CVE-2026-69203 in Http4s
Summary
by MITRE • 09/15/2026
Http4s is a Scala interface for HTTP services. Prior to 0.23.35 and 1.0.0-M47, An Ember server with HTTP/2 enabled through withHttp2 does not enforce SETTINGS_MAX_CONCURRENT_STREAMS for peer-created streams. One unauthenticated connection can open an unbounded number of streams, each retaining per-stream state until heap exhaustion. The same unchecked allocation is reachable in an ember-client through server-initiated PUSH_PROMISE frames because enablePush is not enforced. This issue is fixed in versions 0.23.35 and 1.0.0-M47.
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Analysis
by VulDB Data Team • 09/15/2026
The vulnerability identified in the Http4s library, specifically affecting Ember servers and clients prior to versions 0.23.35 and 1.0.0-M47, represents a critical resource exhaustion flaw rooted in the improper enforcement of HTTP/2 protocol constraints. Http4s serves as a prominent functional interface for building HTTP services in Scala, leveraging its underlying transport layer, Ember, to handle network communications. The core technical deficiency lies within the implementation of the HTTP/2 specification compliance regarding stream management and server push mechanisms. Specifically, when an Ember server is configured with HTTP/2 enabled via the withHttp2 method, it fails to enforce the SETTINGS_MAX_CONCURRENT_STREAMS setting for streams initiated by peers. In a standard HTTP/2 environment, this setting dictates the maximum number of simultaneous open streams allowed per connection, serving as a primary defense against resource abuse. By neglecting this enforcement, the server allows an unauthenticated client to establish an unlimited number of concurrent streams on a single TCP connection.
This lack of concurrency limits has severe operational implications for system stability and availability. Each HTTP/2 stream retains specific state information in memory until it is closed or completed. When an attacker opens an unbounded number of these streams, the server must allocate heap space to maintain this per-stream state data structure. Since there is no upper bound on the number of active streams, the application's memory consumption grows linearly with each new stream opened by the client. This behavior effectively creates a denial-of-service condition where the target system suffers from heap exhaustion. As the garbage collector struggles to reclaim memory or fails entirely due to pressure, the server becomes unresponsive, leading to service degradation for legitimate users and potentially causing the application process to crash if out-of-memory errors are not gracefully handled.
A secondary but equally critical aspect of this vulnerability involves the client-side implementation regarding server-initiated PUSH_PROMISE frames. The HTTP/2 specification allows servers to push resources to clients proactively using these frames, which can be useful for performance optimization in certain scenarios. However, the Ember client prior to the patched versions does not enforce enablePush restrictions or limits on incoming pushed streams. This means that a malicious server could exploit this flaw by sending an excessive number of PUSH_PROMISE frames, forcing the client to allocate memory for resources it did not explicitly request and without any limit on concurrency. Similar to the server-side issue, this unchecked allocation leads to heap exhaustion on the client side, compromising the availability and stability of applications relying on Http4s clients for HTTP/2 communications.
From a classification perspective, this vulnerability aligns with CWE-787: Out-of-bounds Write or CWE-1321: Improperly Controlled Modification of Object Prototype Attributes if viewed through memory corruption lenses, but more accurately fits CWE-400: Uncontrolled Resource Consumption and specifically CWE-693: Protection Mechanism Failure. In the context of attack patterns, this is a classic example of an application-layer denial-of-service vector often categorized under MITRE ATT&CK technique T1499: Endpoint Denial of Service or more precisely within network-based DoS vectors that exploit protocol non-compliance to exhaust resources. The vulnerability highlights the importance of strict adherence to RFC 7540, which defines HTTP/2 semantics and mandates proper handling of flow control and stream limits to prevent such abuse scenarios.
Mitigation for this issue requires an immediate upgrade to Http4s version 0.23.35 or later, including versions 1.0.0-M47 and above where these constraints are properly enforced. For organizations unable to patch immediately due to dependency conflicts, temporary workarounds may include placing a reverse proxy such as Nginx or Apache in front of the Http4s service that strictly enforces HTTP/2 stream limits before requests reach the vulnerable application layer. Additionally, configuring JVM heap size limits and implementing monitoring alerts for memory usage spikes can help detect exploitation attempts early, although these are reactive measures rather than preventive fixes. Ensuring that all instances of Ember-based servers and clients within an infrastructure are updated is essential to eliminate this attack surface entirely.