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Medium, CVSS 5.3amended

h3: SSE Event Injection via Unsanitized Carriage Return (\r) in EventStream Data and Comment Fields (Bypass of CVE Fix)

CVE-2026-86252GHSA-4hxc-9384-m385

What to do

Vendor fix available. Upgrade each affected package to the fixed version on its release line (h3 2.0.1-rc.17, 1.15.9), redeploy, then verify what is actually installed.

This page has no inventory attached, so it cannot say whether you are affected. A vendor fix is not proof that your deployment is fixed.

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Affected packages

  • npm h3

    Affected
    >= 2.0.0-beta.0, <= 2.0.1-rc.16< 1.15.9
    Fixed in
    1.15.9, 2.0.1-rc.17

Sources

Severity and exploitation
Exploitation
No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.
CVSS
5.3 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N
EPSS
  • CVE-2026-86252: 0.36% estimated exploitation activity in the next 30 days; percentile 29.88 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.
Source description

As published by the source; long descriptions are shortened.

Summary The EventStream class in h3 fails to sanitize carriage return (\r) characters in data and comment fields. Per the SSE specification, \r is a valid line terminator, so browsers interpret injected \r as line breaks. This allows an attacker to inject arbitrary SSE events, spoof event types, and split a single push() call into multiple distinct browser-parsed events. This is an incomplete fix bypass of commit 7791538 which addressed \n injection but missed \r-only injection. Details The prior fix in commit 7791538 added sanitizeSingleLine() to strip \n and \r from id and event fields, and changed data formatting to split on \n. However, two code paths remain vulnerable: 1. data field — formatEventStreamMessage() (src/utils/internal/event-stream.ts:190-193) String.prototype.split("\n") does not split on \r. A string like "legit\revent: evil" remains as a single "line" and is emitted as: Per the SSE specification §9.2.6, \r alone is a valid line terminator. The browser parses this…

Dates and revisions
Published by the source
Last modified by the source
Source last checked
  1. Revision 2 · amended ·

    Amended from ghsa record GHSA-4hxc-9384-m385

  2. Revision 1 · created ·

    Published from osv record GHSA-4hxc-9384-m385

Agent brief

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# Patch Next advisory brief: CVE-2026-86252

Advisory: h3: SSE Event Injection via Unsanitized Carriage Return (`\r`) in EventStream Data and Comment Fields (Bypass of CVE Fix)
IDs: CVE-2026-86252, GHSA-4hxc-9384-m385
Record: https://patchnext.com/advisories/cve-2026-86252 (revision 2, source checked 26 Oct 2026, 12:46 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 5.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: h3 2.0.1-rc.17, 1.15.9. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm h3: affected >= 2.0.0-beta.0, <= 2.0.1-rc.16; < 1.15.9; fixed in 1.15.9, 2.0.1-rc.17

## Suggested inspection (read-only)
- `npm ls h3 --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.

## 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 h3.
- 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://osv.dev/vulnerability/GHSA-4hxc-9384-m385 (OSV record GHSA-4hxc-9384-m385 lists affected ranges: npm h3 >=2.0.0-beta.0 =0 <1.15.9 (fixed))
- Verified: https://github.com/advisories/GHSA-4hxc-9384-m385 (GitHub-reviewed advisory GHSA-4hxc-9384-m385 lists affected ranges: npm h3 >=2.0.0-beta.0 =0 <1.15.9)

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