Answer in brief
CVE-2026-54556 records a High severity vulnerability in Http4s: HTTP/2 Denial of Service with Ember Backend. The current sources do not mark it as known exploited. The current feed maps http4s/http4s (generic), org.http4s/http4s-ember-core_2.12 (generic), org.http4s/http4s-ember-core_2.13 (generic), org.http4s/http4s-ember-core_3 (generic) and additional mapped packages. Check affected ranges and fixed versions before updating.
Analysis pending evidence review
HOL Guard separates source facts from reviewed analysis. See the methodology.
A CVSS score is not reported in the current record. The current sources do not mark it as known exploited. Treat this as a source-backed prioritization signal, not a statement about your environment.
Analysis status
Analysis pending evidence review
Factual feed record only; HOL analysis is not approved for indexing. Read the methodology.
The current feed maps http4s/http4s (generic), org.http4s/http4s-ember-core_2.12 (generic), org.http4s/http4s-ember-core_2.13 (generic), org.http4s/http4s-ember-core_3 (generic) and additional mapped packages. Check affected ranges and fixed versions before updating.
| Package | Affected range | Fixed version |
|---|---|---|
| http4s/http4sgeneric | < 0.23.35 || >= 1.0.0-M1, < 1.0.0-M46 | Not reported |
| org.http4s/http4s-ember-core_2.12generic | < 0.23.35 | Not reported |
| org.http4s/http4s-ember-core_2.13generic | >= 1.0.0-M1, < 1.0.0-M46 || < 0.23.35 | Not reported |
| org.http4s/http4s-ember-core_3generic | >= 1.0.0-M1, < 1.0.0-M46 || < 0.23.35 | Not reported |
| org.http4s:http4s-ember-core_2.12maven | <=0.23.34 | 0.23.35 |
| org.http4s:http4s-ember-core_2.13maven | >=1.0.0-M1,<1.0.0-M46 | 1.0.0-M47 |
| org.http4s:http4s-ember-core_2.13maven | <=0.23.34 | 0.23.35 |
| org.http4s:http4s-ember-core_3maven | <=0.23.34 | 0.23.35 |
| org.http4s:http4s-ember-core_3maven | >=1.0.0-M1,<1.0.0-M46 | 1.0.0-M47 |
Published upstream
Aug 26, 2026
Evidence: source:cvelist:source_dates:source-dates:recordSource modified
Aug 26, 2026
Evidence: source:cvelist:source_dates:source-dates:recordFirst seen by HOL
Aug 26, 2026
Http4s is a Scala interface for HTTP services. Prior to 0.23.35 and 1.0.0-M47, an unauthenticated HTTP/2 peer can cause an out-of-memory denial of service in the Ember backend with HTTP/2 enabled. The Hpack wrapper in ember-core/shared/src/main/scala/org/http4s/ember/core/h2/Hpack.scala concatenates HEADERS and CONTINUATION frame fragments and decodes them into a single List, but maxHeaderSize accounting does not include indexed headers or HPACK per-header overhead. A small compressed header block can therefore expand into a much larger decoded representation that remains in memory for processing. Servers exposed to untrusted HTTP/2 traffic and clients directed to an untrusted HTTP/2 server are affected, and concurrent malicious connections can exhaust the process heap. This issue is fixed in versions 0.23.35 and 1.0.0-M47.
Quoted source text, attributed separately from HOL analysis.