Answer in brief
CVE-2026-54556 records a High severity (CVSS 0.0) 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.
CVSS is 0.0. 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_2.12maven | >=0 <0.23.35 | 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_2.13maven | >=1.0.0-M1 <1.0.0-M47 | 1.0.0-M47 |
| org.http4s:http4s-ember-core_2.13maven | >=0 <0.23.35 | 0.23.35 |
| org.http4s:http4s-ember-core_3maven | >=1.0.0-M1 <1.0.0-M47 | 1.0.0-M47 |
| 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 |
| org.http4s:http4s-ember-core_3maven | >=0 <0.23.35 | 0.23.35 |
Published upstream
Aug 26, 2026
Evidence: source:cvelist:source_dates:source-dates:recordSource modified
Sep 9, 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.