{"id":"CVE-2026-55040","kind":"vulnerability","aliases":["CVE-2026-55040"],"related":[],"title":"microsoft sharepoint server: Weak authentication in Microsoft Office SharePoint allows an unauthorized attacker to bypass a security feature over…","description":"Microsoft SharePoint contains a weak authentication vulnerability which allows an unauthorized attacker to bypass a security feature over a network.\n\nCISA required action: Apply mitigations in accordance with vendor instructions, ensuring compliance with CISA’s BOD 26-04 Prioritizing Security Updates Based on Risk (see URL in Notes) guidance and CISA’s “Forensics Triage Requirements” (see URL in Notes). Follow applicable BOD 26-04 guidance for cloud services or discontinue use of the product if mitigations are unavailable. Stakeholders are responsible for evaluating each asset's internet exposure and ensuring adherence to BOD 26-04 patching guidelines.","summary":"Microsoft SharePoint has a weak authentication flaw that lets an unauthenticated remote attacker bypass a security feature over the network. CISA lists it as actively exploited in the wild.","severity":"critical","cvss":9.1,"epss":null,"exploited":true,"exploited_at":"2026-08-18T00:00:00+00:00","products":[{"name":"Microsoft SharePoint","fixed":[],"affected":"","ecosystem":"vendor"},{"name":"microsoft sharepoint server","fixed":["16.0.19725.20434"],"affected":"< 16.0.19725.20434","ecosystem":"vendor"},{"name":"microsoft sharepoint server","fixed":[],"affected":"= 2016","ecosystem":"vendor"},{"name":"microsoft sharepoint server","fixed":[],"affected":"= 2019","ecosystem":"vendor"}],"ecosystems":["vendor"],"tags":["exploited","microsoft-365","self-hosted","sharepoint","vendor"],"not_affected_if":["no-sharepoint-deployment"],"how_to_check":"Confirm whether you run Microsoft SharePoint (on-premises or hosted) and identify its exposure to the internet.\nCheck Microsoft's security advisory for CVE-2026-55040 for affected builds and whether your build is patched.","fix":"Apply the mitigations and patch from Microsoft's vendor instructions for CVE-2026-55040; for cloud-hosted SharePoint follow the vendor-side guidance.\nIf no mitigation is available, discontinue use of the affected SharePoint deployment per CISA BOD 26-04 guidance.","status":"open","published_at":"2026-07-14T18:18:15+00:00","first_seen_at":"2026-10-23T19:11:51+00:00","triaged_at":"2026-10-23T20:31:55+00:00","updated_at":"2026-10-23T23:44:52+00:00","category":"vendor","confidence":0.6,"triage":{"at":"2026-10-23T20:31:55Z","model":"claude-haiku-5-5","output":{"fix":"Apply the mitigations and patch from Microsoft's vendor instructions for CVE-2026-55040; for cloud-hosted SharePoint follow the vendor-side guidance.\nIf no mitigation is available, discontinue use of the affected SharePoint deployment per CISA BOD 26-04 guidance.","summary":"Microsoft SharePoint has a weak authentication flaw that lets an unauthenticated remote attacker bypass a security feature over the network. CISA lists it as actively exploited in the wild.","category":"vendor","who_cares":["sharepoint","microsoft-365","self-hosted"],"confidence":0.6,"analyst_note":"Sources do not give affected or fixed versions or the bypassed security feature; check the Microsoft MSRC advisory for CVE-2026-55040 and the CISA KEV entry.","how_to_check":"Confirm whether you run Microsoft SharePoint (on-premises or hosted) and identify its exposure to the internet.\nCheck Microsoft's security advisory for CVE-2026-55040 for affected builds and whether your build is patched.\nunknown","needs_analyst":true,"not_affected_if":["no-sharepoint-deployment"],"severity_if_unknown":"high"},"cost_usd":0.000434,"input_tokens":2191,"output_tokens":430,"severity_before":"unknown"},"analyst_status":null,"analyst_note":null,"search":"'-04':110C,136C,167C '-2026':20A,23A '-365':38B '-55040':21A,24A '26':109C,135C,166C 'accord':99C 'action':95C 'activ':68B 'adher':163C 'allow':10A,81C 'appli':96C 'applic':133C 'asset':157C 'attack':13A,56B,84C 'authent':5A,49B,78C 'base':114C 'bod':108C,134C,165C 'bypass':15A,57B,86C 'cisa':64B,93C,106C,123C 'cloud':139C 'complianc':104C 'contain':75C 'cve':19A,22A 'discontinu':142C 'ensur':103C,162C 'evalu':155C 'exploit':36B,69B 'exposur':160C 'featur':18A,60B,89C 'flaw':50B 'follow':132C 'forens':125C 'guidanc':121C,137C 'guidelin':169C 'host':41B 'instruct':102C 'internet':159C 'let':52B 'list':65B 'microsoft':1A,7A,25A,27A,30A,33A,37B,44B,73C 'mitig':97C,148C 'network':63B,92C 'note':120C,131C 'offic':8A 'patch':168C 'priorit':111C 'product':146C 'remot':55B 'requir':94C,127C 'respons':153C 'risk':116C 'secur':17A,59B,88C,112C 'see':117C,128C 'self':40B 'self-host':39B 'server':3A,29A,32A,35A 'servic':140C 'sharepoint':2A,9A,26A,28A,31A,34A,42B,45B,74C 'stakehold':151C 'triag':126C 'unauthent':54B 'unauthor':12A,83C 'unavail':150C 'updat':113C 'url':118C,129C 'use':143C 'vendor':43B,101C 'vulner':79C 'weak':4A,48B,77C 'wild':72B","nicknames":[],"mitigations":[],"who_affected":null,"tier":null,"verified":{},"analyst_revisit_at":null,"first_public_at":"2026-07-14T18:18:15+00:00","kev":{"dateAdded":"2026-08-18","dueDate":"2026-08-21","requiredAction":"Apply mitigations in accordance with vendor instructions, ensuring compliance with CISA’s BOD 26-04 Prioritizing Security Updates Based on Risk (see URL in Notes) guidance and CISA’s “Forensics Triage Requirements” (see URL in Notes). Follow applicable BOD 26-04 guidance for cloud services or discontinue use of the product if mitigations are unavailable. Stakeholders are responsible for evaluating each asset's internet exposure and ensuring adherence to BOD 26-04 patching guidelines.","knownRansomwareCampaignUse":"Unknown"},"cvss_vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N","cvss_by":"secure@microsoft.com","poc_count":0,"sources":[{"source":"kev","external_id":"CVE-2026-55040","url":"https://www.cisa.gov/known-exploited-vulnerabilities-catalog?search_api_fulltext=CVE-2026-55040","severity":null,"published_at":"2026-08-18T00:00:00+00:00","modified_at":null,"first_seen_at":"2026-10-23T19:11:51+00:00","fetched_at":"2026-10-25T17:39:54+00:00","note":null},{"source":"nvd","external_id":"CVE-2026-55040","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-55040","severity":null,"published_at":"2026-07-14T18:18:15+00:00","modified_at":"2026-08-19T04:17:23+00:00","first_seen_at":"2026-10-23T23:44:52+00:00","fetched_at":"2026-10-23T23:44:52+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"}