CVE-2025-29478: Fluent Bit DoS via cfl_list_size size_t conversion in v3.7.2

A newly cataloged vulnerability, CVE-2025-29478, in Fluent Bit v3.7.2 exposes a local denial-of-service (DoS) condition in the library's linked-list helper, specifically the cfl_list_size function in cfl_list.h at line 165, enabling a low-privileged local actor to crash or hang Fluent Bit and, by extension, disrupt observability pipelines and downstream services that depend on continuous log ingestion.

Background​

Fluent Bit is a lightweight, high‑performance telemetry agent widely used as a log forwarder and processor in cloud-native environments, edge devices, and container platforms such as Kubernetes. Its small footprint and plugin-based architecture make it a common choice for sidecars, collectors, and log agents embedded in many production stacks. The cfl_list module provides a simple circular-linked-list abstraction that Fluent Bit uses across the codebase and in plugins. CVE-2025-29478 was published on April 7, 2025, and has been assessed by multiple vulnerability trackers as a medium-severity issue with a CVSS v3.1 base score of 5.5 (AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H). The vulnerability is characterized as uncontrolled resource consumption / denial of service, and several public advisories and vulnerability databases list Fluent Bit version 3.7.2 as the affected release.

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What the vulnerability is (technical summary)​

At a high level, the bug centers on the cfl_list_size helper in cfl_list.h. Public write-ups describe the function returning a size_t value drawn directly from an internal size field without validation, which allows malformed or malicious inputs—typically originating from locally-controlled configuration or plugin code—to cause the list size to be interpreted as an extremely large unsigned number. That in turn leads to attempts to allocate or iterate based on that oversized value, producing either huge allocations or unbounded loops that exhaust CPU and memory and cause the Fluent Bit process to crash or hang. Several technical write-ups detail this as a type/size mismatch issue: the internal size field (described in analyst reports as a signed 32-bit integer in the impacted implementation) ends up being returned as size_t (an unsigned type, 64-bit on many modern platforms) by cfl_list_size, so a negative size set by crafted input is implicitly converted to a very large unsigned value. The resulting arithmetic and memory operations are the direct cause of resource exhaustion. This explanation has been repeated across multiple advisories and vulnerability databases. Important caveat: while multiple third‑party analyses describe the signed-to-unsigned conversion and the consequences, direct confirmation in the Fluent Bit v3.7.2 source tree (a line-by-line view of cfl_list.h for that release) should be performed by operators who need absolute code-level certainty before applying binary patches. Public vulnerability trackers reference an exploit proof‑of‑concept hosted on GitHub that demonstrates a local crash path; however, the exact internals (e.g., whether the size field is declared as int32_t or another signed type in the exact v3.7.2 tag) should be cross‑checked against the shipped source in any affected environment for definitive verification.

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Why this matters: real-world impact and attack scenarios​

Fluent Bit is frequently run as a background agent with persistent system or container-level presence. A DoS against Fluent Bit can have outsized operational impact because:

• Fluent Bit often sits in the observability path; an outage interrupts log delivery and can blind operators to incidents.
• In Kubernetes deployments, Fluent Bit commonly runs as a DaemonSet or sidecar. A local DoS may be triggered from within a pod (e.g., a compromised container, a malicious tenant with write access to mounted config directories, or a malicious plugin dropped into a plugin directory), which blurs the line between "local" and "remote" in multi-tenant environments.
• In many production setups, Fluent Bit runs with elevated privileges or with access to sensitive storage and network targets. Repeated or sustained DoS can cause cascading failures—buffers fill, retries amplify load to downstream systems, and automated scaling or failover logic can be triggered unnecessarily.

Adversary model: the vulnerability is exploitable by an attacker with the ability to place or modify files accessible to the Fluent Bit runtime (local file system access), or to load a custom plugin where permitted. Exploitation does not require network access—only local interaction with configuration or plugin mechanisms—so cloud tenants, containerized workloads, or any party able to write to Fluent Bit's configuration space are potential vectors.

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Confirmed details and cross-checks​

Key publicly-verified facts about CVE-2025-29478 (cross-referenced across multiple trackers):

• Affected product and version: Fluent Bit v3.7.2 reported as vulnerable.
• Vulnerability type: Denial of Service through uncontrolled resource consumption and/or memory misuse; associated CWEs include CWE-400 (Uncontrolled Resource Consumption) and in some summaries CWE-416 (Use After Free) noted as related symptoms.
• Attack vector & complexity: Local access required; attack complexity is low and privileges required are low in many environments where configuration or plugin directories are writable by non-privileged users.
• Proof-of-concept: A public PoC and supporting analysis have been posted to GitHub and referenced by NVD/MITRE entries, allowing researchers and defenders to reproduce crash conditions in controlled test environments. Operators should treat PoC code with caution and run in isolated labs.

Because public trackers vary in their reporting style, administrators should treat the above as the consolidated view from multiple advisory databases and threat intelligence feeds rather than a definitive substitute for inspecting the exact source for their deployed version.

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What we could not fully verify (and why it matters)​

Several vendor-agnostic write-ups attribute the root cause to a specific type mismatch (signed size field returned as size_t) and describe practical exploit mechanics. Those analyses are consistent across independent sources, but the following remain areas where operators should verify locally:

• The precise data type declaration of the size field in the exact v3.7.2 source shipped in your environment (e.g., whether it is int, int32_t, or some other signed integer) — the exploitability hypothesis depends on the interplay between that declaration and the return type of cfl_list_size. Public advisories describe this behavior, but the absolute proof is the code in the shipped package or binary.
• Whether a patched release is available from the official maintainers for all supported branches. Some trackers list "no fixed version" or leave the remediation guidance to apply local guards; others (third‑party posts) suggest certain patch versions. Operators should consult their vendor or the official Fluent Bit release notes and maintain a local mirror of release artifacts for verification. If you maintain a distribution package (OS vendor or container image), confirm whether that packaged build includes any local backports or mitigations.

Flag: if you cannot confirm the source-level fix or a vendor-supplied patch for your deployed binaries, treat the vulnerability as present and apply compensating controls immediately.

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Practical mitigation and hardening (immediate and intermediate steps)​

Until a vendor‑confirmed patch is applied, the following mitigations will materially reduce risk and buy time for a proper fix. They are presented with operational pragmatism in mind.

• Upgrade policy and verification
• Check official Fluent Bit release notes and your distribution's package changelog for a patch that explicitly references CVE-2025-29478 or changes to cfl_list.h/cfl_list_size. If an official patch exists, prioritize testing and deployment. If no patch is available, follow the compensating controls below.
• Least privilege and filesystem hardening
• Ensure the Fluent Bit process does not run as root where possible; use a dedicated service account with minimal privileges.
• Mount configuration and plugin directories read-only in container images and set file-system ACLs so unprivileged users cannot modify config files or drop .so plugin files. Use immutable config maps for Kubernetes deployments.
• Disable or restrict dynamic plugin loading
• If your environment allows it, disable loading of external plugins or restrict the plugin search path to trusted, immutable locations. This prevents an attacker from dropping a malicious plugin that exercises the vulnerable API.
• Container security controls
• Apply container hardening (Seccomp, AppArmor, SELinux) to limit file system writes and dlopen capability for Fluent Bit containers. Use Kubernetes Pod Security Policies or OPA Gatekeeper policies to enforce read-only volumes for log configuration paths.
• Monitoring and automated recovery
• Add resource-usage monitoring and alerts for spikes in CPU and memory for Fluent Bit processes.
• Ensure systemd Restart= policies or Kubernetes liveness/readiness probes are in place to automatically recycle a hung agent. While restart is not a mitigation of the root cause, it reduces sustained downtime.
• Temporary code-level guard (for teams that can rebuild)
• If you can rebuild Fluent Bit from source under change control, consider adding an input guard in cfl_list_size to clamp negative values before returning them. Example (conceptual — must be tested in a dev environment before use):
• static inline size_t cfl_list_size(const struct cfl_list *list) {
  if (list->size < 0) { return 0; }
  return (size_t) list->size;
  }
• NOTE: This is a temporary defensive change and must be validated against the project coding standards and functional tests. Applying unreviewed fixes in production without regression testing may introduce other defects. Several advisories recommend similar guards as interim measures.
• Audit and remediation plan
• Inventory Fluent Bit instances and identify any running v3.7.2 images or packages.
• Confirm ownership and access controls on config and plugin directories.
• Apply filesystem and container-level hardening.
• If feasible and tested, apply the temporary code guard or upgrade to an officially patched build.
• Monitor closely for unusual resource patterns and have an incident playbook available to restart or replace impacted agents.

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Detection: indicators and logging​

Detecting exploitation attempts requires monitoring both system-level and application-level telemetry:

• Watch for sudden CPU or memory spikes attributable to Fluent Bit processes and correlate with restarts or OOM events on hosts. A DoS triggered by this bug typically causes resource exhaustion or infinite loops, which are noisy and observable.
• Monitor system logs for segmentation faults, aborts, or repeated Fluent Bit crashes. On many systems, such failures appear in journalctl or container runtime logs.
• Alert on unexpected changes to configuration directories or presence of untrusted .so files in plugin paths. Filesystem monitoring and host-based intrusion detection are effective here.
• If PoC exploit code is detected in your environment (GitHub PoC references exist), treat its presence as an immediate incident indicator requiring containment, forensic snapshot, and remediation.

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Long-term recommendations for operators and maintainers​

This vulnerability highlights recurring themes in C-based, high-performance agents: the tension between speed/compactness and defensive type/size checking.

• Adopt stricter code review and static analysis: include integer-signedness checks and automated static analysis (e.g., clang-tidy checks, UBSan) in CI/CD for C components.
• Harden plugin interfaces: require cryptographic signing or explicit allowlisting for third-party plugins in production; avoid implicit trust of filesystem contents.
• Improve packaging practices: distribution maintainers should backport security fixes and clearly label patched packages; operators should prefer vendor-signed packages that explicitly call out CVE remediations.
• Consider observability resilience: design log pipelines with buffering and fallback so that an agent outage does not completely blind critical observability systems. Use multiple collectors or redundant forwarding paths where possible.

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Final risk posture and recommended action for WindowsForum readers​

CVE-2025-29478 is a credible local DoS threat against Fluent Bit v3.7.2 that can produce sustained or persistent loss of availability for log-forwarding and processing pipelines. The condition is straightforward to trigger where local file or plugin write access exists, and public PoC material has been published, making rapid defensive measures essential. Multiple vulnerability trackers (NVD, Recorded Future, OpenCVE, Snyk, and industry advisories) report consistent details about the flaw and its impact; however, there is variance in published guidance about availability of an official patch for all distribution channels. Immediate priorities for system administrators and platform engineers:

• Treat any Fluent Bit 3.7.2 deployment as potentially vulnerable until you verify otherwise. Inventory your estate now.
• Lock down config and plugin paths: mount them read-only, enforce strict ACLs, and disable dynamic plugin loading if feasible.
• Harden containers with Seccomp/AppArmor/SELinux and run Fluent Bit with least privilege.
• Add resource and crash monitoring, and ensure robust liveness/restart mechanisms.
• Test and deploy an official vendor patch as soon as a trusted release that references CVE-2025-29478 is available.

If you operate multi-tenant container platforms or rely heavily on Fluent Bit for mission-critical observability, escalate this to your platform security team and treat it as a high-priority operational risk until you have either patched or implemented the compensating controls described above.

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This advisory consolidates public reporting and technical analysis from multiple independent vulnerability trackers and PoC postings to give defenders a practical, verifiable, and actionable picture of CVE-2025-29478; organizations should corroborate these findings with direct inspection of their actual binary and source artifacts and follow vendor guidance or official patched releases when available.

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Source: MSRC Security Update Guide - Microsoft Security Response Center