Azure Linux Attestation for CVE-2025-58187: Not a Microsoft Global Guarantee

Microsoft’s public advisory for CVE‑2025‑58187 names Azure Linux as a product that “includes this open‑source library and is therefore potentially affected,” but that statement is a product‑level attestation — not a categorical guarantee that no other Microsoft product can include the same vulnerable Go standard‑library code.

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Background / Overview​

CVE‑2025‑58187 is a vulnerability in the Go standard library’s crypto/x509 certificate‑validation code. The root cause is an algorithmic design issue in name‑constraint checking: for certain crafted certificate inputs the processing time grows non‑linearly (quadratically) with the size of the certificate or its name lists. That behavior can be exploited to cause excessive CPU use during certificate validation, producing a denial‑of‑service condition in services and agents that validate arbitrary certificate chains. The issue was reported to the Go project and tracked in Go’s issue tracker. Multiple downstream distributions and vendors have mapped the vulnerability into their advisories and patch releases; vendor trackers (Ubuntu, Amazon Linux, NVD/third‑party aggregators) show the issue and list remediation timelines. The Go project’s own issue page and the security mailing lists confirm the technical root cause and planned fixes.

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What Microsoft actually said — and how to read it​

Microsoft’s MSRC entry for CVE‑2025‑58187 includes the language many administrators have seen: “Azure Linux includes this open‑source library and is therefore potentially affected.” In the same advisory Microsoft also stated it began publishing machine‑readable CSAF/VEX attestations for third‑party components in October 2025 and that it will update the CVE if additional Microsoft products are discovered to carry the same component. This is an explicit operational policy: publish the attestations you have completed and expand them as product inventories are finished. Put plainly:

• The Azure Linux mention is an inventory attestation — Microsoft has scanned the Azure Linux images and found the implicated component.
• Microsoft’s promise to update the CVE‑level attestation is a procedural commitment to widen the published product mapping as further internal scans complete.
• The wording is not equivalent to a technical claim that other Microsoft products do not ship the same vulnerable component. Absence from the published attestation list is absence of evidence, not evidence of absence.

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Technical summary (concise)​

• Affected component: Go standard library — crypto/x509 (name‑constraint checking).
• Symptom: Quadratic runtime complexity for certain certificate inputs (e.g., unusually large or crafted SAN/name lists or constraint constructs).
• Primary impact: Availability/DoS — CPU/time exhaustion during validation of certificate chains.
• Who is at risk: any program or agent that uses the Go standard library to validate arbitrary or untrusted certificate chains (network-facing services, mTLS endpoints, agent software, CI/CD tooling, etc.. This includes artifacts running on Windows if they embed Go‑built binaries.

Multiple vendors have issued advisories and packaged fixes for their distributions and images; operators must rebuild and redeploy statically compiled Go binaries with patched toolchains rather than rely solely on a source‑only fix.

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Is Azure Linux the only Microsoft product potentially affected?​

Short answer: No — not necessarily. Azure Linux is the only Microsoft product Microsoft has publicly attested as including the vulnerable open‑source library at the time of the advisory, but that does not technically preclude other Microsoft products or artifacts from shipping the same vulnerable code. Microsoft’s wording is an attestation of scope for Azure Linux only; it also commits to update that scope if other Microsoft product inventories show inclusion of the component.
Why that distinction matters:

• Microsoft ships many Linux artifacts and images: Azure Linux images, linux‑azure kernels, Marketplace VM images, AKS node images, curated container images, and the WSL2 kernel. Each artifact is an independent build with its own configuration and may or may not include the Go runtime or the specific crypto/x509 symbols in question.
• Go binaries are often statically compiled. A single prebuilt binary embedded in an image, container, or appliance remains vulnerable until it is rebuilt with a fixed Go toolchain and redeployed — updating a system golang package does not magically patch prebuilt, statically linked binaries.
• Marketplace or partner images hosted via Microsoft may be built by third parties and can carry divergent libraries and toolchain versions; these are not automatically covered by Microsoft’s Azure Linux attestation.

In operational terms: treat Microsoft’s Azure Linux attestation as authoritative and actionable for Azure Linux customers, but perform per‑artifact verification for any other Microsoft‑provided images or services you run.

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Evidence and cross‑checks​

Key authoritative signals available at disclosure:

• Go project issue and tracker confirm the problem and its assignment as CVE‑2025‑58187; the issue is tracked in Go’s repository and referenced in OS and vendor advisories.
• OSS‑security / openwall and other security lists included the name‑constraints entry in their Go vulnerability roundups.
• Ubuntu, Amazon Linux (ALAS), and other distributors published advisories mapping affected packages and remediation guidance. These independent vendor entries corroborate the root cause, affected versions, and the need to rebuild/redeploy compiled artifacts.
• Microsoft’s MSRC advisory and the CSAF/VEX attestation for Azure Linux are the vendor’s published inventory signals; Microsoft has explicitly committed to update the product mapping if additional Microsoft products are found to include the component.

Together these sources converge on the same operational message: the vulnerability is real, the fix path is via patched Go releases and rebuilt artifacts, and Microsoft’s Azure Linux attestation is an inventory datum for one product family rather than a global guarantee.

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Which Microsoft artifacts could include the vulnerable code?​

The following Microsoft‑supplied artifacts are plausible carriers of a vulnerable Go crypto/x509 implementation — each requires artifact‑level verification:

• Azure Linux images (attested by Microsoft) — authoritative signal to treat as in‑scope.
• WSL2 kernel builds and Windows‑shipped WSL kernel images — Microsoft maintains WSL kernel code and could ship kernel or agent images that include Go binaries in userland or as sidecars.
• Marketplace VM images and partners’ prebuilt appliances hosted by Microsoft — these may be built by third parties and include their own prebuilt binaries.
• AKS node images and curated container images used in Azure services — container images can carry language runtimes and prebuilt agents built with older toolchains.
• Windows or cross‑platform agent binaries shipped by Microsoft that embed Go runtimes (for logging, telemetry, or management) — if built with an affected toolchain they will carry the vulnerable stdlib.

Operational takeaway: the only reliable way to confirm presence is to check each artifact’s build provenance, binary metadata, or SBOM/VEX attestation.

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Practical verification checklist (artifact‑level)​

• Inventory: locate all Go‑built artifacts in your environment.
• Search container registries for images that include Go runtimes.
• Scan installed binaries and packages for Go build metadata (strings like go1.24.x, go1.25.x in binary build info).
• Identify static binaries: list statically compiled Go executables deployed on servers, appliances, and images.
• Static Go binaries contain their stdlib; they must be rebuilt to remediate.
• Inspect Microsoft artifacts you run:
• If you run Azure Linux images: consume Microsoft’s CSAF/VEX attestation and follow its guidance.
• For Marketplace images, WSL kernels, and AKS node images: request SBOMs or rebuild attestations from Microsoft or the image publisher.
• For each candidate binary or image:
• Extract build info (where available) and confirm toolchain version.
• If toolchain is in the affected range (per Go advisories), schedule a rebuild with a fixed toolchain and redeploy.
• If you cannot rebuild immediately, apply short‑term mitigations (below).

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Short‑term mitigations and compensating controls​

While the correct remediation is to upgrade toolchains and rebuild artifacts, these operational mitigations reduce exposure during rollout:

• Limit or reject untrusted certificate chains at the network perimeter — where possible terminate TLS at a patched proxy that performs certificate validation outside the vulnerable Go process.
• Apply rate‑limits and CPU quotas to processes that perform certificate validation to reduce amplification of attack attempts.
• Enforce stricter CA and issuance policies in internal PKI — avoid acceptance of large or unusually crafted SAN lists from subordinate CAs.
• Monitor for anomalous CPU spikes in processes that perform certificate validation and add alerts tied to certificate parsing timeouts or long‑running validation calls.
• Request SBOMs or signed rebuild attestations from vendors for any Microsoft or third‑party images you deploy in production.

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Long‑term remediation steps (recommended)​

• Upgrade the Go toolchain to a fixed release (follow Go project and vendor advisories for exact fixed release numbers).
• Rebuild and redeploy all static Go binaries and container images compiled with affected toolchain versions. Source‑only upgrades are insufficient for deployed, compiled artifacts.
• Adopt CI/CD policies that pin and record the Go toolchain version used to produce production builds; require signed rebuild attestations for third‑party images.
• Add unit and integration tests that exercise certificate validation behavior (including name‑constraint edge cases).
• Harden logging and diagnostics to avoid building unbounded strings in hot paths (use builders, truncate long name lists, and avoid full stringify of huge structures).

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Critical analysis: strengths and residual risks​

Strengths of Microsoft’s approach

• Publishing a CSAF/VEX attestation for Azure Linux is a positive transparency step: it provides a machine‑readable inventory signal to Azure Linux customers and automates triage for that product family.
• Microsoft’s public commitment to update the CVE mapping if additional products are identified provides a single authoritative channel for follow‑up product mappings.

Limitations and residual operational risk

• Inventory timing lag: vendor attestations are phased. Microsoft starting with Azure Linux does not imply other Microsoft SKUs are unaffected — there is a time window during which additional artifacts may be discovered and attested. Absence of attestations for other SKUs is not proof of safety.
• Rebuild inertia: prebuilt static Go binaries embedded in images or appliances remain vulnerable until rebuilt and redeployed. This is the principal operational friction: many environments rely on long‑lived images and appliances that are not rebuilt frequently.
• Marketplace / partner image long tail: third‑party images hosted in Azure can have varying patch cadences and may not be covered by Microsoft’s attestation. Operators must insist on SBOMs and rebuild attestations from image publishers.
• Detection blind spots: exploitation can be silent (CPU exhaustion may look like transient load), and without telemetry specifically tied to certificate validation time, incidents may be missed.

Unverifiable claims to flag

• It is not possible from outside Microsoft to definitively confirm whether every Microsoft product or internal binary includes the vulnerable crypto/x509 code without Microsoft publishing VEX/CSAF attestations or SBOMs for those artifacts. Any statement that “no other Microsoft product is affected” is therefore unverifiable until Microsoft provides explicit attestations for those products.

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Practical questions administrators should resolve now (action list)​

• Do we run Azure Linux images? If yes, consume the published CSAF/VEX for the CVE and follow Microsoft’s instructions immediately.
• Do we run any Microsoft Marketplace images, AKS node images, or WSL2 kernels in our estate? If yes, request SBOMs or confirm build provenance with Microsoft or the image publisher.
• Do we have statically compiled Go agents, telemetry collectors, or sidecars? Inventory and schedule rebuilds using patched Go releases.
• Can we short‑circuit certificate validation to a patched proxy for high‑risk flows while rebuilding artifacts?
• Do our incident monitors include CPU anomalies during certificate validation operations? If not, add those signals.

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Conclusion​

Microsoft’s wording that “Azure Linux includes this open‑source library and is therefore potentially affected” is an important and authoritative inventory attestation for Azure Linux customers, but it should not be read as a universal guarantee that no other Microsoft product can include the vulnerable Go crypto/x509 code. The correct operational posture for defenders is to treat Azure Linux as in‑scope (absorbing Microsoft’s attestation), while independently verifying other Microsoft images, appliances, and binaries in their environment at the artifact level.
The vulnerability itself is an algorithmic complexity flaw in Go’s certificate validation: the fix path is straightforward but operationally demanding — upgrade toolchains, rebuild static binaries and images, and redeploy. Until that rebuilding is complete, teams must rely on compensating controls (proxies, rate limits, CPU quotas) and on vendor attestations and SBOMs from Microsoft and any third‑party image publishers. Stay tuned to vendor VEX/CSAF updates — Microsoft has pledged to expand its product mapping as further inventories complete — and treat absence from that mapping as not proof of safety.

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Source: MSRC Security Update Guide - Microsoft Security Response Center