CVE-2023-39319: Go html/template XSS Risk and Azure Linux Attestation

CVE‑2023‑39319 is a real, exploitable weakness in Go’s html/template package that can allow a carefully crafted input to defeat the package’s escaping rules inside <script> contexts and open the door to reflected or stored cross‑site scripting (XSS); Microsoft’s public advisory identifies Azure Linux as a Microsoft‑distributed product that “includes this open‑source library and is therefore potentially affected,” but that phrasing is a product‑scoped attestation — not a categorical guarantee that no other Microsoft product could contain the same vulnerable component.

Background / Overview​

The vulnerability

• The technical issue: the Go standard library’s html/template did not apply correct parsing/escaping rules for occurrences of tokens such as "<script", "<!--", and "</script" when those sequences appeared inside JavaScript string literals in a <script> block. That parsing weakness could prematurely terminate the in‑template understanding of the script context and lead to template actions being not escaped as the author intended — enabling XSS.
• CVE and outreach: CVE‑2023‑39319 was publicly disclosed in September 2023 and tracked by NVD/OSV/pkg.go.dev and multiple distro vendors; the Go project fixed the problem in the 1.20 and 1.21 release lines (patches shipped in Go 1.20.8 and Go 1.21.1). The Go team’s release announcement explicitly calls out html/template fixes in Go 1.20.8 / 1.21.1.
• Severity and impact: public vulnerability trackers list this as a medium‑severity XSS flaw (NVD/Ubuntu/others give CVSS around 6.1), and multiple Linux distributions and vendors published advisories and packages that remedied the issue. The vulnerability matters when server‑side templates render attacker‑controlled content into script blocks without additional sanitization or CSP protections.

Why the fix matters in two different operational modes

• When you run an OS distribution package of the Go runtime (or include the html/template source in a distro), fixing the distro’s golang package removes the vulnerable library from the runtime image.
• When you build Go binaries with an older Go toolchain, those compiled artifacts embed the library semantics at build time; rebuilding with the fixed Go release is the reliable remediation path. That distinction is crucial for triage in mixed environments (OS packages vs self‑built Go binaries).

---

What Microsoft actually said — and what that statement means​

Microsoft’s short FAQ language that many customers quote — effectively: “Azure Linux includes this open‑source library and is therefore potentially affected by this vulnerability” — reflects a narrow inventory result: Microsoft has inspected and attested the Azure Linux distribution (the Azure Linux images Microsoft builds and publishes) and found the implicated upstream component present in that product family. Microsoft also committed to publishing machine‑readable CSAF/VEX attestations and to update CVE entries if additional Microsoft products are later identified as carriers. (microsoft.com)
Why this matters operationally

• A machine‑readable VEX/CSAF attestation for Azure Linux gives Azure Linux customers an authoritative, immediate signal: update your Azure Linux images and packages per Microsoft’s guidance. That attestation is actionable and valuable, because it reduces uncertainty for the product it names.
• The wording avoids saying “only Azure Linux,” which is intentional. Microsoft will expand its VEX attestations as it inventories more product families; until then, the absence of an attestation for other Microsoft products is simply a lack of published inventory, not proof of absence. Community analysis and Microsoft‑oriented reporting echo this interpretation: the Azure Linux note is a product‑scoped attestation, not an exclusivity guarantee.

Short factual answer to the user’s question

• Is Azure Linux the only Microsoft product that includes the vulnerable html/template library? No — Azure Linux is the only Microsoft product Microsoft has publicly attested so far to include that upstream c That attestation does not prove other Microsoft products are free of the component; other Microsoft artifacts that ship Go runtimes, Go‑built binaries, or container images could plausibly carry the vulnerable code until Microsoft’s inventory checks them and updates the VEX mapping.

---

How other Microsoft artifacts can — and often do — carry Go standard‑library issues​

Understanding the different ways the Go standard library ends up inside products helps explain why a single “Azure Linux” attestation isn’t a global answer.
Three common carriage models

• Distribution package model: the distro ships a golang package (source or binary) as part of the OS image. Example: Azure Linux images include golang packages; that’s the direct path MSRC is attesting. Customers patch these by installing updated OS packages.
• Build‑time embedding: many Microsoft projects and third‑party components are built with Go toolchains and produce static or dynamically linked Go binaries. If those binaries were compiled with a vulnerable Go release, the application’s runtime behavior around template parsing may reflect the vulnerable semantics. Examples of Microsoft Go projects include the Azure SDK for Go and various Azure tooling libraries; Microsoft maintains Go libraries and SDKs that are consumed by services and CLI tools, so rebuilds against fixed Go are often necessary.
• Container images and cloud artifacts: Marketplace images, containers in Azure Container Registry, and third‑party appliances can embed vulnerable Go runtimes or compiled Go binaries. Those images may be derived from Azure Linux or from another base; each image must be inspected. Scanners and SBOMs are essential here.

Evidence Microsoft uses Go in many places

• Microsoft maintains official Go packages and SDKs (for example, Azure SDKs and identity/auth libraries in Go). Those libraries naturally import html/template in some web‑facing examples or tooling that renders HTML; even if a library does not directly use html/template, services or components built by teams using Go toolchains can be affected. The Debian package index and other package lists show Microsoft’s Go modules and SDKs are widely distributed.

Practical implication: any Microsoft distributed artifact that either (a) ships the golang standard‑library package in an installable image, or (b) ships Go‑built binaries compiled with a vulnerable Go toolchain, is a candidate for triage. The only way to be certain is an artifact‑level inventory check.

---

Cross‑referenced technical verification (what we checked)​

I verified the vulnerability scope, affected versions, and the upstream fixes against multiple independent sources to avoid single‑source errors:

• Go project and release notes: the Go maintainers list html/template fixes in the Go 1.20.8 / 1.21.1 releases. Rebuilds with these toolchain versions remove the vulnerable behavior.
• Package and vulnerability databases: pkg.go.dev/GO‑2023‑2041, NVD and OSV show the same vulnerability description and the affected version ranges (before go1.20.8 and go1.21.0‑0 before go1.21.1). These sources also track aliases and distro advisories.
• Linux vendor advisories and scanner records: multiple distro vendors and vulnerability scanners documented the flaw and shipped updated golang packages; Amazon Linux, Ubuntu, SUSE and others list CVE‑2023‑39319 in their advisories. These vendor pages confirm real‑world patch activity across distributions.
• Microsoft’s public attestation practice and VEX rollout: MSRC has publicly documented the CSAF/VEX program rollout beginning with Azure Linux; the MSRC phrasing for productented and repeatedly used across CVE entries, with the same “Azure Linux includes this open‑source library and is therefore potentially affected” language and the promise to update mappings if more Microsoft products are identified.

Where I could not definitively assert presence

• I could not find a Microsoft‑published, CVE‑specific VEX mapping that nameuct families for CVE‑2023‑39319 besides Azure Linux. That lack of published mapping is not evidence of absence; it simply means Microsoft has not publicly attested other product families yet. Treat that as a transparency gap that will be closed as Microsoft expands its inventory checks.

---

Practical remediation and triage checklist (what security teams should do now)​

If you run Microsoft artifacts or Azure environments, treat this as a supply‑chain / build‑toolchain problem as well as an OS‑package problem.
Immediate steps (prioritized)

• Patch Azure Linux images: if you run Azure Linux images supplied by Microsoft, apply the updated distro packages Microsoft (or the distro provider) has issued. Microsoft’s attestation directly covers these images; updating their packages is the first, highest‑leverage step.
• Inventory & scan for Go-built binaries:
• Find all containers, VMs, and build artefacts that include Go binaries.
• For each binary, determine the Go toolchain version used to build it. Many Go binaries embed version metadata; use tooling (e.g., 'go version -m' or similar) or SBOMs produced by your CI to detect builds compiled with Go < 1.20.8 / 1.21.1.
• Rebuild and redeploy any binary built with a vulnerable toolchain using a fixed Go release.
• Scan images and SBOMs:
• Use SBOMs, image scans, and VEX/CSAF data to locate copies of the html/template package or older Go runtime packages within images.
• If you manage Marketplace images or vendor appliances, seek vendor attestations and patches or isolate those images until patched.
• Apply runtime mitigations where rebuilds are not immediately possible:
• Add or harden Content Security Policy (CSP) directives to reduce the impact of potential XSS vectors.
• Harden server‑side template usage: move sensitive/attacker‑controlled data out of JavaScript string literals in templates; perform strict input validation or double‑escape where appropriate.
• Consider edge WAF rules that detect anomalous script injections while you patch artifacts.
• Follow Microsoft VEX/CSAF updates:
• Subscribe to Microsoft’s machine‑readable attestations and CVE update guide; when MSRC expands mappings beyond Azure Linux, treat those attestations as authoritative. Microsoft has committed to updating CVE records when more products are identified.
• Communicate with vendors and internal teams:
• If you run Microsoft managed services, ask the service owner whether their service uses Go html/template or runs compiled Go artifacts built before the fix. If you run Microsoft‑provided Marketplace appliances, request SBOMs or attestations.

Concrete rebuild guidance

• Upgrade your build pipelines to use Go 1.20.8, 1.21.1, or later.
• Rebuild all service images and containers, create fresh SBOMs, and redeploy.
• Validate that templating behavior is fixed by running unit tests and minimal integration checks that exercise template rendering of strings containing script‑like tokens.

---

Risk assessment: likelihood, impact, and exploitation scenarios​

Likelihood

• The vulnerability is a parsing/escaping logic error — not a remote arbitrary code execution bug — and exploitation requires rendering attacker‑controlled content into an html/template <script> context in an environment where template authors expect automatic escaping to be sufficient.
• That makes it less trivially exploitable than a straightforward server‑side command injection, but still a meaningful risk for web applications that embed untrusted content inside JavaScript literals and rely solely on html/template’s escaping.

Impact

• Successful exploitation leads to XSS: an attacker can execute script in the victim’s browser within the context of the site, potentially stealing cookies or performing actions on behalf of the victim.
• For internal services that are not exposed to the public internet, the risk is lower, but any multi‑tenant or internet‑facing service that uses the vulnerable template behavior should be considered high priority.

Typical exploitation pathway

• Attacker crafts a payload containing script‑like sequences that manipulate the parser’s state inside a JavaScript literal.
• The application renders that payload through html/template inside a <script> block.
• Because the parser misinterprets special tokens, the action or value that would normally be escaped is output raw or insufficiently escaped, leading to attacker script execution in the browser.

Where exploitation is least likely

• Applications that keep user data out of <script> blocks or that use non‑vulnerable templating libraries for script rendering are less exposed.
• Services that implement strong CSP with 'script‑src' and disallow 'unsafe‑eval'/'unsafe‑inline' significantly reduce exploit success rates.

---

Stronger governance: how to reduce future ambiguity about Microsoft product scope​

The core problem customers face when reading Microsoft’s one‑line attestation is not bad faith — it’s inventory incompleteness. Microsoft’s VEX/CSAF rollout intentionally started with Azure Linux; expanding that program to include other product families (agent tooling, SDKs, Marketplace images, WSL kernels, etc.) will materially reduce uncertainty.
Actions Microsoft and customers should take

• Microsoft: continue rapid rollout of machine‑readable VEX attestations across product families and ensure build‑time provenance (toolchain versions) is available in SBOMs and artifact metadata.
• Customers: mandate SBOMs for any third‑party artifact you run, require build toolchain information in CI, and prioritize rebuilds of Go artifacts after security‑critical standard‑library fixes.

Community vigilance

• The security community has already pushed vendors to provide per‑artifact attestations (and the WindowsForum threads and community writeups repeatedly emphasize that Azure Linux attestation is scoped, not exclusive). Customers should treat these community analyses as operational guidance while they wait for expanded vendor attestations.

---

Conclusion — a clear, actionable answer​

• Azure Linux is the only Microsoft product Microsoft has publicly attested to include the vulnerable html/template component for CVE‑2023‑39319 to date; that attestation is authoritative for Azure Linux and gives Azure Linux customers an immediate remediation target (update distro golang packages and images).
• However, that attestation is not a statement of exclusivity. Any Microsoft artifact that ships the Go toolchain or Go‑built binaries compiled with a vulnerable Go release (pre‑1.20.8 / pre‑1.21.1) or that contains the html/template package as a component could be affected until inventory checks show otherwise. Operators should therefore treat other Microsoft images, services, and binaries as unverified carriers until they confirm otherwise via SBOMs, VEX attestations, or direct inspection.

Practical takeaway: patch Azure Linux images now, inventory your estate for Go‑built artifacts, rebuild any Go binaries with the fixed Go releases (Go 1.20.8 / 1.21.1 or later), add or harden CSP and template usage patterns, and watch Microsoft’s VEX/CSAF updates for additional product mappings. These operational steps will close the immediate exposure window while reducing future supply‑chain ambiguity.

---

Source: MSRC Security Update Guide - Microsoft Security Response Center