YAFNET has Unauthenticated Stored Second-Order XSS in Admin Event Log via Reflected User-Agent Header

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

8.1 · CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:N

EPSS

• CVE-2026-43938: 0.28% estimated exploitation activity in the next 30 days; percentile 20.84 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Description: Stored (second-order) Cross-Site Scripting (XSS) occurs when attacker-controlled input is persisted through one component of an application and later rendered, without proper sanitization or contextual output encoding, by a completely different component — often one that implicitly trusts the stored data. Because the dangerous sink is typically a privileged administrative interface, the payload executes in the browser of a high-value user (such as an administrator) and inherits their authenticated session. This class of issue is especially severe when the entry point is an HTTP header on an unauthenticated endpoint, since the attack surface extends to any anonymous attacker on the internet with no prerequisites. Issue Details: The application's database logger (YAFNET.Core/Logger/DbLogger.cs) captures the incoming request's User-Agent header into a JObject, serializes it with JsonConvert, and stores the result in the EventLog.Description column whenever an event (e.g.,…

Published by the source

5 May 2026, 20:31 UTC

Last modified by the source

13 May 2026, 16:24 UTC

Source last checked

26 Oct 2026, 17:04 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-33gv-fc78-qgf5

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# Patch Next advisory brief: CVE-2026-43938

Advisory: YAFNET has Unauthenticated Stored Second-Order XSS in Admin Event Log via Reflected `User-Agent` Header
IDs: CVE-2026-43938, GHSA-33gv-fc78-qgf5
Record: https://patchnext.com/advisories/cve-2026-43938 (revision 1, source checked 26 Oct 2026, 17:04 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 8.1)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: yafnet.core 4.0.5, 3.2.12. A vendor fix does not mean a deployment is fixed.

## Affected components
- NuGet yafnet.core: affected >= 4.0.0-beta01, <= 4.0.4; <= 3.2.11; fixed in 3.2.12, 4.0.5

## Suggested inspection (read-only)
- NuGet yafnet.core: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on yafnet.core.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-33gv-fc78-qgf5 (GitHub-reviewed advisory GHSA-33gv-fc78-qgf5 lists affected ranges: NuGet yafnet.core >=4.0.0-beta01 =0 <=3.2.11)

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