CVE-2026-69213 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, Ember HTTP/2 serializes outbound frames through one unbounded queue consumed by writeLoop. When the peer stops reading, an unauthenticated HTTP/2 client can continue sending PING, SETTINGS, or DATA frames that cause Ember to enqueue acknowledgments or WINDOW_UPDATE frames faster than the writer drains them, exhausting heap memory on a server built with withHttp2. The shared behavior also affects an ember-client connected to a hostile HTTP/2 server, and the patch replaces the unbounded path with bounded, backpressured outbound queues. 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 within the Http4s library, specifically affecting its Ember HTTP/2 implementation prior to versions 0.23.35 and 1.0.0-M47, represents a critical resource exhaustion flaw rooted in improper flow control mechanisms for outbound frame serialization. Http4s serves as a prominent Scala interface for building HTTP services, and the Ember module provides an asynchronous HTTP client and server implementation built on top of Cats Effect. The core technical defect lies in how the library handles outgoing data frames when communicating over HTTP/2 connections that have been negotiated or forced to use version 2 protocols. In these affected versions, outbound frames such as PING, SETTINGS, WINDOW_UPDATE, and DATA are serialized through a single unbounded queue. This architectural choice assumes that the underlying network connection will always be able to drain data at a rate sufficient to keep pace with production, which is a dangerous assumption in real-world networking scenarios where latency spikes or peer-side congestion can occur.
The operational impact of this flaw manifests as a Denial of Service (DoS) condition caused by heap memory exhaustion on the server side. When an HTTP/2 peer stops reading from the connection, typically due to its own resource constraints or malicious intent, it ceases to acknowledge incoming frames or send WINDOW_UPDATE signals that would permit further data transmission under standard HTTP/2 flow control rules. However, because the Ember implementation uses an unbounded queue for outbound serialization, the server continues to enqueue acknowledgments and window update frames generated in response to inbound traffic faster than the write loop can drain them into the socket buffer. This discrepancy leads to a rapid accumulation of objects in memory, eventually triggering OutOfMemoryError exceptions that crash the JVM process hosting the Http4s service. An unauthenticated attacker can exploit this by initiating an HTTP/2 connection and then deliberately stalling or slowing down their read operations, effectively forcing the server into an infinite loop of generating internal frames that consume all available heap space without any backpressure mechanism to halt production.
This vulnerability is not limited strictly to servers; it also affects ember-client instances connecting to hostile HTTP/2 servers. In this client-side scenario, a malicious server can similarly exploit the unbounded nature of the outbound queue by refusing to read acknowledgments or window updates sent by the client. This causes the client application to exhaust its own memory resources when attempting to maintain protocol compliance and manage connection state. The shared behavior across both client and server implementations highlights a systemic design flaw in how Ember handles backpressure for HTTP/2 control frames, rather than an isolated bug in one component. The lack of bounded queues means there is no natural limit on the number of pending operations that can accumulate during periods of network asymmetry or peer non-compliance with flow control standards.
The resolution implemented in versions 0.23.35 and 1.0.0-M47 addresses this issue by replacing the unbounded outbound queues with bounded, backpressured alternatives. This architectural change ensures that when the write loop cannot keep up with production rates due to network congestion or peer behavior, the system applies backpressure upstream. This prevents the unchecked growth of memory usage and allows the application to gracefully handle slow consumers without crashing. By enforcing limits on queue depth, the library aligns more closely with robust resource management practices required for stable long-running services exposed to untrusted networks.
From a classification perspective, this vulnerability maps directly to CWE-400: Uncontrolled Resource Consumption, as it involves the unchecked allocation of system resources (memory) leading to service degradation or failure. It also relates to CWE-770: Allocation of Resources Without Limits or Throttling, specifically in the context of internal queues and buffers that grow without bound under specific network conditions. In terms of adversarial tactics, this exploit aligns with MITRE ATT&CK technique T1499: Endpoint Denial of Service, where an attacker aims to exhaust computational resources such as memory or CPU cycles on a target system. The attack vector is classified as Network-based and requires no authentication, making it particularly dangerous for public-facing services that accept HTTP/2 connections from arbitrary sources.
To mitigate the risk associated with this vulnerability in environments running affected versions of Http4s, immediate upgrade to version 0.23.35 or later, or version 1.0.0-M47 and later, is required. For organizations unable to patch immediately due to dependency constraints, implementing a reverse proxy such as NGINX or Apache HTTP Server in front of the Scala application can provide an additional layer of defense. These proxies often have their own robust flow control mechanisms that can absorb traffic spikes and manage connection states more effectively than the underlying application server during periods of peer non-compliance. Additionally, configuring strict timeouts for idle connections and monitoring heap memory usage with alerts for sudden increases can help detect potential exploitation attempts early. Security teams should also review network configurations to ensure that HTTP/2 is properly negotiated only when necessary, as disabling HTTP/2 entirely removes the attack surface associated with this specific frame serialization flaw, although it may impact performance characteristics required by modern web applications.