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Cover image for CVE-2025-9086 | Out of bounds read for cookie path
Aakash Rahsi
Aakash Rahsi

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CVE-2025-9086 | Out of bounds read for cookie path

#cve20259086 #cve #ai #azure

I’m dropping something a little quiet today for the Azure and cloud world – but it goes straight into the bloodstream of our platforms.

CVE-2025-9086 | Out of bounds read for cookie path is not “just another curl bug”. It’s a flaw in how cookie path comparison is handled: a bug in the logic that decides whether a cookie should be sent on a given request can push curl/libcurl into an out-of-bounds read on the heap when processing certain cookie paths.

That means your “boring” HTTP client suddenly becomes a memory-safety primitive sitting inside:

  • API gateways and reverse proxies
  • AKS workloads and sidecars
  • Azure App Service and Functions runtimes that shell out to curl
  • Container images built on distro bases that just updated “everything”
  • DevOps/CI agents, security scanners, and CLIs that quietly rely on libcurl
  • Network appliances and OS components that ship curl as plumbing

No drama, no fear-mongering – this is about owning the quiet places where we’ve been assuming “HTTP just works”.

What actually goes wrong?

At a high level, CVE-2025-9086 is:

“Curl/libcurl: out of bounds read for cookie path due to a bug in path comparison logic when processing cookies.”

When certain cookie path conditions are met, curl’s internal comparison can walk beyond the allocated heap buffer. In practice:

  • The cookie jar contains a cookie whose path and scheme history put it on the edge of a boundary condition.
  • A later request re-evaluates whether that cookie should be attached.
  • The logic that compares request path vs cookie path miscalculates, and reads off the end of the buffer.

On many platforms this is “only” denial of service. In the wrong place, it becomes:

  • A repeatable crash primitive on demand
  • A potential side-channel on heap layout or adjacent memory
  • A building block inside more complex exploit chains

In other words: this is control over the error surface of your HTTP stack, not “just” a bug in a CLI tool.

Where this quietly hits in real Azure estates

If you live in Azure, the first step is to assume curl is everywhere, then prove where it isn’t.

Typical blast radius in a modern cloud estate:

  • AKS / Kubernetes

    • Sidecars that handle callbacks, token exchanges or webhooks
    • Init containers that pull configuration or secrets over HTTP(S)
    • Operators and controllers using libcurl via language bindings

  • Azure App Service & Functions

    • Images based on distro baselines (Ubuntu, Debian, SUSE, RHEL) that updated curl
    • Background jobs and bindings that call out to internal APIs with cookie-based state

  • API gateways, WAFs, and load balancers

    • Management or telemetry components that use curl/libcurl under the hood
    • Health-checks and redirect-heavy flows where cookies matter for liveness

  • DevOps and security infrastructure

    • CI runners (GitHub Actions, Azure Pipelines, self-hosted agents)
    • Vulnerability scanners and policy engines that talk REST everywhere over curl

Anywhere you have:

  • Redirect-heavy flows
  • Shared service domains
  • Non-trivial cookie scopes and paths

…you have a good candidate for “this bug could be reachable here”.

Threat model shift: from “curl bug” to “estate primitive”

A lot of patch reports will treat this as:

“libcurl: Curl out of bounds read for cookie path (CVE-2025-9086). Fixed in distro package X.”

That’s technically correct and operationally incomplete.

From a cloud security lens, what this actually does is:

  1. Turns cookie routing into a crash lever
    • Crafted cookie/path combinations can deliberately push a service into a fault state.
  2. Moves instability into key control-plane paths
    • Token brokers, sidecars, and management agents become choke points.
  3. Creates asymmetric impact in mixed-patch estates
    • Some nodes crash on a given request, some don’t – incident patterns get weird.
  4. Smuggles memory-safety risk into places we treat as “safe plumbing”
    • We rarely threat model curl itself. We threat model the APIs it calls.

This is why Azure-scale environments need to treat CVE-2025-9086 as more than a checkbox. It’s not about panic; it’s about being honest about where our trust actually lives.

What “done” actually looks like (beyond a green tick)

If you tell your CISO “we’re done with CVE-2025-9086”, they should be able to ask:

“Done where? For which flows? How do you know?”

A more complete definition of “done” for a modern Azure + multi-cloud estate:

1. Versions and packages

  • All base images and host OSes that back App Service, AKS nodes, VM scale sets, and Function runtimes have moved to curl/libcurl builds where CVE-2025-9086 is fixed.
  • Distros with layered advisories (SUSE, RHEL, Ubuntu, etc.) are aligned – no stragglers in “legacy” pools.

2. Cookie & redirect patterns

  • Redirect chains and cookie scopes for internal APIs are understood and documented.
  • You can point to which apps rely on:
    • Cross-scheme flows
    • Shared cookie domains
    • Wide / path cookies

…and have a clear story on their curl/libcurl dependency.

3. Runtime telemetry

  • You’re ingesting:
    • curl/libcurl error codes and logs where exposed
    • Application logs around cookie handling and redirect failures
    • Crash dumps or OOM signals on services that use curl for HTTP flows
  • You have at least one dashboard that can answer: > “Show me 30 days of curl/libcurl faults filtered around CVE-2025-9086 indicators.”

4. SBOM + CI/CD controls

  • Your SBOMs (Syft/Grype, Trivy, Dependency-Track, etc.) tell you exactly which artefacts ship curl/libcurl and which version.
  • Pipelines fail (or at least hard-warn) when:
    • A new image pulls in vulnerable curl
    • A legacy base image with unfixed curl is referenced

5. Cloud-native governance

  • Azure Policy / workload governance has at least one rule or query that can help find:
    • Nodes or workloads built from unpatched images
    • Custom appliances/VMs that skip regular OS patching for curl

When those five dimensions line up, “we patched curl” becomes “we understand and governed the risk”.

If you work in Azure today, here’s the checklist I’d personally run

You can treat this as a quiet morning exercise with your platform and security teams:

  1. Inventory

    • Pull a list of all images and OS SKUs underpinning your Azure workloads.
    • Map which ones ship curl/libcurl and collect their versions.

  2. Intersect with critical paths

    • Tag where these are part of:
      • Authentication/token flows
      • Control-plane APIs
      • Tenant-facing gateways and developer endpoints

  3. Align with advisories

    • Cross-check against MSRC, distro advisories, and vendor bulletins for CVE-2025-9086.

  4. Patch with intent

    • Prioritise:
      • Internet-facing APIs
      • Shared-domain services
      • Anything doing complex cookie juggling across redirects

  5. Wire signals

    • Make sure curl faults, crashes, and weird cookie behavior have a home in:
      • Azure Monitor / Log Analytics
      • Your SIEM (Sentinel, Splunk, Elastic, etc.)
      • Your runbooks

  6. Prove it

    • Run a simple before/after scenario on a non-prod cluster:
      • Exercise cookie paths and redirects designed to stress the bug.
      • Watch behaviour pre-patch vs post-patch.

Not because you expect a Hollywood exploit – but because this is how you build muscle memory around memory-safety issues in your estate.

Why I’m sharing this quietly

I’m sharing this as someone who has also lived in the assumption that HTTP plumbing “just works” – that if we’re using TLS correctly and building APIs cleanly, the underlying client libraries are a solved problem.

CVE-2025-9086 is a reminder that:

  • The smallest details in cookie path logic can become security-relevant.
  • Our control-plane trust often flows through tools and libraries we never threat-modeled.
  • “Just patch curl” is technically accurate, but strategically incomplete for a hyperscale Azure estate.

If this nudges even a few teams to look at their curl/libcurl blast radius with fresh eyes – especially in Azure, where we lean heavily on shared HTTP plumbing – it has done its job.

If you want to go deeper, the starting point is here:

Read Complete Analysis: https://www.aakashrahsi.online/post/cve-2025-9086

Humbly shared, with a lot of respect for everyone quietly keeping the pipes running.

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