{"id":"CVE-2026-54556","state":"PUBLISHED","title":"Http4s: HTTP/2 Denial of Service with Ember Backend","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.","publishedAt":"2026-08-26T14:10:22.349Z","updatedAt":"2026-08-29T02:38:44.725Z","severity":{"score":8.2,"label":"High","version":"4.0","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","source":"GitHub_M"},"assigner":"GitHub_M","products":["http4s http4s","org.http4s http4s-ember-core_2.12","org.http4s http4s-ember-core_2.13","org.http4s http4s-ember-core_3"],"affected":[{"vendor":"http4s","product":"http4s","versions":[{"version":"< 0.23.35","status":"affected"},{"version":">= 1.0.0-M1, < 1.0.0-M46","status":"affected"}]},{"vendor":"org.http4s","product":"http4s-ember-core_2.12","versions":[{"version":"< 0.23.35","status":"affected"}]},{"vendor":"org.http4s","product":"http4s-ember-core_2.13","versions":[{"version":">= 1.0.0-M1, < 1.0.0-M46","status":"affected"},{"version":"< 0.23.35","status":"affected"}]},{"vendor":"org.http4s","product":"http4s-ember-core_3","versions":[{"version":">= 1.0.0-M1, < 1.0.0-M46","status":"affected"},{"version":"< 0.23.35","status":"affected"}]}],"references":["https://github.com/http4s/http4s/security/advisories/GHSA-vmm3-xgcx-67hm","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"],"weaknesses":["CWE-409"],"ingestedAt":"2026-10-29T20:12:24.861Z","source":{"repository":"CVEProject/cvelistV5","path":"cves/2026/54xxx/CVE-2026-54556.json","commit":"556c9d3a8cd17e218bb8e452ba7cd619a384ff47"},"raw":{"dataType":"CVE_RECORD","dataVersion":"5.2","cveMetadata":{"cveId":"CVE-2026-54556","state":"PUBLISHED","datePublished":"2026-08-26T14:10:22.349Z","dateUpdated":"2026-08-29T02:38:44.725Z","assignerShortName":"GitHub_M","assignerOrgId":"a0819718-46f1-4df5-94e2-005712e83aaa","dateReserved":"2026-06-15T19:04:14.456Z"},"containers":{"cna":{"title":"Http4s: HTTP/2 Denial of Service with Ember Backend","descriptions":[{"lang":"en","value":"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."}],"affected":[{"vendor":"http4s","product":"http4s","versions":[{"version":"< 0.23.35","status":"affected"},{"version":">= 1.0.0-M1, < 1.0.0-M46","status":"affected"}]},{"vendor":"org.http4s","product":"http4s-ember-core_2.12","versions":[{"version":"< 0.23.35","status":"affected"}]},{"vendor":"org.http4s","product":"http4s-ember-core_2.13","versions":[{"version":">= 1.0.0-M1, < 1.0.0-M46","status":"affected"},{"version":"< 0.23.35","status":"affected"}]},{"vendor":"org.http4s","product":"http4s-ember-core_3","versions":[{"version":">= 1.0.0-M1, < 1.0.0-M46","status":"affected"},{"version":"< 0.23.35","status":"affected"}]}],"references":[{"url":"https://github.com/http4s/http4s/security/advisories/GHSA-vmm3-xgcx-67hm","name":"https://github.com/http4s/http4s/security/advisories/GHSA-vmm3-xgcx-67hm","tags":["x_refsource_CONFIRM"]},{"url":"https://github.com/http4s/http4s/commit/6e8eccd64a6a74ab4811897881e95e0e1b3a818e","name":"https://github.com/http4s/http4s/commit/6e8eccd64a6a74ab4811897881e95e0e1b3a818e","tags":["x_refsource_MISC"]},{"url":"https://github.com/http4s/http4s/releases/tag/v0.23.35","name":"https://github.com/http4s/http4s/releases/tag/v0.23.35","tags":["x_refsource_MISC"]},{"url":"https://github.com/http4s/http4s/releases/tag/v1.0.0-M47","name":"https://github.com/http4s/http4s/releases/tag/v1.0.0-M47","tags":["x_refsource_MISC"]}],"metrics":[{"cvssV4_0":{"attackVector":"NETWORK","attackComplexity":"LOW","attackRequirements":"PRESENT","privilegesRequired":"NONE","userInteraction":"NONE","vulnConfidentialityImpact":"NONE","vulnIntegrityImpact":"NONE","vulnAvailabilityImpact":"HIGH","subConfidentialityImpact":"NONE","subIntegrityImpact":"NONE","subAvailabilityImpact":"NONE","baseScore":8.2,"baseSeverity":"HIGH","vectorString":"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","version":"4.0"}}],"problemTypes":[{"descriptions":[{"cweId":"CWE-409","lang":"en","description":"CWE-409: Improper Handling of Highly Compressed Data (Data Amplification)","type":"CWE"}]}],"providerMetadata":{"shortName":"GitHub_M","orgId":"a0819718-46f1-4df5-94e2-005712e83aaa","dateUpdated":"2026-08-26T14:10:22.349Z"},"source":{"advisory":"GHSA-vmm3-xgcx-67hm","discovery":"UNKNOWN"}},"adp":[{"title":"CISA ADP Vulnrichment","metrics":[{"other":{"type":"ssvc","content":{"timestamp":"2026-08-29T02:38:20.242464Z","id":"CVE-2026-54556","options":[{"Exploitation":"none"},{"Automatable":"yes"},{"Technical Impact":"partial"}],"role":"CISA Coordinator","version":"2.0.3"}}}],"providerMetadata":{"shortName":"CISA-ADP","orgId":"134c704f-9b21-4f2e-91b3-4a467353bcc0","dateUpdated":"2026-08-29T02:38:44.725Z"}}]}}}