CVE-2026-45498
CVE-2026-45498 is a security flaw in Microsoft Defender, the antivirus built into Windows. Attackers can crash Microsoft Defender or stop it working. The US Cybersecurity and Infrastructure Security Agency (CISA) added it to its catalog of flaws known to be used in attacks on 20 May 2026.[1]
Microsoft Defender is in things households have, such as phones, computers, browsers and everyday apps, so the fix matters at home as much as at work. Fixed in version 4.18.26040.7 of Microsoft Defender Antimalware Platform.
Description
CISA calls it a denial of service in Microsoft Defender. In its words: “Microsoft Defender contains an unspecified vulnerability that allows for denial of service.”[1]
The underlying weakness is resource exhaustion (CWE-400), in MITRE’s list of common software weaknesses.[2]
It scores 7.5 out of 10 on the Common Vulnerability Scoring System, which rates it high, scored by NVD using CVSS 3.1.[2]
Affected versions
NVD’s record lists one fixed version: everything before it is affected.[2]
| Product | Fixed in |
|---|---|
| Microsoft Defender Antimalware Platform | 4.18.26040.7 |
Every device with Microsoft Defender needs the update.
Exploitation
CISA added it to its catalog of known exploited vulnerabilities on 20 May 2026, within a day of it being made public. CISA lists a flaw only when it has reliable evidence of it being exploited, and gave US federal agencies until 3 Jun 2026 to fix it.[1]
FIRST’s Exploit Prediction Scoring System, which estimates how likely a flaw is to be used in attacks in the next 30 days from what it sees across the internet, puts it at 63%. That’s in the top 1% of the flaws it scores.[4]
CISA assesses it as not automatable: each attack takes some work.
CISA doesn’t say who was attacked, or how.
Fix
Microsoft has published a fix or advice.[3] CISA’s required action: “Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.”[1]
- Update Microsoft Defender now.Install the latest update from Microsoft on every device that has it, and restart when asked. Then turn on automatic updates, so the next fix arrives on its own.
- Check you’re on 4.18.26040.7 or later.Fixed in version 4.18.26040.7 of Microsoft Defender Antimalware Platform. Anything older is what attackers are going after.
- If there’s no update, stop using it.CISA’s advice when a flaw being used in attacks has no fix is to stop using the product until it does.
Timeline
| Date | Event |
|---|---|
| The flaw is made public in the National Vulnerability Database, with a severity score of 7.5.[2] | |
| CISA adds it to its catalog of flaws known to be used in attacks.[1] | |
| CISA’s deadline for US government agencies to fix it.[1] | |
| FIRST puts the odds of it being used in attacks in the next 30 days at 63%.[4] |
See also
Other Microsoft Defender flaws
- CVE-2026-41091: link following ·
- CVE-2026-33825: insufficient granularity of access control ·
- CVE-2022-44698: smartScreen security feature bypass ·
- CVE-2021-1647: remote code execution ·
Other Microsoft flaws
- CVE-2026-96951, Microsoft Windows: command injection ·
- CVE-2026-65660, Microsoft SharePoint: code injection ·
- CVE-2026-85880, Microsoft Windows: heap-based buffer overflow ·
- CVE-2026-81963, Microsoft Windows: link following ·
- CVE-2019-1068, Microsoft SQL Server: remote code execution ·
Other resource exhaustion flaws
- CVE-2026-28318, SolarWinds Serv-U: uncontrolled resource consumption ·
- CVE-2023-44487, IETF HTTP/2: rapid reset attack ·
- CVE-2004-1464, Cisco IOS: denial-of-service ·
- CVE-2020-3569, Cisco IOS XR: software DVMRP memory exhaustion ·
- CVE-2020-3566, Cisco IOS XR: software DVMRP memory exhaustion ·
Added the same day
- CVE-2026-41091, Microsoft Defender: link following ·
- CVE-2010-0806, Microsoft Internet Explorer: use-after-free ·
- CVE-2010-0249, Microsoft Internet Explorer: use-after-free ·
- CVE-2009-3459, Adobe Acrobat and Reader: heap-based buffer overflow ·
- CVE-2009-1537, Microsoft DirectX: NULL byte overwrite ·
Every Microsoft flaw on CISA’s list · Everything added in May 2026
References
- ^ Cybersecurity and Infrastructure Security Agency. “Microsoft Defender Denial of Service Vulnerability”. Known Exploited Vulnerabilities Catalog, added 20 May 2026.
- ^ National Institute of Standards and Technology. “CVE-2026-45498 Detail”. National Vulnerability Database, published 20 May 2026.
- ^ Microsoft. Advisories and fixes: msrc.microsoft.com, msrc.microsoft.com.
- ^ FIRST. “Exploit Prediction Scoring System”. EPSS 0.631, percentile 99.2, 31 Oct 2026.
- CVE Program. “CVE-2026-45498”. The official CVE record.
This article is built from CISA’s catalog (version 2026.10.30), NVD’s record and FIRST’s scores, kept in HARD’s database and checked every morning. The plain English is HARD’s.