Go net/mail Vulnerability CVE-2025-61725: Azure Linux Attestation and Mitigation

The short answer is: No — Azure Linux is not necessarily the only Microsoft product that can include the vulnerable code, but it is the only Microsoft product Microsoft has publicly attested as including the affected Go standard‑library component so far; absence of additional attestations is not proof that other Microsoft images, kernels, or binaries do not carry the same vulnerable code.

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Background / overview​

On October 2025 the Go project and downstream vulnerability trackers published an advisory for CVE‑2025‑61725: an excessive CPU consumption (denial‑of‑service) bug in the Go standard library package net/mail, specifically in ParseAddress. The problem arises because ParseAddress (and related routines) assembled domain‑literal address parts with repeated immutable string concatenation; when fed large or adversarially crafted domain‑literal components, that algorithmic pattern can produce quadratic time behavior and drive very high CPU usage in the victim process. The upstream remediation changed the accumulation logic to use an amortized buffer (strings.Builder / equivalent) to restore linear performance. This vulnerability affects Go runtimes or binaries that include the vulnerable net/mail symbols. Upstream fixes were included in Go releases 1.24.8 and 1.25.2 (and later releases). Multiple distribution and vendor advisories tracked the issue and backported fixes into package builds and container base images. Microsoft’s public advisory language for CVE‑2025‑61725 states that Azure Linux includes this open‑source library and is therefore potentially affected, and also notes Microsoft began publishing machine‑readable CSAF/VEX attestations for product components in October 2025 and will update the CVE record if additional Microsoft products are found to ship the same component. That phrasing should be read as a product‑scoped inventory attestation (definitive for Azure Linux) rather than a vendor‑wide exclusion of all other Microsoft artifacts.

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What the bug is — technical summary​

• Affected component: Go standard library, package net/mail (ParseAddress and related symbols).
• Root cause: repeated concatenation of immutable strings when constructing domain‑literal address components; repeated concatenation in a loop produces O(n^2) time and memory copy work for n fragments.
• Real‑world impact: availability — an attacker who can provide or cause parsing of specially crafted email address text (for example, via SMTP responses, crafted headers, or other line‑oriented flows that call ParseAddress) can trigger sustained, high CPU usage in the affected process. This is a denial‑of‑service vector, not a memory‑safety or arbitrary code‑execution flaw.

Key upstream facts verified against multiple sources:

• The Go project’s vulnerability record (GO‑2025‑4006) lists the affected symbols and the fixed releases.
• Distribution trackers (Debian Security Tracker, SUSE, Amazon Linux ALAS entries) reflect the same affected ranges and fixed package versions.

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How to read Microsoft’s wording (attestation ≠ exclusivity)​

Microsoft’s sentence — that “Azure Linux includes this open‑source library and is therefore potentially affected” — is an authoritative statement about the inventory Microsoft completed and published for the Azure Linux product family. That is valuable because CSAF/VEX attestations are machine‑readable and suitable for automated triage in large estates.
But there are three important clarifications:

• An attestation is product‑scoped: it describes what Microsoft has validated for that product family at the time of publication. Treat it as a high‑confidence “yes” for Azure Linux images.
• It is not an assertion of absence for other Microsoft products. Microsoft explicitly committed to update the CVE/VEX mapping if other Microsoft artifacts are discovered to ship the same code; that implies the initial rollout of attestations is phased and may not yet cover all Microsoft SKUs. Absence of another product in MSRC/VEX output is absence of attestation, not evidence of absence.
• Practically, whether any Microsoft‑shipped artifact (image, kernel, WSL binary, Marketplace appliance) includes the vulnerable code depends on per‑artifact build choices: which Go version was used to compile included binaries, whether a given image contains a packaged Go runtime, and how CI/builds embed or statically link libraries. Those are artifact‑level properties that require per‑artifact verification.

In short: Microsoft is telling you what it has checked (Azure Linux). It is promising to expand that list if it finds the component in other Microsoft products. Until then, operators must not assume other Microsoft artifacts are automatically safe.

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Is Azure Linux the only Microsoft product that includes net/mail and is therefore potentially affected?​

• Short, pragmatic answer: Not necessarily — Azure Linux is the only Microsoft product Microsoft has publicly attested as including the vulnerable net/mail component so far, but other Microsoft artifacts could also carry the same vulnerable code until verified and/or attested.

Why the nuance matters:

• Microsoft ships multiple Linux‑based artifacts and binaries (Azure Marketplace images, Azure‑tuned kernels, CBL‑Mariner derivatives, WSL2 kernel binaries, container base images, partner appliances). Each artifact is built independently and can include different third‑party components. If a Microsoft artifact was built with an older Go toolchain or contains a binary statically linked to a vulnerable Go runtime, it could be a carrier.
• The most reliable signals for your estate are: Microsoft’s machine‑readable VEX/CSAF attestations, your own artifact inventories / SBOMs, and binary provenance checks (which Go version was used to build the binary). Do not rely on silence in a vendor advisory as proof of safety.

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Which Microsoft products are plausible carriers (operational checklist)​

Treat the following Microsoft artifacts as plausible places to find the vulnerable net/mail code until proven otherwise:

• Azure Linux (explicitly attested by Microsoft) — highest priority for triage and patching.
• Azure Marketplace images and curated VM images that include pre‑installed Go runtimes or vendor binaries compiled with older Go toolchains.
• Managed service artifacts that bundle Go binaries (for example, telemetry/agent binaries, logging agents, credential helpers) shipped by Microsoft or partners.
• Windows Subsystem for Linux (WSL/WSL2): WSL ships a Microsoft‑built Linux kernel and can also include distribution images; if a WSL distro or included binaries were compiled with an affected Go version, they may be vulnerable.
• Any Microsoft container images, AKS node base images, or Marketplace appliances that include Go programs or packaged Go runtimes built from vulnerable Go versions.

This is not an exhaustive list; the general rule is: any artifact that includes a Go runtime or contains binaries compiled with pre‑fix Go versions must be inventoried and verified.

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Detection guidance — how to tell if you’re exposed​

Because this is a performance/DoS bug rather than a memory‑safety flaw, detection centers on identifying affected binaries and anomalous CPU behavior.

• Inventory / SCA / SBOM:
• Search your SBOMs and software composition analysis results for Go versions prior to 1.24.8 and in the range 1.25.0 .. 1.25.1 (those ranges contain the vulnerable builds). Use build provenance where available to map Go version used to compile binaries.
• Codebase search:
• Grep or search repositories for imports of net/mail, net/textproto, and usages of ParseAddress / AddressParser symbols. These are the direct call sites. Example search:
• grep -R "import .*net/mail" .
• grep -R "ParseAddress(" .
• Review indirect dependencies: some libraries (SMTP clients, mail agents, credential helpers) call into net/mail via higher‑level packages.
• Runtime signals:
• Monitor process‑level CPU metrics and alert on sustained high CPU for processes that perform email parsing or handle large multi‑line responses.
• Use Go tooling (pprof, runtime/trace) to capture CPU profiles during a spike; samples showing significant time in net/mail or net/textproto functions indicate possible exploitation or accidental triggering.
• Network correlation:
• If you operate SMTP/NNTP clients or services that parse remote multi‑line responses, correlate remote endpoints sending large or unusually long domain‑literal parts with CPU spikes.
• Behavioral sandboxing:
• In a test environment, feed large domain‑literal address strings to your parsing endpoints to confirm whether the behavior reproduces and whether the patched Go runtime eliminates the CPU amplification.

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Immediate mitigation and remediation steps (prioritized)​

• Apply the upstream fixes
• The definitive remediation is to upgrade the Go toolchain and rebuild affected binaries with a patched Go release (Go 1.24.8 or 1.25.2 or later). Binaries linked/compiled with vulnerable Go versions remain at risk until rebuilt.
• Apply distribution updates
• If you rely on OS‑packaged Go (for example, Debian, SUSE, Amazon Linux, or Azure Linux packages), install your vendor’s security updates for Go and rebuild or redeploy as required. Distribution trackers and vendor advisories list fixed package versions and backports.
• For containerized deployments
• Pull updated base images that include the patched Go runtime or rebuilt binaries and rebuild your application images. Re‑deploy node pools and containers after verification.
• Short‑term hardening if you can’t rebuild immediately
• Limit the ability of a single process to saturate host CPU using cgroups or container CPU quotas.
• Add per‑request CPU/timeouts and circuit breakers around external parsing operations.
• Wrap untrusted readers with io.LimitReader or similar to reduce the maximum number of bytes a single parse can consume.
• Increase observability (profiling, metrics) to detect and throttle anomalous parsing workloads.
• Prioritize Azure Linux artifacts
• Because Microsoft attested Azure Linux as including the component, treat Azure Linux images and services in your estate as the highest‑priority Microsoft artifacts for triage and patching; apply the Microsoft guidance and patched images as they become available.

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Enterprise checklist — practical steps you can take today​

• Inventory: gather SBOMs, container manifests, and build metadata; identify any artifact that includes Go or Go‑built binaries.
• Search and tag: find imports and binaries that reference net/mail / net/textproto / ParseAddress.
• Prioritize: rank artifacts by exposure (Internet‑facing, parsing untrusted inputs, high‑throughput services).
• Patch & rebuild: update Go toolchains to fixed releases and rebuild, or apply distro/vendor packages that include the fixes.
• Deploy & verify: use profiling and smoke tests to confirm CPU behavior is normalized post‑update.
• Automate: ingest Microsoft’s CSAF/VEX attestations into your triage pipelines to automatically mark Azure Linux artifacts as in‑scope and watch for later updates to the VEX records.

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Risk assessment and attacker model​

• Attack vector: a remote attacker who can cause your application to parse large or crafted domain‑literal email address strings (for example, via SMTP server replies, crafted email headers, or other text protocols) can trigger the bug.
• Likelihood: moderate for exposed parsers that accept untrusted input. The vulnerability is algorithmic rather than memory‑corruption, so it is relatively easy to construct test inputs that cause CPU amplification.
• Impact: primarily availability — CPU exhaustion at process or host level; secondary operational impacts include latency and potential downstream outages.
• Exploitability: relatively straightforward in appropriate contexts, but a successful denial‑of‑service depends on the attacker’s ability to feed untrusted inputs into the parsing code path and any mitigating resource controls you have in place. Real‑world exploitation has not been widely reported publicly as of initial disclosure, but the EPSS and CVSS assessments reflect non‑trivial risk and warrant rapid remediation.

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Cross‑checks and independent corroboration​

To avoid single‑source errors, the key claims have been verified across multiple independent sources:

• Upstream Go advisory and vulnerability tracker (GO‑2025‑4006): details affected symbols and fixed Go releases.
• Distribution security trackers (Debian, SUSE, Amazon Linux) and vendor advisories list affected package versions and fixes, confirming mapped remediations and release timelines.
• Industry vulnerability databases and security vendors document the same technical root cause (quadratic string concatenation) and recommend upgrades to Go 1.24.8 / 1.25.2.
• Microsoft’s product attestation wording and the VEX/CSAF rollout approach are discussed in Microsoft’s public Security Update Guide text and clarified by independent analysis: Microsoft explicitly listed Azure Linux as including the library, and the company pledged to update CVE records if other Microsoft products are identified. That attestation is product‑scoped and phased.

Where statements could not be independently confirmed (for example, the internal Microsoft inventories for all product families), that limitation is noted — MSRC’s public attestation is authoritative for Azure Linux, but it does not reveal the full state of Microsoft’s internal inventories for other product families until Microsoft publishes further VEX/CSAF attestations.

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Practical examples and commands (operational)​

• Find local Go binaries and check build info (if the binary exposes goVersion):
• If your Go binaries were built with module build info compiled in, you can use:
• go version -m /path/to/binary
• strings /path/to/binary | grep "go1."
• Search for net/mail import usage in repos:
• grep -R --line-number "net/mail" ./src
• Capture a CPU profile when you see a spike:
• pprof: import net/http/pprof in your service, collect CPU profile, and inspect top frames for net/mail or net/textproto heavy CPU time.
• Container images:
• Inspect image layers and SBOMs for Go runtime packages.
• Rebuild images on patched base images (Go 1.24.8 / 1.25.2) and run integration tests.

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Conclusion — what to act on now​

• Treat Microsoft’s Azure Linux assertion as a high‑confidence signal to act immediately on any Azure Linux images you run: patch, rebuild, and redeploy as guided by Microsoft’s updates.
• Do not assume other Microsoft products are automatically unaffected. Perform artifact‑level inventory and verification across your estate (Marketplace images, AKS node images, WSL kernels, third‑party appliances) because inclusion of the vulnerable component is an artifact‑level property that depends on build‑time choices and packaging.
• The definitive technical fix is to upgrade to patched Go releases (Go 1.24.8, Go 1.25.2, or later) and rebuild affected binaries; vendor/distro patches are equally valid if you rely on packaged runtimes.
• While you work through rebuilding and patching, apply short‑term mitigations — CPU quotas, parsing limits, increased observability, and request throttles — to reduce the risk of a denial‑of‑service event.

Microsoft’s public approach (product‑scoped VEX/CSAF attestations starting with Azure Linux) improves transparency and automation for defenders, but it also creates a practical responsibility for operators to verify their own artifacts. Use Microsoft’s VEX output to automate triage for Azure Linux images and continue artifact discovery and binary provenance checks elsewhere in your environment until the vendor’s attestations cover the full product set you rely on.

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Source: MSRC Security Update Guide - Microsoft Security Response Center