GHSA-mvh2-crg5-v77c CVE-2026-55833

Netty SPDY zlib header block continues decoded expansion after maxHeaderSize truncation

High Maven

Severity: 7.5 / 10; High · CVSS 3.1
Exploit chance (EPSS): 0.66%; Chance of exploitation in the next 30 days. Higher than 51.81% of scored vulnerabilities.
Known exploited: No; Not in CISA's catalog
Weakness: CWE-400 Uncontrolled Resource Consumption
Fix: 4.2.16.Final; Upgrade io.netty:netty-codec-http to this version or later

Description

Summary

Netty SPDY header decoding continues inflating zlib-compressed header blocks after the raw header parser has already exceeded maxHeaderSize and marked the frame truncated. At commit b2d2137c4404af425bf9d5d601a62576f5c06925, a 12,253-byte compressed SPDY header block can declare and inflate a 12 MiB header-name field with maxHeaderSize=16, forcing compression-amplified decode and skip work in a reachable SpdyFrameCodec pipeline.

PoC

poc.zip

run with:

bash ./poc/run.sh

expected output:

NETTY_SPDY_ZLIB_DECODED_AFTER_LIMIT_TRIGGERED compressed_bytes=12253 declared_name_length=12582912 max_header_size=16 truncated=true invalid=false

The fingerprint means the compressed input was fully consumed while the raw header parser ended with truncated=true and invalid=false after processing the oversized decoded name. That specific state distinguishes this bug from a generic setup failure: the maxHeaderSize guard fired, but the zlib/raw decode path still inflated and skipped the full 12 MiB declared name.

Impact

A remote unauthenticated peer that can speak SPDY to a Netty pipeline containing SpdyFrameCodec can send a small compressed HEADERS block that expands into much larger raw header data after the configured maxHeaderSize limit has already been exceeded. The attack requires a reachable SPDY codec, ordinary transport setup such as TCP and optional TLS, and no independent compressed-frame-size or connection-rate limit ahead of SpdyFrameCodec. The satisfied protocol guards are straightforward: the HEADERS frame uses a nonzero stream id and length >= 4, the decoder factory selects the zlib decoder, the payload uses the SPDY dictionary, and the raw block appends a zero-length value so the already-truncated frame reaches END_HEADER_BLOCK. The user-visible effect is denial of service through compression-amplified CPU and allocation churn.

Affected versions

Maven

Package	Vulnerable	Fixed in
io.netty:netty-codec-http	`>= 4.2.0.Final, < 4.2.16.Final`	`4.2.16.Final`
io.netty:netty-codec-http	`>= 4.1.0.Final, < 4.1.136.Final`	`4.1.136.Final`

How to fix

Move to the fixed version or later with your package manager, update the lockfile, and run your tests. If the package only arrives through another dependency, upgrade that dependency first. If none of its releases pulls a fixed version yet, force the version with your package manager's override.

io.netty:netty-codec-http Maven → `4.2.16.Final`

maven

<dependency><groupId>io.netty</groupId><artifactId>netty-codec-http</artifactId><version>4.2.16.Final</version></dependency>

gradle

implementation("io.netty:netty-codec-http:4.2.16.Final")

Severity breakdown

CVSS 3.1 7.5

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Attack vector: Network
Attack complexity: Low
Privileges required: None
User interaction: None
Scope: Unchanged
Confidentiality: None
Integrity: None
Availability: High

Weakness

CWE-400: Uncontrolled Resource Consumption

The product does not properly control the allocation and maintenance of a limited resource.

How to prevent it

Architecture and Design: Design throttling mechanisms into the system architecture. The best protection is to limit the amount of resources that an unauthorized user can cause to be expended. A strong authentication and access control model will help prevent such attacks from occurring in the first place. The login application should be protected against DoS attacks as much as possible. Limiting the database access, perhaps by caching result sets, can help minimize the resources expended. To further limit the potential for a DoS attack, consider tracking the rate of requests received from users and blocking requests that exceed a defined rate threshold.
Architecture and Design: Mitigation of resource exhaustion attacks requires that the target system either: recognizes the attack and denies that user further access for a given amount of time, or uniformly throttles all requests in order to make it more difficult to consume resources more quickly than they can again be freed. The first of these solutions is an issue in itself though, since it may allow attackers to prevent the use of the system by a particular valid user. If the attacker impersonates the valid user, they may be able to prevent the user from accessing the server in question. The second solution is simply difficult to effectively institute -- and even when properly done, it does not provide a full solution. It simply makes the attack require more resources on the part of the attacker.
Architecture and Design: Ensure that protocols have specific limits of scale placed on them.
Implementation: Ensure that all failures in resource allocation place the system into a safe posture.

References

Advisories

nvd.nist.gov/vuln/detail/CVE-2026-55833

Packages

github.com/netty/netty

Timeline

Jul 21, 2026 Published
Sep 10, 2026 Last updated by the source
Oct 31, 2026 Latest EPSS score

Advisory	Severity	Why it's related
GHSA-8c42-7qj2-3j46 Netty Vulnerable to Cache Poisoning and Information Disclosure via CORS Vary Header Overwrite	High	Also affects io.netty:netty-codec-http
GHSA-gcjf-9mgh-3p7g Netty: CRLF Injection via Multipart Filename in Netty HttpPostRequestEncoder	Medium	Also affects io.netty:netty-codec-http
GHSA-q4f6-jm68-57ww Netty: [HttpContentEncoder] Unbounded Per-Connection Queue Growth via HTTP/1.1 Pipelining Leads to Denial of Service	High	Also affects io.netty:netty-codec-http
GHSA-4mp9-239f-g9hg Netty: WebSockets V07/V08 handshaker missing Connection/Upgrade validation	High	Also affects io.netty:netty-codec-http
GHSA-6cqp-g7gg-8hr5 Netty: Security Control Bypass via CORS Short-Circuit Failure	Medium	Also affects io.netty:netty-codec-http
GHSA-jppx-w49h-x2qq Netty: [SpdyHttpDecoder] ByteBuf Reference Leak on RST_STREAM Leads to Native Memory Exhaustion	High	Also affects io.netty:netty-codec-http
GHSA-6jqx-86gh-f27w Netty SPDY SETTINGS frame count materializes unbounded settings map	High	Also affects io.netty:netty-codec-http
GHSA-hvcg-qmg6-jm4c Netty: HttpObjectDecoder skips arbitrary initial control characters when only initial CRLF characters are permitted	Medium	Also affects io.netty:netty-codec-http

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