Summary
Http4s Ember HTTP/2: unbounded continuation frame accumulation
When Ember receives an HTTP/2 HEADERS or PUSH_PROMISE frame without the END_HEADERS flag, it buffers the header block fragment and waits for subsequent CONTINUATION frames. These accumulate unbounded until the connection closes.
Prerequisites
EmberServerBuilderconfigured with.withHttp2.- For the server: the attacker can establish an HTTP/2 connection
- For the client: the application makes a request to an attacker-controlled origin.
HTTP/2 incoming headers exceeding the configured size limit are buffer in order to generate an informative HTTP 413 response. If a client does not stop sending headers, this leads to memory exhaustion.
Workarounds
- Disable HTTP/2 (
.withHttp2) until upgraded (default). It is off by default. - If HTTP/2 must remain enabled, place ember behind a reverse proxy that terminates HTTP/2 and speaks HTTP/1.1 to ember.
Impact
A remote, unauthenticated peer can exhaust the heap on any Ember endpoint that has HTTP/2 enabled:
- ember-server with
.withHttp2: any HTTP/2 client can trigger this against any reachable path (including paths that return 404). No authentication is required because the attack completes before the request is decoded. - ember-client with
.withHttp2: a malicious or compromised origin server can trigger this via the response header block. A single in-flight request is sufficient.
Memory consumption is bounded only by the attacker's upload bandwidth and the connection lifetime.
The application allocates resources such as memory, threads, or file descriptors based on untrusted input without enforcing a cap. Typical impact: resource exhaustion leading to denial of service.
CVE-2026-69218 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (0.23.35, 1.0.0-M47); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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The fix bounds the accumulated header-block size at SETTINGS_MAX_HEADER_LIST_SIZE (derived from EmberServerBuilder.maxHeaderSize / EmberClientBuilder.maxResponseHeaderSize). When a CONTINUATION frame would push the accumulated block over that limit, the connection is terminated with GOAWAY. The receiveHeadersTimeout additionally bounds how long an incomplete header block may remain open.
Frequently Asked Questions
- What is CVE-2026-69218? CVE-2026-69218 is a high-severity allocation of resources without limits or throttling vulnerability in org.http4s:http4s-ember-core_2.12 (maven), affecting versions <= 0.23.34. It is fixed in 0.23.35, 1.0.0-M47. The application allocates resources such as memory, threads, or file descriptors based on untrusted input without enforcing a cap.
- How severe is CVE-2026-69218? CVE-2026-69218 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which packages are affected by CVE-2026-69218?
org.http4s:http4s-ember-core_2.12(maven) (versions <= 0.23.34)org.http4s:http4s-ember-core_2.13(maven) (versions <= 0.23.34)org.http4s:http4s-ember-core_3(maven) (versions <= 0.23.34)
- Is there a fix for CVE-2026-69218? Yes. CVE-2026-69218 is fixed in 0.23.35, 1.0.0-M47. Upgrade to this version or later.
- Is CVE-2026-69218 exploitable, and should I be worried? Whether CVE-2026-69218 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-69218 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-69218?
- Upgrade
org.http4s:http4s-ember-core_2.12to 0.23.35 or later - Upgrade
org.http4s:http4s-ember-core_2.13to 0.23.35 or later - Upgrade
org.http4s:http4s-ember-core_3to 0.23.35 or later - Upgrade
org.http4s:http4s-ember-core_2.13to 1.0.0-M47 or later - Upgrade
org.http4s:http4s-ember-core_3to 1.0.0-M47 or later
- Upgrade