CVE-2026-73493 is a high-severity (CVSS 7.5) CWE-770 vulnerability in org.http4s:http4s-blaze-server_2.13. A fix is available for org.http4s:http4s-blaze-server_2.13 — see the affected versions and patch details below.
http4s-blaze-server: Unbounded WebSocket message aggregation
Exploitation Status
No confirmed exploitation observed yet
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-73493.
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
CVE-2026-73493 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 379,842 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
Real-World Exposure
org.http4s:http4s-blaze-server_2.13☕org.http4s:http4s-blaze-server_2.13☕org.http4s:http4s-blaze-server_2.12☕org.http4s:http4s-blaze-server_3Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Maven packages — download data is not available via public APIs for these ecosystems.
Description
Summary
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.
Impact
Any http4s application serving WebSocket routes over
BlazeServerBuilder 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.
Workarounds
- No blaze-server configuration bounds the aggregate;
maxWebSocketBufferSizeis 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).
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| ☕Maven | org.http4s:http4s-blaze-server_2.13 | all versions | 0.23.18org.http4s:http4s-blaze-server_2.13:0.23.18 |
| ☕Maven | org.http4s:http4s-blaze-server_2.13 | ≥ 1.0.0-M1&&< 1.0.0-M42 | 1.0.0-M42org.http4s:http4s-blaze-server_2.13:1.0.0-M42 |
| ☕Maven | org.http4s:http4s-blaze-server_2.12 | all versions | 0.23.18org.http4s:http4s-blaze-server_2.12:0.23.18 |
| ☕Maven | org.http4s:http4s-blaze-server_3 | ≥ 1.0.0-M1&&< 1.0.0-M42 | 1.0.0-M42org.http4s:http4s-blaze-server_3:1.0.0-M42 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for org.http4s:http4s-blaze-server_2.13, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update org.http4s:http4s-blaze-server_2.13 to 0.23.18 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-73493 is resolved across your whole dependency graph.
Workarounds
Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2026-73493 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2026-73493. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2026-73493 in your dependencies?
O3 Security finds CVE-2026-73493 across Maven dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.