Vulnerability · Published Aug 26, 2026 · Assigned by GitHub_M
CVE-2026-54556Http4s: HTTP/2 Denial of Service with Ember Backend
High 8.2
In plain English
A high-severity security flaw in http4s from http4s. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.5% (higher than 45% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.
Official 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.
Affected packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| Maven | org.http4s:http4s-ember-core_2.12 | 0.23.35 | GHSA-vmm3-xgcx-67hm |
| Maven | org.http4s:http4s-ember-core_2.13 | 1.0.0-M47 | GHSA-vmm3-xgcx-67hm |
| Maven | org.http4s:http4s-ember-core_2.13 | 0.23.35 | GHSA-vmm3-xgcx-67hm |
| Maven | org.http4s:http4s-ember-core_3 | 0.23.35 | GHSA-vmm3-xgcx-67hm |
| Maven | org.http4s:http4s-ember-core_3 | 1.0.0-M47 | GHSA-vmm3-xgcx-67hm |
Affected products (as reported)
| Vendor | Product | Versions |
|---|---|---|
| http4s | http4s | < 0.23.35>= 1.0.0-M1, < 1.0.0-M46 |
| org.http4s | http4s-ember-core_2.12 | < 0.23.35 |
| org.http4s | http4s-ember-core_2.13 | >= 1.0.0-M1, < 1.0.0-M46< 0.23.35 |
| org.http4s | http4s-ember-core_3 | >= 1.0.0-M1, < 1.0.0-M46< 0.23.35 |
Weakness: CWE-409 · 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
References
- https://github.com/http4s/http4s/security/advisories/GHSA-vmm3-xgcx-67hm x_refsource_CONFIRM
- https://github.com/http4s/http4s/commit/6e8eccd64a6a74ab4811897881e95e0e1b3a818e x_refsource_MISC
- https://github.com/http4s/http4s/releases/tag/v0.23.35 x_refsource_MISC
- https://github.com/http4s/http4s/releases/tag/v1.0.0-M47 x_refsource_MISC