CVE-2026-21438 — webtransport-go
MEDIUMCVE-2026-21438 is a medium-severity (CVSS 5.3) CWE-401 vulnerability in github.com/quic-go/webtransport-go. A fix is available for github.com/quic-go/webtransport-go — see the affected versions and patch details below.
webtransport-go affected by a Memory Exhaustion Attack due to Missing Cleanup of Streams Map
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-2026-21438.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
Where this sits among everything scored
Real-World Exposure
github.com/quic-go/webtransport-goReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects Go packages — download data is not available via public APIs for these ecosystems.
Description
Summary
An attacker can cause unbounded memory consumption repeatedly creating and closing many WebTransport streams. Closed streams were not removed from an internal session map, preventing garbage collection of their resources.
Details
webtransport-go maintains an internal map tracking WebTransport streams (both unidirectional and bidirectional) belonging to a session. In affected versions, entries for closed streams were not removed from this map, causing the map to grow indefinitely as streams were created and closed.
A malicious peer can exploit this by opening large numbers of streams and closing them, leading to steady memory growth proportional to the number of closed streams.
The Fix
webtransport-go now removes closed streams from the internal map upon closure. This allows the associated resources to be garbage collected, bounding memory usage to active streams only.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐹Go | github.com/quic-go/webtransport-go | all versions | 0.10.0go get github.com/quic-go/webtransport-go@v0.10.0 |
Affected Products
webtransport-goquic-goDetection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/quic-go/webtransport-go, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update github.com/quic-go/webtransport-go to 0.10.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-21438 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
Frequently Asked Questions
Is CVE-2026-21438 in your dependencies?
Find it across Go, including transitive dependencies.