Critical Vulnerability in Fluent Bit Exposes Cloud Services to Potential Cyber Attacks

A critical security flaw in Fluent Bit, a widely adopted log processing and metrics collection tool part of the Cloud Native Computing Foundation (CNCF), has exposed enterprise cloud infrastructures to denial-of-service (DoS) attacks.

Designated as CVE-2024-50608 and CVE-2024-50609, these vulnerabilities—scoring 8.9 on the CVSS v3.1 severity scale—stem from improper handling of HTTP headers in the Prometheus Remote Write and OpenTelemetry input plugins.

Researchers at Ebryx discovered that attackers could exploit these flaws by sending malicious payloads with Content-Length: 0, triggering a null pointer dereference (CWE-476) and crashing the service1.

Technical Breakdown of the Vulnerabilities

The vulnerabilities reside in Fluent Bit’s HTTP server implementation, which processes incoming requests for metrics ingestion.

When the affected plugins receive a POST request with a Content-Length header set to zero, the parser fails to validate the input, dereferencing an uninitialized memory pointer.

For example, the following curl command reliably crashes unpatched instances:

bashcurl --path-as-is -i -s -k -X POST \
  -H "Host: localhost:8080" \
  -H "Content-Length: 0" \
  --data-binary 'message "RkFSQU46TUVHQUNIQVIweDAx"' \
  http://127.0.0.1:9090/api/prom/push

The crash occurs in the cfl_sds_len function, which attempts to calculate the length of a null pointer passed from the HTTP payload processing layer.

Backtrace analysis revealed the flawed logic in the header_lookup function, where Content-Length validation lacked checks for zero or negative values1:

celse if (i == MK_HEADER_CONTENT_LENGTH) {
  // ... 
  if (val <= 0) {  // Pre-patch: only checked for val < 0
    return -1;
  }
  p -> header_content_length = val;
}

A second vulnerability, an out-of-bounds write (CWE-787), was identified in systems with >32 CPU cores.

Fluent Bit’s ne_utils_file_read_uint64 function improperly indexed a statically allocated array core_throttles_set[256], causing segmentation faults on multi-core servers1.

Fuzzing Methodology and Impact

Ebryx employed boofuzz, a network protocol fuzzer, to probe Fluent Bit’s input handlers.

The team targeted three critical plugins—HTTP, Prometheus Remote Write, and OpenTelemetry—using mutational fuzzing strategies.

For the OpenTelemetry plugin, this involved manipulating protocol buffers (Protobuf) and HTTP headers:

pythonfrom boofuzz import *
session = Session(target=Target(connection=TCPSocketConnection("localhost", 4318)))
s_initialize("HTTP POST")
s_static("Content-Length: 0\r\n\r\n")  # Crash-inducing header
session.connect(s_get("HTTP POST"))
session.fuzz()

The fuzzing campaign revealed that all OpenTelemetry endpoints (/v1/traces, /v1/logs, /v1/metrics) were susceptible to crashes.

In production environments, this could disrupt log aggregation pipelines, impairing visibility into application performance and security events.

Mitigation and Patch Deployment

The Fluent Bit maintainers have released patches validating Content-Length headers and adding bounds checks for CPU core indices.

Administrators must upgrade to:

• Fluent Bit v3.0.4 (stable)
• Fluent Bit v2.2.2 (LTS)

Configuration hardening is also advised:

text[INPUT]
  name opentelemetry
  listen 0.0.0.0
  port 4318
  # Enable TLS to mitigate unauthenticated attacks
  tls on
  tls.cert_file /etc/ssl/certs/fluent-bit.crt
  tls.key_file /etc/ssl/private/fluent-bit.key

Industry Implications

With over 15 billion downloads and 10 million daily deployments, Fluent Bit underpins observability stacks at major cloud providers.

Unpatched instances risk cascading failures in Kubernetes clusters, serverless platforms, and SaaS monitoring tools.

Ebryx’s findings underscore the critical need for protocol-level fuzz testing in CNCF projects—especially for components processing untrusted network data1.

As enterprises accelerate cloud adoption, securing the “pipes” of telemetry data becomes as crucial as safeguarding application code.

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