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Fencer Intel
high 8.2

CVE-2026-54556 json

Http4s: HTTP/2 Denial of Service with Ember Backend

published 2mo ago modified 2mo ago also EUVD-2026-66558 , GHSA-vmm3-xgcx-67hm

cvss
8.2
version 4.0, CNA
cisa kev
Not listed
not in CISA's catalog
epss
0.5%
EPSS 0.5% chance of exploitation in 30 days, percentile 44%
public exploits
0
none known

Summary

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.

Patch / fix

  • maven org.http4s:http4s-ember-core_2.12 0.23.35
  • maven org.http4s:http4s-ember-core_2.13 0.23.35, 1.0.0-M47
  • maven org.http4s:http4s-ember-core_3 0.23.35, 1.0.0-M47
  • typelevel http4s 0.23.35, 1.0.0-M47
  • org.http4s http4s-ember-core_2.12 0.23.35
  • org.http4s http4s-ember-core_2.13 0.23.35, 1.0.0-M47
  • org.http4s http4s-ember-core_3 0.23.35, 1.0.0-M47

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CISA SSVC

Exploitation
none
Automatable
yes
Technical impact
partial
Assessed
2mo ago

Exploit prediction

0.52% chance of exploitation in the next 30 days, higher than 44% of all scored CVEs (FIRST EPSS, 2026-11-11).

CVSS scores

assigned by version score vector
CNA 4.0 8.2 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
CNA 4.0 8.2 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

Affected packages

ecosystem package affected fixed in
maven org.http4s:http4s-ember-core_2.12
<0.23.35
0.23.35
maven org.http4s:http4s-ember-core_2.13
>=1.0.0-M1, <1.0.0-M47
<0.23.35
1.0.0-M47
0.23.35
maven org.http4s:http4s-ember-core_3
<0.23.35
>=1.0.0-M1, <1.0.0-M47
0.23.35
1.0.0-M47

Affected products

  • http4s http4s
  • org.http4s http4s-ember-core_2.12
  • org.http4s http4s-ember-core_2.13
  • org.http4s http4s-ember-core_3

Classification

Weakness
  • CWE-409 Improper Handling of Highly Compressed Data (Data Amplification)

References

Sources

Fencer Intel keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.

CPEs and fixed versions use Anchore's correction of NVD's data: Added CPE configurations because not yet analyzed by NVD.