Summary
webtransport-go: Memory Exhaustion Attack due to Missing Length Check in WTCLOSESESSION Capsule
Full technical description
An attacker can cause excessive memory consumption in webtransport-go's session implementation by sending a WT_CLOSE_SESSION capsule containing an excessively large Application Error Message. The implementation does not enforce the draft-mandated limit of 1024 bytes on this field, allowing a peer to send an arbitrarily large message payload that is fully read and stored in memory.
This allows an attacker to consume an arbitrary amount of memory. The attacker must transmit the full payload to achieve the memory consumption, but the lack of any upper bound makes large-scale attacks feasible given sufficient bandwidth.
Details
WebTransport over HTTP/3, as defined in draft-ietf-webtrans-http3, uses the WT_CLOSE_SESSION capsule to signal session termination with an optional detailed error. The draft specifies that the length of the Application Error Message in this capsule MUST NOT exceed 1024 bytes.
In affected versions of webtransport-go, the parser does not enforce this 1024-byte maximum when processing incoming WT_CLOSE_SESSION capsules. A peer can send a capsule with an excessively large payload, forcing the recipient to allocate and buffer the full amount of transmitted data without bound.
The Fix
webtransport-go now limits the length of the parsed Application Error Message to 1024 bytes in WT_CLOSE_SESSION capsules by reading no more than this amount. This prevents excessive memory consumption.
Impact
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.
CVE-2026-21434 has a CVSS score of 5.3 (Medium). 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 (0.10.0); 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.
Remediation advice
Kodem Kai can prioritize this vulnerability in your dependency tree and generate a fix recommendation.
Frequently Asked Questions
- What is CVE-2026-21434? CVE-2026-21434 is a medium-severity allocation of resources without limits or throttling vulnerability in github.com/quic-go/webtransport-go (go), affecting versions >= 0.3.0, <= 0.9.0. It is fixed in 0.10.0. The application allocates resources such as memory, threads, or file descriptors based on untrusted input without enforcing a cap.
- How severe is CVE-2026-21434? CVE-2026-21434 has a CVSS score of 5.3 (Medium). 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 versions of github.com/quic-go/webtransport-go are affected by CVE-2026-21434? github.com/quic-go/webtransport-go (go) versions >= 0.3.0, <= 0.9.0 is affected.
- Is there a fix for CVE-2026-21434? Yes. CVE-2026-21434 is fixed in 0.10.0. Upgrade to this version or later.
- Is CVE-2026-21434 exploitable, and should I be worried? Whether CVE-2026-21434 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 CVE-2026-21434 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 CVE-2026-21434? Upgrade
github.com/quic-go/webtransport-goto 0.10.0 or later.