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Vulnerability · Published Aug 12, 2026 · Assigned by GitHub_M

CVE-2026-73493http4s-blaze-server: Unbounded WebSocket message aggregation

High 7.5

In plain English

A high-severity resource allocation without limits in blaze from http4s. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.6% (higher than 50% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.

Severity (CVSS 3.1)7.5High
Exploit odds (30 days)0.6%top 49.6% of all CVEs
Exploited in the wildNo reportsCISA SSVC: none
Attack toolingNoneno public tooling known

Official description

Http4s (http4s-blaze-server) is a minimal, idiomatic Scala interface for HTTP services. Prior to 0.23.18 and 1.0.0-M42, http4s-blaze-server aggregates fragments of an incoming WebSocket message with no limit on total size or fragment count. A client that completes a WebSocket handshake can send an unterminated fragmented message and drive unbounded heap growth in the server JVM, resulting in denial of service through OutOfMemoryError. Any http4s application serving WebSocket routes over BlazeServerBuilder is affected, no non-default configuration is required, and maxWebSocketBufferSize does not bound the aggregate because it bounds only individual frames. A single connection sending continuation frames that never set FIN forces the server to buffer every fragment until the heap is exhausted, terminating the JVM with OutOfMemoryError on the blaze selector thread. Small fragments amplify the cost through per-frame object overhead, so a modest volume of wire bytes is sufficient. This issue is fixed in versions 0.23.18 and 1.0.0-M42.

Affected packages & fixed versions

EcosystemPackageFixed inAdvisory
Mavenorg.http4s:http4s-blaze-server_2.120.23.18GHSA-7ppr-r889-mcf2
Mavenorg.http4s:http4s-blaze-server_2.130.23.18GHSA-7ppr-r889-mcf2
Mavenorg.http4s:http4s-blaze-server_2.131.0.0-M42GHSA-7ppr-r889-mcf2
Mavenorg.http4s:http4s-blaze-server_31.0.0-M42GHSA-7ppr-r889-mcf2

Affected products (as reported)

VendorProductVersions
http4s blaze < 0.23.18>= 1.0.0-M1, < 1.0.0-M42

Weakness: CWE-770 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

References