CVE-2026-54556Http4s: HTTP/2 Denial of Service with Ember Backend
- 8.2HighCVSS base score
- 0.52%Exploitation in 30 daysEPSS · top 55% of all CVEs
- NoKnown exploitednot in CISA's KEV catalog
- NonePublic exploitsnone known yet
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.
Timeline
ID reserved
By GitHub_M
- 2 months later
· 2 months later
Published
- 3 days later
· 3 days later
Record updated
Changes after publication (a new score, a public exploit, EPSS crossing 1, 10 or 50 %, leaving KEV) are kept since November 2026; earlier ones are not known.
Scores
| Scored by | CVSS | Score | Vector |
|---|---|---|---|
| GitHub_MCNA (assigner) | 4.0 | High8.2 | 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 |
| NVDNVD · Deferred | 4.0 | High8.2 | 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/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X |
- EPSS (exploitation in 30 days)
- 0.52% · top 55% of all CVEs (13 Nov 2026)
- CISA SSVC
- Exploitation: none · Automatable: yes · Technical impact: partial
The list uses the assigner's score when there is one, then CISA's, then NVD's: High 8.2.
Affected products
- affected < 0.23.35
- affected >= 1.0.0-M1, < 1.0.0-M46
org.http4s · http4s-ember-core_2.12
- affected < 0.23.35
org.http4s · http4s-ember-core_2.13
- affected >= 1.0.0-M1, < 1.0.0-M46
- affected < 0.23.35
org.http4s · http4s-ember-core_3
- affected >= 1.0.0-M1, < 1.0.0-M46
- affected < 0.23.35
Affected packages
From reviewed advisories of the GitHub Advisory Database.
- Mavenorg.http4s:http4s-ember-core_2.12GHSA-vmm3-xgcx-67hm (opens in a new tab)
- Mavenorg.http4s:http4s-ember-core_2.13GHSA-vmm3-xgcx-67hm (opens in a new tab)
- Mavenorg.http4s:http4s-ember-core_3GHSA-vmm3-xgcx-67hm (opens in a new tab)
References
References4
- https://github.com/http4s/http4s/security/advisories/GHSA-vmm3-xgcx-67hm (opens in a new tab)x_refsource_CONFIRM
- https://github.com/http4s/http4s/commit/6e8eccd64a6a74ab4811897881e95e0e1b3a818e (opens in a new tab)x_refsource_MISC
- https://github.com/http4s/http4s/releases/tag/v0.23.35 (opens in a new tab)x_refsource_MISC
- https://github.com/http4s/http4s/releases/tag/v1.0.0-M47 (opens in a new tab)x_refsource_MISC
Sources
The same CVE in the primary databases and aggregators, to check against.
- CVE.org record (opens in a new tab)
- CVE List JSON (GitHub) (opens in a new tab)
- NVD (opens in a new tab)
- FIRST EPSS (opens in a new tab)
- EUVD EUVD-2026-66558 (opens in a new tab)
- OSV.dev (opens in a new tab)
- GitHub Advisory GHSA-vmm3-xgcx-67hm (opens in a new tab)
- CIRCL Vulnerability-Lookup (opens in a new tab)
Badge for a README or a ticket
[](https://cveledger.app/cve/CVE-2026-54556)Record last updated .