Skip to content
Fencer Vulns
high 8.2

CVE-2026-54556 json

Http4s: HTTP/2 Denial of Service with Ember Backend

published 2026-08-26 2 mo ago modified 2026-09-09 1 mo ago also GHSA-vmm3-xgcx-67hm , EUVD-2026-66558

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

CISA SSVC

Exploitation
none
Automatable
yes
Technical impact
partial
Assessed
2 mo ago

Exploit prediction

0.52% chance of exploitation in the next 30 days, higher than 44% of all scored CVEs (FIRST EPSS, 2026-11-08).

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 Vulns 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.