Summary
blaze: Unbounded WebSocket message aggregation in http4s-blaze-server
http4s-blaze-server aggregates the 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 via OutOfMemoryError.
Workarounds
- No blaze-server configuration bounds the aggregate;
maxWebSocketBufferSize
is not a mitigation. - Terminate/limit WebSocket traffic at a fronting layer that enforces
message-size and fragment limits, or disable WebSocket routes. - Longer term: blaze is EOL upstream; plan migration to a maintained
backend (e.g. ember).
Impact
Any http4s application serving WebSocket routes overBlazeServerBuilder is affected; no non-default configuration is required,
and maxWebSocketBufferSize does not bound the aggregate (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. Where the
WebSocket endpoint is reachable without authentication the attacker is
unauthenticated and remote; where the handshake requires a principal, any
authenticated client can still trigger it.
The application allocates resources such as memory, threads, or file descriptors based on untrusted input without enforcing a cap. Typical impact: resource exhaustion leading to denial of service.
GHSA-7PPR-R889-MCF2 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (0.23.18, 1.0.0-M42); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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org.http4s:http4s-blaze-server_2.13 to 0.23.18 or later; org.http4s:http4s-blaze-server_2.13 to 1.0.0-M42 or later; org.http4s:http4s-blaze-server_2.12 to 0.23.18 or later; org.http4s:http4s-blaze-server_3 to 1.0.0-M42 or later
Kodem Kai can prioritize this vulnerability in your dependency tree and generate a fix recommendation.
Frequently Asked Questions
- What is GHSA-7PPR-R889-MCF2? GHSA-7PPR-R889-MCF2 is a high-severity allocation of resources without limits or throttling vulnerability in org.http4s:http4s-blaze-server_2.13 (maven), affecting versions < 0.23.18. It is fixed in 0.23.18, 1.0.0-M42. The application allocates resources such as memory, threads, or file descriptors based on untrusted input without enforcing a cap.
- How severe is GHSA-7PPR-R889-MCF2? GHSA-7PPR-R889-MCF2 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which packages are affected by GHSA-7PPR-R889-MCF2?
org.http4s:http4s-blaze-server_2.13(maven) (versions < 0.23.18)org.http4s:http4s-blaze-server_2.12(maven) (versions < 0.23.18)org.http4s:http4s-blaze-server_3(maven) (versions >= 1.0.0-M1, < 1.0.0-M42)
- Is there a fix for GHSA-7PPR-R889-MCF2? Yes. GHSA-7PPR-R889-MCF2 is fixed in 0.23.18, 1.0.0-M42. Upgrade to this version or later.
- Is GHSA-7PPR-R889-MCF2 exploitable, and should I be worried? Whether GHSA-7PPR-R889-MCF2 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether GHSA-7PPR-R889-MCF2 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix GHSA-7PPR-R889-MCF2?
- Upgrade
org.http4s:http4s-blaze-server_2.13to 0.23.18 or later - Upgrade
org.http4s:http4s-blaze-server_2.13to 1.0.0-M42 or later - Upgrade
org.http4s:http4s-blaze-server_2.12to 0.23.18 or later - Upgrade
org.http4s:http4s-blaze-server_3to 1.0.0-M42 or later
- Upgrade