{"description":"Trending threats, MITRE ATT\u0026CK coverage, and detection metadata. Fed continuously.","feed_url":"https://feed.craftedsignal.io/products/http4s-blaze-server_2.13-vulnerable--1.0.0-m1--1.0.0-m42/feed.json","home_page_url":"https://feed.craftedsignal.io/","items":[{"_cs_actors":[],"_cs_cpes":[],"_cs_cves":[],"_cs_exploited":false,"_cs_has_poc":false,"_cs_poc_references":[],"_cs_products":["http4s-blaze-server_2.13 (vulnerable: \u003c 0.23.18)","http4s-blaze-server_2.13 (vulnerable: \u003e= 1.0.0-M1, \u003c 1.0.0-M42)","http4s-blaze-server_2.12 (vulnerable: \u003c 0.23.18)","http4s-blaze-server_3 (vulnerable: \u003e= 1.0.0-M1, \u003c 1.0.0-M42)"],"_cs_severities":["medium"],"_cs_tags":["denial-of-service","webserver","jvm","websocket"],"_cs_type":"advisory","_cs_vendors":["http4s"],"content_html":"\u003cp\u003eA critical vulnerability exists in \u003ccode\u003ehttp4s-blaze-server\u003c/code\u003e versions prior to 0.23.18 and within versions 1.0.0-M1 to 1.0.0-M41, stemming from unbounded WebSocket message aggregation. Attackers can exploit this by establishing a WebSocket connection and sending fragmented messages without setting the final (FIN) bit. The server, particularly when using \u003ccode\u003eBlazeServerBuilder\u003c/code\u003e, buffers these fragments without any size limit, irrespective of the \u003ccode\u003emaxWebSocketBufferSize\u003c/code\u003e setting which only applies to individual frames. This leads to continuous and uncontrolled heap memory growth in the server's Java Virtual Machine (JVM). Ultimately, this resource exhaustion results in an \u003ccode\u003eOutOfMemoryError\u003c/code\u003e and the termination of the JVM process, effectively causing a Denial of Service (DoS) for the affected http4s application. Both unauthenticated and authenticated clients can trigger this vulnerability, depending on the WebSocket endpoint's exposure, making it a severe threat to the availability of affected services.\u003c/p\u003e\n\u003ch2 id=\"attack-chain\"\u003eAttack Chain\u003c/h2\u003e\n\u003col\u003e\n\u003cli\u003eAn attacker establishes a WebSocket connection with a vulnerable http4s-blaze-server application.\u003c/li\u003e\n\u003cli\u003eThe attacker sends an initial WebSocket message fragment.\u003c/li\u003e\n\u003cli\u003eThe attacker continues to send a series of subsequent WebSocket message fragments.\u003c/li\u003e\n\u003cli\u003eThese fragments intentionally do not set the \u003ccode\u003eFIN\u003c/code\u003e (final fragment) bit, indicating an unterminated message.\u003c/li\u003e\n\u003cli\u003eThe \u003ccode\u003ehttp4s-blaze-server\u003c/code\u003e buffers all incoming fragments of this unterminated message without an aggregate size limit.\u003c/li\u003e\n\u003cli\u003eDue to the lack of an aggregate size limit, the server's JVM heap memory grows uncontrollably as fragments accumulate.\u003c/li\u003e\n\u003cli\u003eThe server's JVM encounters an \u003ccode\u003eOutOfMemoryError\u003c/code\u003e as it exhausts available memory resources.\u003c/li\u003e\n\u003cli\u003eThis \u003ccode\u003eOutOfMemoryError\u003c/code\u003e leads to the termination of the server JVM, resulting in a Denial of Service for the affected http4s application.\u003c/li\u003e\n\u003c/ol\u003e\n\u003ch2 id=\"impact\"\u003eImpact\u003c/h2\u003e\n\u003cp\u003eAny http4s application that serves WebSocket routes over \u003ccode\u003eBlazeServerBuilder\u003c/code\u003e is directly impacted by this vulnerability. Successful exploitation leads to an \u003ccode\u003eOutOfMemoryError\u003c/code\u003e within the server's JVM, causing it to terminate. This results in complete unavailability of the affected application, leading to a Denial of Service. The \u003ccode\u003emaxWebSocketBufferSize\u003c/code\u003e configuration parameter does not mitigate this issue as it only limits individual frame sizes, not the aggregated message size. A single malicious connection can exhaust the server's heap, even with a modest volume of wire bytes if many small fragments are used, amplifying the per-frame object overhead. The attacker can be unauthenticated if the WebSocket endpoint is publicly reachable or an authenticated client if the handshake requires authentication.\u003c/p\u003e\n\u003ch2 id=\"recommendation\"\u003eRecommendation\u003c/h2\u003e\n\u003cul\u003e\n\u003cli\u003eTerminate or limit WebSocket traffic at a fronting layer (e.g., a reverse proxy or load balancer) that enforces message-size and fragment count limits, or disable WebSocket routes entirely on affected \u003ccode\u003ehttp4s-blaze-server\u003c/code\u003e instances.\u003c/li\u003e\n\u003cli\u003ePlan migration to a maintained backend (e.g., http4s-ember) as the upstream \u003ccode\u003eblaze\u003c/code\u003e server is End-of-Life, addressing the root cause in affected \u003ccode\u003ehttp4s-blaze-server\u003c/code\u003e versions.\u003c/li\u003e\n\u003c/ul\u003e\n","date_modified":"2026-07-24T22:35:42Z","date_published":"2026-07-24T22:35:42Z","id":"https://feed.craftedsignal.io/briefs/2026-07-unbounded-websocket-dos-http4s-blaze/","summary":"A vulnerability in `http4s-blaze-server` allows an attacker to cause a denial of service by exploiting unbounded WebSocket message aggregation, enabling an attacker to drive unbounded heap growth in the server's JVM by sending an unterminated fragmented WebSocket message, leading to an `OutOfMemoryError` and server termination, affecting any http4s application serving WebSocket routes over `BlazeServerBuilder` and triggerable by unauthenticated or authenticated clients.","title":"Unbounded WebSocket Message Aggregation Leads to Denial of Service in http4s-blaze-server","url":"https://feed.craftedsignal.io/briefs/2026-07-unbounded-websocket-dos-http4s-blaze/"}],"language":"en","title":"CraftedSignal Threat Feed - Http4s-Blaze-Server_2.13 (Vulnerable: \u003e= 1.0.0-M1, \u003c 1.0.0-M42)","version":"https://jsonfeed.org/version/1.1"}