Summary
Netty SPDY SETTINGS frame count materializes unbounded settings map
Netty's SPDY SETTINGS decoder accepts a peer-declared SETTINGS entry count up to the 24-bit frame-length limit and materializes every unique setting ID in DefaultSpdySettingsFrame without an implementation-level count cap. A remote SPDY/3.1 peer can send one syntactically valid roughly 2 MiB SETTINGS frame that creates 262144 map entries, amplifying network input into heap growth and ordered-map insertion work.
Details
Inbound SPDY bytes enter SpdyFrameCodec.decode() and are passed directly to the frame decoder. The decoder reads the peer-controlled flags and 24-bit frame length from the common header, then accepts SETTINGS frames with only length >= 4. For SETTINGS payloads, it reads the peer-controlled numSettings field and validates only that the remaining payload is divisible into 8-byte entries and exactly matches that count. Each accepted entry then supplies an attacker-controlled 24-bit ID and value, and the normal delegate path forwards it into spdySettingsFrame.setValue(). The sink is DefaultSpdySettingsFrame: it backs settings with a TreeMap, checks only that IDs fit the SPDY 24-bit maximum, and inserts a new Setting for each previously unseen ID. There is no count budget between the wire-format count validation and the TreeMap insertion site.
PoC
run with
bash ./poc/run.sh
expected output:
NETTY_SPDY_SETTINGS_COUNT_MAP_TRIGGERED settings_count=262144 wire_bytes=2097164 approx_heap_delta=17692272 first_value=1 last_value=262144
The NETTY_SPDY_SETTINGS_COUNT_MAP_TRIGGERED line means the harness decoded the crafted SETTINGS frame and observed all 262144 peer-selected IDs in the resulting settings map. The wire_bytes=2097164, first_value=1, and last_value=262144 fields distinguish this from a setup failure: they show the exact oversized frame was accepted and fully materialized.
Impact
remote unauthenticated network peer that can speak SPDY/3.1 to a Netty pipeline containing SpdyFrameCodec can trigger resource-exhaustion denial of service. The required guards are satisfied by a complete valid SETTINGS frame using the expected SPDY version, a length of 4 + numSettings * 8, and IDs within the accepted 24-bit range; the verified PoC uses numSettings=262144 and wire_bytes=2097164. On that input, Netty materializes 262144 attacker-controlled entries in a TreeMap-backed DefaultSpdySettingsFrame, with local runs observing about 17-18 MiB of heap growth per decoded frame plus CPU work for ordered-map insertion.
Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service. Typical impact: denial of service.
CVE-2026-55831 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 (4.2.16.Final, 4.1.136.Final); 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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io.netty:netty-codec-http to 4.2.16.Final or later; io.netty:netty-codec-http to 4.1.136.Final or later
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Frequently Asked Questions
- What is CVE-2026-55831? CVE-2026-55831 is a high-severity uncontrolled resource consumption vulnerability in io.netty:netty-codec-http (maven), affecting versions >= 4.2.0.Final, <= 4.2.15.Final. It is fixed in 4.2.16.Final, 4.1.136.Final. Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service.
- How severe is CVE-2026-55831? CVE-2026-55831 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 versions of io.netty:netty-codec-http are affected by CVE-2026-55831? io.netty:netty-codec-http (maven) versions >= 4.2.0.Final, <= 4.2.15.Final is affected.
- Is there a fix for CVE-2026-55831? Yes. CVE-2026-55831 is fixed in 4.2.16.Final, 4.1.136.Final. Upgrade to this version or later.
- Is CVE-2026-55831 exploitable, and should I be worried? Whether CVE-2026-55831 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-2026-55831 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-2026-55831?
- Upgrade
io.netty:netty-codec-httpto 4.2.16.Final or later - Upgrade
io.netty:netty-codec-httpto 4.1.136.Final or later
- Upgrade