{"id":"CVE-2021-28799","kind":"vulnerability","aliases":["CVE-2021-28799"],"related":[],"title":"QNAP Network Attached Storage (NAS): QNAP NAS Improper Authorization Vulnerability","description":"An improper authorization vulnerability has been reported to affect QNAP NAS running HBS 3 (Hybrid Backup Sync. ) If exploited, the vulnerability allows remote attackers to log in to a device. This issue affects: QNAP Systems Inc. HBS 3 versions prior to v16.0.0415 on QTS 4.5.2; versions prior to v3.0.210412 on QTS 4.3.6; versions prior to v3.0.210411 on QTS 4.3.4; versions prior to v3.0.210411 on QTS 4.3.3; versions prior to v16.0.0419 on QuTS hero h4.5.1; versions prior to v16.0.0419 on QuTScloud c4.5.1~c4.5.4. This issue does not affect: QNAP Systems Inc. HBS 2 . QNAP Systems Inc. HBS 1.3 .","summary":"QNAP NAS devices running HBS 3 have an improper authorization flaw that lets remote attackers log in to the device. It is listed in CISA's known-exploited catalog and tagged for ransomware use.","severity":"critical","cvss":10.0,"epss":0.7825,"exploited":true,"exploited_at":"2022-03-31T00:00:00+00:00","products":[{"name":"QNAP Network Attached Storage (NAS)","fixed":[],"affected":"","ecosystem":"vendor"}],"ecosystems":["vendor"],"tags":["exploited","hbs-3","infrastructure","nas","qnap","ransomware","self-hosted"],"not_affected_if":["no-hbs-3-installed"],"how_to_check":"Check whether a QNAP NAS runs the HBS 3 (Hybrid Backup Sync) app, which the sources name as affected; the sources do not give a command or version range.","fix":"Apply the QNAP update per vendor instructions, as CISA requires; the fixed version is unknown from these sources.\nUpdate HBS 3 and the NAS firmware, then review login logs.","status":"open","published_at":"2021-05-13T03:15:06+00:00","first_seen_at":"2026-10-23T19:11:52+00:00","triaged_at":"2026-10-23T21:19:40+00:00","updated_at":"2026-10-23T21:57:35+00:00","category":"infrastructure","confidence":0.6,"triage":{"at":"2026-10-23T21:19:40Z","model":"claude-haiku-5-5","output":{"fix":"Apply the QNAP update per vendor instructions, as CISA requires; the fixed version is unknown from these sources.\nUpdate HBS 3 and the NAS firmware, then review login logs.","reach":"internet","impact":"high","summary":"QNAP NAS devices running HBS 3 have an improper authorization flaw that lets remote attackers log in to the device. It is listed in CISA's known-exploited catalog and tagged for ransomware use.","category":"infrastructure","who_cares":["qnap","nas","hbs-3","self-hosted"],"confidence":0.6,"needs_auth":"no","worst_case":"Remote attacker logs in to the NAS without valid credentials, possibly with ransomware follow-on.","analyst_note":"Find the QNAP advisory for CVE-2021-28799 to confirm affected HBS 3 versions, the fixed build, and the detection method.","how_to_check":"Check whether a QNAP NAS runs the HBS 3 (Hybrid Backup Sync) app, which the sources name as affected; the sources do not give a command or version range.\nunknown","needs_analyst":true,"not_affected_if":["no-hbs-3-installed"],"user_interaction":"no","needs_special_setup":"unknown","severity_if_unknown":"high"},"cost_usd":0.000507,"input_tokens":2841,"output_tokens":446,"severity_before":"unknown"},"analyst_status":null,"analyst_note":null,"search":"'-2021':12A,15A '-28799':13A,16A '-3':24B '1.3':163C '2':158C '3':37B,80C,104C '4.3.3':132C '4.3.4':125C '4.3.6':118C '4.5.2':111C 'affect':75C,99C,153C 'allow':88C 'attach':3A 'attached':19A 'attack':46B,90C 'author':9A,41B,69C 'backup':82C 'c4.5.1':147C 'catalog':61B 'cisa':56B 'cve':11A,14A 'devic':34B,51B,96C 'exploit':22B,60B,85C 'flaw':42B 'h4.5.1':140C 'hbs':23B,36B,79C,103C,157C,162C 'hero':139C 'host':31B 'hybrid':81C 'improp':8A,40B,68C 'inc':102C,156C,161C 'infrastructur':25B 'issu':98C,150C 'known':59B 'known-exploit':58B 'let':44B 'list':54B 'log':47B,92C 'nas':5A,7A,21A,26B,33B,77C 'network':2A,18A 'prior':106C,113C,120C,127C,134C,142C 'qnap':1A,6A,17A,27B,32B,76C,100C,154C,159C 'qts':110C,117C,124C,131C 'qut':138C 'qutscloud':146C 'ransomwar':28B,65B 'remot':45B,89C 'report':73C 'run':35B,78C 'self':30B 'self-host':29B 'storag':4A 'storage':20A 'sync':83C 'system':101C,155C,160C 'tag':63B 'use':66B 'v16.0.0415':108C 'v16.0.0419':136C,144C 'v3.0.210411':122C,129C 'v3.0.210412':115C 'version':105C,112C,119C,126C,133C,141C 'vulner':10A,70C,87C '~c4.5.4':148C","nicknames":[],"mitigations":[],"who_affected":null,"tier":null,"verified":{},"analyst_revisit_at":null,"first_public_at":"2021-05-13T03:15:06+00:00","kev":{"dateAdded":"2022-03-31","dueDate":"2022-04-21","requiredAction":"Apply updates per vendor instructions.","knownRansomwareCampaignUse":"Known"},"cvss_vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H","cvss_by":"security@qnapsecurity.com.tw","poc_count":0,"sources":[{"source":"kev","external_id":"CVE-2021-28799","url":"https://www.cisa.gov/known-exploited-vulnerabilities-catalog?search_api_fulltext=CVE-2021-28799","severity":null,"published_at":"2022-03-31T00:00:00+00:00","modified_at":null,"first_seen_at":"2026-10-23T19:11:52+00:00","fetched_at":"2026-10-25T10:39:07+00:00","note":null},{"source":"epss","external_id":"CVE-2021-28799","url":"https://api.first.org/data/v1/epss?cve=CVE-2021-28799","severity":null,"published_at":null,"modified_at":null,"first_seen_at":"2026-10-23T19:13:44+00:00","fetched_at":"2026-10-23T19:13:44+00:00","note":null},{"source":"nvd","external_id":"CVE-2021-28799","url":"https://nvd.nist.gov/vuln/detail/CVE-2021-28799","severity":null,"published_at":"2021-05-13T03:15:06+00:00","modified_at":"2026-06-17T03:46:53+00:00","first_seen_at":"2026-10-23T21:57:35+00:00","fetched_at":"2026-10-23T21:57:35+00:00","note":null}],"documents":[],"notice":"Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai"}