CVE-2024-6874 Explained: macidn Bug in libcurl and Azure Linux Attestations

The macidn/punycode bug tracked as CVE-2024-6874 is real, but the short answer to the question is: Microsoft’s public attestation names Azure Linux as the product that includes the affected upstream component, but that attestation is an inventory statement — not proof that no other Microsoft product could contain the same vulnerable code. In plain terms, Azure Linux is the only Microsoft product Microsoft has publicly listed as potentially affected so far, but absence of a public attestation is not the same as a technical guarantee that every other Microsoft image, kernel artifact, or appliance is free of the vulnerable libcurl build. This article explains what the vulnerability actually is, why Microsoft singled out Azure Linux in its early attestations, how to determine whether your systems are affected, and what administrators and security teams should do now.

Background / Overview​

CVE-2024-6874 was disclosed by the curl project on July 24, 2024 and fixed in the next release of curl on the same day. The bug is a stack-based buffer over-read / missing NUL termination that appears in libcurl’s URL API when libcurl is built to use the Apple-native IDN backend commonly called macidn. The vulnerable code path is triggered when an IDN (internationalized domain name) punycode conversion is asked to process a name that is exactly 256 bytes long; the conversion fills a provided buffer exactly but fails to NUL-terminate it, which can cause stack contents to be exposed to the caller.
Two characteristics make this bug important to understand:

• The bug is specific to libcurl versioning and build-time choices: it was introduced in curl 8.8.0 and corrected in curl 8.9.0, and — crucially — it is only exploitable when libcurl is built to use the macidn backend.
• The vulnerability is an information disclosure (stack contents being returned in a string), not a remote code execution bug; its severity is therefore lower than many memory-corruption bugs but still meaningful where sensitive data could be leaked.

Microsoft’s security guidance about this CVE has followed a pragmatic model: publish machine-readable attestations (CSAF/VEX) for product families where inventory mapping has been completed (Azure Linux was an early, clearly scoped product), and update those attestations if additional Microsoft product SKUs are later discovered to include the implicated upstream component. The wording Microsoft uses — that Azure Linux “includes the upstream open‑source component and is therefore potentially affected” — is an inventory statement, not a blanket assurance that other Microsoft products cannot contain the same upstream code.

---

What exactly is CVE-2024-6874?​

Technical summary​

• The bug affects the libcurl URL API function that performs punycode/IDN conversions.
• It occurs when converting an input name that is exactly 256 bytes long.
• When libcurl is built with the macidn IDN backend, the conversion routine fills a caller-provided buffer without writing a terminating NUL character. That missing NUL means the returned string can include bytes from adjacent stack memory.
• The issue was introduced in curl 8.8.0 and fixed in curl 8.9.0; the curl project issued an advisory and a patch on July 24, 2024.
• The flaw is classified as a buffer over-read (CWE-126) and is an information-disclosure issue rather than code execution.

Exploitability and impact​

• Exploitability depends on the application using libcurl’s URL API (the CLI curl tool is not commonly affected by this path) and on libcurl being built with macidn.
• An attacker would need to trick the vulnerable API into converting the specific 256-byte input. Success can result in sensitive stack data appearing in the conversion result and thus being leaked to the application or logs that consume it.
• The vulnerability’s CVSS rating is in the low-to-medium range because of its narrow trigger (exact 256-byte input) and build-time dependency on a specific IDN backend.

---

Why Microsoft named Azure Linux — and what that wording actually means​

When large vendors publish machine-readable vulnerability attestations (CSAF/VEX), they normally follow a phased process:

• Inventory first the product families they can fully enumerate and trace to their binary components.
• Publish attestations for those product families, indicating which versions include the vulnerable component, which versions are known not affected, and which have been fixed.
• Continue scanning and extend attestations as more product SKUs are inspected.

Microsoft’s public attestation for this CVE lists Azure Linux as a product that included the affected curl/cmake packages in specific package versions until vendor fixes were applied. That statement is narrow and operational by design: Microsoft is telling Azure Linux customers, in machine-readable form, that this upstream component appeared in the Azure Linux images Microsoft ships and that those images needed vendor-provided updates.
Why that does not mean “only Azure Linux is affected”

• Attestation ≠ exhaustive proof. An attestation says “we have confirmed this product includes the upstream component.” It does not say Microsoft has completed a binary-by-binary proof that no other product could possibly have the component.
• Microsoft explicitly commits to updating CVE/VEX entries if impact to additional products is identified. The product attestation is part of a living inventory effort rather than a one-time absolute claim.
• Many Microsoft artifacts reuse upstream open-source components in different forms: container images, Marketplace appliances, CBL-Mariner-based images, WSL kernels, linux-azure builds, and other Linux artifacts. Whether each of those artifacts contains a vulnerable libcurl build depends on package choices and build-time flags.

Put simply: Azure Linux is the one Microsoft product Microsoft has publicly attested includes the vulnerable upstream component; other Microsoft artifacts may or may not include the same library depending on how they were built and packaged.

---

Independent verification: what multiple sources say​

The curl project published the advisory and patch on the disclosure date, and multiple independent vulnerability databases and vendors cataloged the CVE and its properties. Cross-referencing the curl advisory with vendor DBs and Linux distribution notices yields the following consistent facts:

• The problem is a build-time, backend-dependent bug in libcurl’s punycode conversion. It is only reachable when the macidn backend is used.
• The bug was fixed in curl 8.9.0; upgrades or rebuilds of libcurl are the recommended remedies.
• Multiple downstream vendors and distribution maintainers treated the issue as part of their package-update work; at least one vendor attested that their images were not affected because their libcurl builds did not use the macidn backend.
• Microsoft published machine-readable VEX/CSAF attestations that specifically add Azure Linux to the product tree for the CVE and list the package versions that were fixed.

These cross-checks show that the technical characterization and remediation approach are well understood across both upstream and downstream communities.

---

How to determine whether your Microsoft-hosted or Microsoft-supplied systems are affected​

Because the vulnerability depends on a combination of libcurl version and the IDN backend used at build time, the only reliable way to determine exposure is to inspect the actual packages or binaries on the systems you run. The following steps give a practical, prioritized approach for administrators and security teams.

Quick triage checklist (high-level)​

• Identify Linux images and appliances you run that originate from Microsoft or run on Microsoft cloud services (Azure Marketplace images, Azure Linux VMs, Microsoft-supplied container images, WSL distributions provided by Microsoft).
• Check installed curl/libcurl package versions. If libcurl is older than 8.9.0 and exactly version 8.8.0 is present, further inspection is required.
• Determine whether libcurl was built with an IDN backend that is vulnerable (macidn); if the build uses libidn2, ICU, or another non-macidn backend, the canonical CVE code path is not reachable.
• Apply vendor-supplied package updates (or rebuild libcurl) where required.

Commands and methods to inspect a system​

On a Linux system:

• Get the installed curl/libcurl package version (distribution-specific):
• Debian/Ubuntu: dpkg -l curl OR apt-cache policy curl
• RHEL/CentOS/Fedora: rpm -q curl
• Azure Linux: use the distro’s package manager and inventory tools to list curl/curl-libs package versions
• Use the curl binary to get its reported features and linked libraries:
• curl --version (or curl -V)
• This prints the curl version and lists supported features and sometimes linked libraries. It can show whether libidn2 or other IDN libraries were linked.
• Check libcurl details programmatically:
• curl-config --version and curl-config --features can show what features were compiled in.
• A short C program that calls curl_version_info(CURLVERSION_NOW) can be compiled against libcurl to learn the feature flags at runtime.
• Inspect the binary for references to macidn (low precision but pragmatic):
• strings $(ldd $(which curl) | grep libcurl | awk '{print $3}') | grep -i macidn
• This attempts to locate textual hints that macidn was used. It is not guaranteed but can flag obvious builds.
• Check distribution advisories and the vendor’s CSAF/VEX entries:
• Microsoft’s machine-readable VEX for the CVE includes product entries and remediation dates for Azure Linux; consult your vendor-supplied vulnerability attestation and package release notes.

What to look for and how to interpret findings​

• If package version < 8.8.0: the bug did not exist in libcurl before 8.8.0 — you are not affected by CVE-2024-6874 because of version alone.
• If package version == 8.8.0: this is the potentially vulnerable release; you must verify the IDN backend.
• If package version >= 8.9.0: the upstream fix is present; you are not vulnerable to CVE-2024-6874 (unless a downstream backport reintroduced the same logic — consult your vendor advisory).
• If libcurl was built with macidn: the code path that triggers the bug exists and the package must be updated or rebuilt with a safe backend.
• If libcurl is built with libidn2, ICU, or another non-macidn backend: the specific macidn-triggered bug is not reachable.

---

Mitigation and remediation options​

If you determine you may be affected, take the following actions in order of preference:

• Upgrade libcurl / curl to a fixed vendor package that incorporates curl 8.9.0 or later (or the vendor backport that corrects the flaw).
• If a vendor package is not available, rebuild libcurl from a safe source:
• Apply the official curl patch or upgrade the source to 8.9.0+ and rebuild your distribution package.
• Alternatively, rebuild libcurl using an IDN backend that is not macidn (for Linux builds, libidn2 or ICU are the usual choices).
• Where rebuilding or upgrading is impractical, mitigate by limiting or monitoring usage of the particular libcurl API that performs punycode conversions, and ensure inputs that might trigger the exact 256-byte conversion are sanitized or rejected.
• Monitor vendor-supplied CSAF/VEX attestations and Microsoft’s security guidance for updates to the product inventory.

Additional best practices

• Use your organization’s software inventory and SBOM tooling to identify which images and appliances include libcurl packages and what versions are present.
• If you run Microsoft Marketplace images or vendor-supplied appliances, track the vendor advisories; vendors often publish updated images with the patched packages.
• Where possible, automate package updates for cloud images and container builds to reduce the window of exposure.

---

Practical examples: what to run on a Linux host​

(These are example commands administrators can adapt. They are intended to be executed on a Linux shell with privileges appropriate to query package and file metadata.)

• Check curl binary and features:
• curl --version
• curl-config --version
• curl-config --features
• Check package information:
• Debian/Ubuntu:
• apt list --installed | grep curl
• dpkg -s curl
• RHEL/Fedora:
• rpm -q curl
• dnf info curl
• Inspect linked libcurl and strings:
• ldd $(which curl) | grep libcurl
• strings /usr/lib64/libcurl.so | grep -i 'idn|macidn'
• Where you manage many systems, script these checks in a central inventory sweep and correlate results with your patch-management system.

---

Risk analysis and practical implications​

Strengths in the supply-chain response​

• The curl project published a fast, clear advisory and released a corrected version quickly.
• Downstream vendors and distributions have tracked the CVE and produced patches or remediation steps.
• Microsoft’s CSAF/VEX attestations provide machine-readable mapping for products the vendor has completed inventory for, making automation-based triage easier for customers who run Azure Linux.

Risks and blind spots to watch​

• Attestations are incremental: a vendor’s public list of products that include a component is only as good as the vendor’s completed inventory. Silence in a vendor’s published attestation is not a reliable proof-of-absence.
• Build-time differences matter: two images with the same package name and version could be built with different flags and therefore have different exposures.
• The bug’s trigger conditions are narrow (exact 256-byte input) and the affected backend (macidn) is platform-specific, which reduces exploitation probability — but the potential exposure of stack memory can be valuable to attackers in some scenarios (credential leakage, secrets in memory, etc.).
• Automated scanners that only check package versions (without confirming build-time backends) can produce false positives or false negatives; careful inspection is required.

Real-world impact scenarios​

• A web application or service embedded with an application-level use of libcurl’s URL API might call the IDN conversion routine on user-supplied names; if the binary uses macidn and the application accepts attacker-controlled domain names, stack data could be leaked.
• For many Linux server deployments, the typical libcurl builds are not macidn-based and therefore are not vulnerable; however, any custom-built or vendor-supplied binary built with macidn (for example, an image built on a macOS toolchain or cross-compiled with a macidn shim) needs direct verification.

---

What administrators should tell stakeholders​

• Transparency: tell stakeholders that Microsoft has attested Azure Linux includes the upstream component and issued VEX/CSAF details that list fixed package versions and remediation dates.
• Actionable instruction: if you run Azure Linux images, apply the vendor-supplied security updates that contain the patched curl and cmake packages. If you host Microsoft-supplied Marketplace images, verify vendor patch status and apply updates.
• Investigation: for other Microsoft artifacts, do not assume “not listed means safe.” Treat the vendor attestation as a starting point and carry out the binary-level inspection steps outlined above for each image or appliance in your environment.
• Monitoring: enable logging and monitoring for applications that parse or convert IDNs — unusual long input or precisely-sized inputs should be considered suspicious during triage.

---

Why build-time choices and SBOMs matter​

This CVE is a textbook example of why modern software supply-chain hygiene must include:

• Reproducible builds and clear build-time flags recorded in an SBOM (software bill of materials).
• Machine-readable vulnerability attestations (CSAF/VEX) that map fixed product SKUs and package versions.
• Automated scans that go beyond package version checks and also collect metadata about how packages were compiled (IDN backend, feature flags, etc).

Microsoft’s approach — publishing CSAF/VEX attestations for products as inventory work completes — is a positive step that aligns with those principles. But the practical limitation remains: customers must pair vendor attestations with their own inventory and binary-level verification to reach a high-confidence posture.

---

Closing recommendations (what to do now)​

• If you run Azure Linux images: apply the patch levels your vendor advises without delay. Treat Microsoft’s attestation as a direct signal for remediation.
• Inventory your environment: locate all occurrences of libcurl/curl packages across VMs, containers, Marketplace images, and appliances from Microsoft and third parties.
• For each occurrence where libcurl version equals 8.8.0 (or where vendor advisory indicates potential exposure), determine the IDN backend used at build time; if it is macidn or unknown, plan an immediate update or rebuild.
• Where possible, automate detection: script the curl --version and package-version checks and correlate with your configuration management and patch orchestration systems.
• If patches are not available for a specific image or appliance, work with the vendor to obtain updated images. As an interim protection, harden inputs to the libcurl URL API and block or monitor suspicious IDN inputs.
• Track vendor CSAF/VEX feeds and your vendor security advisories: Microsoft and other vendors will update attestations if new product families are found to ship the vulnerable component.

---

CVE-2024-6874 is not a global emergency, but it demonstrates how a small, build-time configuration detail can turn a modest library bug into a vendor-specific incident. Microsoft’s Azure Linux attestation is useful and actionable for Azure customers, but it must be read as a partial inventory disclosure — not a complete proof that every Microsoft product is unaffected. Organizations should combine vendor attestations with their own binary-level checks, SBOM-backed inventories, and rapid patching processes to reduce exposure from this and similar supply-chain vulnerabilities.

---

Source: MSRC Security Update Guide - Microsoft Security Response Center