active-exploitationExploit, Third-party advisory
    • cisa.govknown-exploited-vulnerabilities-catalog?field_cve=CVE-2021-20090government-resource, US government resource
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