{"cve":"CVE-2026-54556","nvd":{"published":"2026-08-26T15:16:49.527","modified":"2026-09-09T21:09:13.080","status":"Deferred","description":"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.","cvss_v4":{"vector":"CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N","score":8.2},"cvss_v3":null,"cwes":["CWE-409"],"references":["https://github.com/http4s/http4s/commit/6e8eccd64a6a74ab4811897881e95e0e1b3a818e","https://github.com/http4s/http4s/releases/tag/v0.23.35","https://github.com/http4s/http4s/releases/tag/v1.0.0-M47","https://github.com/http4s/http4s/security/advisories/GHSA-vmm3-xgcx-67hm"],"products":[],"source":"nvd","synced_at":"2026-10-19T22:50:17.453294893+00:00"},"epss":{"cve":"CVE-2026-54556","score":0.00524,"percentile":0.44438,"date":"2026-10-31"},"kev":null,"osv":[{"id":"GHSA-vmm3-xgcx-67hm","url":"https://osv.dev/vulnerability/GHSA-vmm3-xgcx-67hm","summary":"http4s has HTTP/2 Denial of Service with Ember Backend","modified":"2026-08-26T14:26:28.260052Z","packages":[{"ecosystem":"Maven","name":"org.http4s:http4s-ember-core_2.12","purl":"pkg:maven/org.http4s/http4s-ember-core_2.12","ranges":["<0.23.35"],"fixed":["0.23.35"]},{"ecosystem":"Maven","name":"org.http4s:http4s-ember-core_2.13","purl":"pkg:maven/org.http4s/http4s-ember-core_2.13","ranges":[">=1.0.0-M1 <1.0.0-M47"],"fixed":["1.0.0-M47"]},{"ecosystem":"Maven","name":"org.http4s:http4s-ember-core_3","purl":"pkg:maven/org.http4s/http4s-ember-core_3","ranges":["<0.23.35"],"fixed":["0.23.35"]},{"ecosystem":"Maven","name":"org.http4s:http4s-ember-core_3","purl":"pkg:maven/org.http4s/http4s-ember-core_3","ranges":[">=1.0.0-M1 <1.0.0-M47"],"fixed":["1.0.0-M47"]},{"ecosystem":"Maven","name":"org.http4s:http4s-ember-core_2.13","purl":"pkg:maven/org.http4s/http4s-ember-core_2.13","ranges":["<0.23.35"],"fixed":["0.23.35"]}]}],"attribution":["This product uses the NVD API but is not endorsed or certified by the NVD.","EPSS scores: FIRST Exploit Prediction Scoring System, https://www.first.org/epss","Known exploited vulnerabilities: CISA Known Exploited Vulnerabilities Catalog, https://www.cisa.gov/known-exploited-vulnerabilities-catalog","Package advisories: OSV (https://osv.dev), from the GitHub Advisory Database, PyPA, Go, RustSec and other ecosystem databases, under CC-BY 4.0 or CC0","CVE records: the CVE Program's CVE List (https://www.cve.org), under its terms of use (https://www.cve.org/Legal/TermsOfUse). CVE is a registered trademark of The MITRE Corporation; this product is not affiliated with the CVE Program."]}