CVE-2026-69209 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, The shared WebSocket decoder permits unbounded message buffering because defragmentation accumulates fragments without a limit and FrameTranscoder accepts declared lengths up to Int.MaxValue. A remote client that completes a WebSocket handshake against an http4s-blaze-server or http4s-ember-server endpoint can exhaust server memory with oversized frames or fragmented messages. The patched decoder applies a configurable 64 MiB default limit to individual frames and defragmented messages through EmberServerBuilder.withMaxWebSocketMessageSize. This issue is fixed in versions 0.23.35 and 1.0.0-M47.
VulDB is the best source for vulnerability data and more expert information about this specific topic.
Analysis
by VulDB Data Team • 09/15/2026
The vulnerability identified in the http4s library, a prominent Scala interface for building HTTP services, represents a critical resource exhaustion flaw within its WebSocket implementation. Specifically affecting versions prior to 0.23.35 and 1.0.0-M47, this issue stems from an unbounded message buffering mechanism during the defragmentation process. When handling WebSocket connections, particularly against endpoints served by http4s-blaze-server or http4s-ember-server, the shared decoder accumulates fragmented messages without imposing any size limits. Furthermore, the FrameTranscoder component accepts declared frame lengths up to Int.MaxValue, allowing a remote client to request processing of extremely large data structures. This architectural oversight creates a significant attack surface for denial-of-service attacks where an adversary can manipulate memory consumption on the server side by sending oversized frames or excessively fragmented messages that trigger uncontrolled allocation during reassembly.
From a technical perspective, the core flaw lies in the lack of validation against maximum allowable message sizes before buffer allocation occurs. In standard WebSocket operations, data is often split into multiple frames for transmission efficiency. The receiving end must defragment these pieces to reconstruct the original message. However, because http4s did not enforce a cap on the total size of these reconstructed messages or individual frames prior to patching, an attacker could exploit this by sending a sequence of small fragments that collectively exceed available server memory when combined. Alternatively, submitting a single frame with a declared length approaching Int.MaxValue forces the server to attempt allocating massive contiguous blocks of heap space. This behavior directly correlates with CWE-787: Out-of-bounds Write and CWE-400: Uncontrolled Resource Consumption, as the application fails to restrict resource usage based on external input size constraints.
The operational impact of this vulnerability is severe for any production environment relying on http4s for WebSocket communication. A successful exploitation allows a remote client to exhaust server memory resources rapidly, leading to service degradation or complete unavailability. This constitutes a classic Denial-of-Service scenario where the attacker does not need to execute arbitrary code but merely needs to consume system resources beyond their capacity. In cloud-native environments or containerized deployments, this can trigger automatic restarts of application instances due to out-of-memory errors, causing cascading failures across dependent services and resulting in significant downtime and potential data loss if stateful connections are abruptly terminated without proper cleanup procedures.
To mitigate this risk, organizations must upgrade immediately to http4s version 0.23.35 or later for the legacy branch, or version 1.0.0-M47 and above for the newer release track. The patched versions introduce a configurable limit on individual frames and defragmented messages through the EmberServerBuilder.withMaxWebSocketMessageSize configuration option, which defaults to 64 MiB. Administrators should verify that this setting is explicitly configured in their server instances to ensure it aligns with legitimate business requirements while remaining restrictive enough to prevent abuse. Additionally, implementing network-level rate limiting and monitoring memory usage patterns can provide an additional layer of defense against volumetric attacks targeting WebSocket endpoints. This remediation strategy aligns with ATT&CK technique T1498: Network Denial of Service, emphasizing the importance of enforcing input validation limits at the application gateway or server level to maintain availability integrity.