Http4s: Ember HTTP/2 buffers a frame's declared payload before checking SETTINGS_MAX_FRAME_SIZE

Exploitation

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

CVSS

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

EPSS

• CVE-2026-88975: 0.63% estimated exploitation activity in the next 30 days; percentile 50.23 (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.

Summary An unauthenticated peer can make Ember's HTTP/2 read loop hold 16 MiB of a single frame in memory on a connection where Ember advertised a 16 KiB limit. The declared length is readable from the frame's first 9 bytes, but it is not compared against SETTINGSMAXFRAMESIZE until the whole payload has been read into a contiguous buffer. That is 1024x amplification per connection, bounded by the protocol's 24-bit length field rather than by any setting, and at the default maxConnections of 1024 it puts about 16 GiB of heap in reach. Details H2Frame.RawFrame.fromByteVector (ember-core/.../h2/H2Frame.scala:66-69) reads the length, then refuses to produce a frame until 9 + length bytes are present: readNextFrame in H2Connection.readLoop (H2Connection.scala:246-266) reads that None as "need more" and concatenates further socket reads into one accumulator until it is satisfied. The only check of an inbound frame against the connection's own SETTINGSMAXFRAMESIZE is in processFrame at…

Published by the source

15 Sep 2026, 20:01 UTC

Last modified by the source

15 Sep 2026, 20:15 UTC

Source last checked

26 Oct 2026, 16:55 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-gq9p-f254-h286

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-gq9p-f254-h286

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# Patch Next advisory brief: CVE-2026-88975

Advisory: Http4s: Ember HTTP/2 buffers a frame's declared payload before checking SETTINGS_MAX_FRAME_SIZE
IDs: CVE-2026-88975, GHSA-gq9p-f254-h286
Record: https://patchnext.com/advisories/cve-2026-88975 (revision 2, source checked 26 Oct 2026, 16:55 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 7.5)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: org.http4s:http4s-ember-core_2.12 0.23.37; org.http4s:http4s-ember-core_3 1.0.0-M48, 0.23.37; org.http4s:http4s-ember-core_2.13 1.0.0-M48, 0.23.37. A vendor fix does not mean a deployment is fixed.

## Affected components
- Maven org.http4s:http4s-ember-core_2.12: affected <= 0.23.36; fixed in 0.23.37
- Maven org.http4s:http4s-ember-core_3: affected <= 0.23.36; >= 1.0.0-M1, <= 1.0.0-M47; fixed in 1.0.0-M48, 0.23.37
- Maven org.http4s:http4s-ember-core_2.13: affected <= 0.23.36; >= 1.0.0-M1, <= 1.0.0-M47; fixed in 1.0.0-M48, 0.23.37

## Suggested inspection (read-only)
- Maven org.http4s:http4s-ember-core_2.12: check the installed version with your package manager.
- Maven org.http4s:http4s-ember-core_3: check the installed version with your package manager.
- Maven org.http4s:http4s-ember-core_2.13: 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 org.http4s:http4s-ember-core_2.12, org.http4s:http4s-ember-core_3, org.http4s:http4s-ember-core_2.13.
- 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-gq9p-f254-h286 (OSV record GHSA-gq9p-f254-h286 lists affected ranges: Maven org.http4s:http4s-ember-core_2.13 >=0 =1.0.0-M1 =0 =0 =1.0.0-M1 <1.0.0-M48 (fixed))
- Verified: https://github.com/advisories/GHSA-gq9p-f254-h286 (GitHub-reviewed advisory GHSA-gq9p-f254-h286 lists affected ranges: Maven org.http4s:http4s-ember-core_2.13 >=0 =1.0.0-M1 =0 =0 =1.0.0-M1 <=1.0.0-M47)

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