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NVD / CVE Program· CNA GitHub_M

CVE-2026-54556: Http4s: HTTP/2 Denial of Service with Ember Backend

Noroxi assessment

Priority: Monitor (action score 32/100)

  • The sources we track (CISA KEV, Exploit-DB, Metasploit, Nuclei, GitHub) show no public exploit code or exploitation in the wild.
  • Probability of exploitation in the next 30 days: 0.52% (FIRST EPSS).
  • Anyone who can reach it over the internet can trigger it. No account or password is required. No user action is required. No special conditions are required; it is repeatable.
  • According to CISA the attack is automatable: targets can be found by mass scanning.
  • Fixed version: org.http4s:http4s-ember-core_2.12 (Maven) 0.23.35; org.http4s:http4s-ember-core_2.13 (Maven) 1.0.0-M47, 0.23.35; org.http4s:http4s-ember-core_3 (Maven) 1.0.0-M47, 0.23.35.

Low priority for now. Upgrade to org.http4s:http4s-ember-core_2.12 (Maven) 0.23.35; org.http4s:http4s-ember-core_2.13 (Maven) 1.0.0-M47, 0.23.35; org.http4s:http4s-ember-core_3 (Maven) 1.0.0-M47, 0.23.35 or later.

Generated automatically from the record's data: CVSS (NVD and CNA), CISA KEV and SSVC, FIRST EPSS, exploit sources, fixed versions. How we calculate it

Primary sources: NVD · CVE.org · FIRST EPSS

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.

HighCVSS 8.2 · v4.0—No exploit Fix available
Published
Aug 26, 2026
Updated
Sep 9, 2026
EPSS
0.5% · 44th percentile

Report tools

JSON

Action score

32

Monitor

Low priority for now.

CVSS
32 / 40 · 8.2 / 10
CISA KEV
0 / 30 · Not listed
EPSS
0 / 30 · 0.5%

CISA SSVC decision

Exploitation
none
Automatable
yes
Technical impact
partial

Vulnrichment: CISA's decision-tree inputs.

CNA vs NVD score

NVD
—
CNA · GitHub_M
8.2
NVD has not scored this yet; the score shown is the CNA’s.

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

The score given by the assigning authority versus NVD’s independent score. A gap means the severity is contested.

Get told when this record changes

  • Not in KEV: no record of exploitation in the wild
  • No public exploit code on record
  • A fix has been released

If any of these changes, we'll tell you in the daily email digest.

Follow CVE-2026-54556

When you'll hear from us

  • when it enters CISA KEV
  • when exploit code appears or is weaponized
  • when the record is updated (including a fixed version)
  • when EPSS rises 20 points in a week

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Noroxi analysis

What does it mean in your environment?

The assessment above is generated automatically from the record's data; it doesn't know your environment. Our team verifies on your systems whether this vulnerability is actually exploitable and which mitigation is enough.

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Affected systems

Is your version affected?

Pick the product and type the version you run. It's compared with this record's version ranges in your browser; the version isn't sent anywhere.

—

Versions reported by the vendor

Affected version ranges reported by the assigning authority (GitHub_M). Independent of NVD's CPE analysis and usually ahead of it.

  • http4s http4s

    • < 0.23.35, >= 1.0.0-M1, < 1.0.0-M46affected

  • org.http4s http4s-ember-core_2.12

    • < 0.23.35affected

  • org.http4s http4s-ember-core_2.13

    • >= 1.0.0-M1, < 1.0.0-M46, < 0.23.35affected

  • org.http4s http4s-ember-core_3

    • >= 1.0.0-M1, < 1.0.0-M46, < 0.23.35affected

Package-level exposure

OSV and GitHub Advisory data: ecosystem, package and range. SBOM matching uses this table.

EcosystemPackageAffected rangeFix
Mavenorg.http4s:http4s-ember-core_2.12before 0.23.350.23.35
Mavenorg.http4s:http4s-ember-core_2.13from 1.0.0-M1 · before 1.0.0-M471.0.0-M47
Mavenorg.http4s:http4s-ember-core_2.13before 0.23.350.23.35
Mavenorg.http4s:http4s-ember-core_3from 1.0.0-M1 · before 1.0.0-M471.0.0-M47
Mavenorg.http4s:http4s-ember-core_3before 0.23.350.23.35

Same product

http4s: all records

Other highest-scoring records for the same primary product.

  • CVE-2026-69204Http4s: Ember accepts Transfer-Encoding combined with Content-Length (CL.TE request smuggling)
    36Monitor
  • CVE-2026-69217Http4s: Ember Server accepts duplicate Content-Length headers
    34Monitor
  • CVE-2026-69205Http4s: Ember Transfer-Encoding value parsing (TE.CL / TE.0 request smuggling)
    34Monitor

Published before and after this one in the same product

  • Later →CVE-2026-69201Sep 15, 2026 · Http4s: ResourceService and Webjar Service path escape via percent-encoded separators
    23Monitor
  • Later →CVE-2026-69208Sep 15, 2026 · Http4s: DigestAuth nonce map grows unbounded
    30Monitor
  • Later →CVE-2026-69209Sep 15, 2026 · Http4s: WebSocket decoder accepts unbounded message sizes
    30Monitor

Remediation

Which version to upgrade to

Fix versions compiled from the vendor, package registries and Microsoft. Verify the vendor's note before upgrading.

Product / packageFixed versionSource
Maven:org.http4s:http4s-ember-core_2.120.23.35Package registry (OSV)
Maven:org.http4s:http4s-ember-core_2.130.23.35Package registry (OSV)
Maven:org.http4s:http4s-ember-core_2.131.0.0-M47Package registry (OSV)
Maven:org.http4s:http4s-ember-core_30.23.35Package registry (OSV)
Maven:org.http4s:http4s-ember-core_31.0.0-M47Package registry (OSV)

Exploit status

No known public exploit

No public exploit has been observed yet. That doesn't mean you're safe, only that the bar is a little higher.

Research context

For pentesters and researchers: attack profile, score disagreement, timeline, patch commits, credits, variant and chain candidates, bug bounty scope. All derived from existing data; no exploit code.

Timeline

From publication to today: proof of concept, Metasploit module, CISA KEV and fix record. Dates are as reported by the sources.

No dated events beyond publication.

EPSS, last 120 days

FIRST EPSS daily score; only changes of 0.01 or more are recorded (step chart).

Patch and commit links

Commit, PR and diff links among the references. A starting point for patch-diffing and variant hunting; fixes, not exploits.

Credits

All researchers

Finders, reporters and analysts named in the CNA record. Click a name for that researcher’s other records.

No credits in the CNA record.

Variant candidates

Same product, same weakness class, within 18 months. If the patch missed the root cause, the sibling bug is here.

No nightly-computed relations.

Chain candidates

An authentication bypass and a privilege-requiring bug in the same product, published close together: combined they may become an unauthenticated path.

—

Bug bounty scope

No known public program.

Source: bounty-targets-data (public HackerOne, Bugcrowd, Intigriti, YesWeHack listings).

Technical details

Attack conditions

  • Anyone who can reach it over the internet can trigger it.
  • No account or password is required.
  • No user action is required.
  • No special conditions are required; it is repeatable.

If successful

Confidentiality
Integrity
Availability
Attack vector
Network
Attack complexity
Low
Privileges required
None
User interaction
None
Scope
X

Weakness class (CWE)

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

CVSS 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/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

nvd-secondary

Attack context

MITRE CAPEC attack patterns and ATT&CK techniques for this weakness class (CWE). A starting point for detection rules and threat hunting.

MITRE has no CAPEC/ATT&CK mapping for this CWE.

Change log

  1. Fix✗ → ✓

For records you follow, these changes also arrive as notifications. →

References

Show all references (3 more)

Vendor advisories and official records. Exploit/PoC links are deliberately left out.

All records