CVE-2024-42516: Apache HTTPD Patch and Azure Linux Attestation Explained

A critical HTTP response splitting vulnerability in the Apache HTTP Server — tracked as CVE-2024-42516 — has been confirmed and fixed upstream, but Microsoft’s public advisory language that “Azure Linux includes this open‑source library and is therefore potentially affected” has caused understandable confusion: that attestation confirms Azure Linux contains the vulnerable httpd package, but it does not, by itself, prove that Azure Linux is the only Microsoft product that could carry the same vulnerable component.

Background / Overview​

Apache’s security team describes CVE-2024-42516 as an HTTP response splitting vulnerability in the core of Apache HTTP Server that can be triggered when an attacker is able to manipulate Content-Type response values emitted by backend applications or proxied services. The underlying problem is insufficient sanitization of header values: crafted input that includes CRLF sequences can cause the server to emit additional, attacker-controlled responses, enabling cache poisoning, cross-site scripting (XSS), request smuggling/desynchronization, and related attacks. Apache recommends upgrading to httpd 2.4.64, which contains the fix; versions 2.4.0 through 2.4.63 are affected.
The National Vulnerability Database (NVD) and multiple vulnerability-tracking vendors echo Apache’s summary and remediation guidance: upgrade to 2.4.64 (or deploy vendor-supplied patched packages) as the primary corrective action. Those independent listings provide consistent confirmation of the affected version range and the remedial release.

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Why this matters: the technical risk in plain language​

• What the attacker needs: the ability to control or influence certain response header values — typically via a backend application or a proxied upstream that accepts attacker-controllable input. If an upstream service can be induced to set a Content-Type (or similar header) that includes CRLF characters, the Apache server could treat that header as multiple headers/responses. (openwall.com)
• Likely attack outcomes: cache poisoning (poisoning shared caches or reverse proxies), page hijacking, session confusion, or delivering malicious content to downstream clients. In proxying environments these attacks can be especially damaging because a single poisoned response may affect many downstream clients or intermediary caches.
• Exploitability: the flaw’s exploitability depends on the environment. If your deployment uses Apache as a reverse proxy (mod_proxy, mod_proxy_fcgi, mod_proxy_http2) or hosts backends that might echo unsanitized header values into responses, the risk is real and present. Apache’s advisory and public discussions describe this vector and the need for careful input/response validation. (openwall.com)

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Microsoft’s advisory language: what it actually says — and what it doesn’t​

Microsoft’s Security Response Center (MSRC) entries for a number of Linux/open-source CVEs have adopted a standard FAQ paragraph: they confirm that Azure Linux images include the open source library in question and therefore are potentially affected, note Microsoft’s commitment to publish machine-readable CSAF/VEX attestations (starting October 2025), and promise to update the CVE entry if impact to additional Microsoft products is identified. That text appears repeatedly across multiple Microsoft CVE pages. That wording is an affirmative inventory statement for Azure Linux, not a denial of presence in other Microsoft products.
Put differently: MSRC’s message says, in effect, “we have verified Azure Linux contains the component and we will publish attestations; if we later identify other Microsoft products that ship the same component, we will update the advisory.” It does not assert that Azure Linux is the exclusive Microsoft carrier of the library.

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No — Azure Linux is not necessarily the only Microsoft product that could be affected​

Short answer: No. Azure Linux being explicitly listed by MSRC is a confirmed hit, but it’s not proof of exclusivity. Treat Microsoft’s attestation as product‑specific inventory for the single product named (Azure Linux), not as a global statement about every Microsoft image, service, or artifact. Operational teams should therefore assume other Microsoft-supplied artifacts are unverified until Microsoft publishes a VEX/CSAF mapping for them or until you inspect them yourself.
Why this caution is warranted:

• Microsoft’s ecosystem includes many places where Linux software can appear: Marketplace VM images, vendor-supplied VM templates, managed service runtime images, container base images published to Azure Container Registry or Marketplace, CI/CD runner images (GitHub/GitHub Actions and Azure DevOps/hosted agents — note: GitHub is Microsoft‑owned), and even third‑party appliances offered through the Marketplace. Any of those artifacts can embed an httpd package, sometimes by design and sometimes indirectly. The presence of the Apache package in a Microsoft product other than Azure Linux would be a packaging/inventory outcome, not an inevitability MSRC lists proactively for every artifact.
• The security community’s standard practice is to publish attestations when a vendor has audited specific products. Microsoft’s MSRC entry for CVE-2024-42516 follows that pattern: Azure Linux was confirmed; Microsoft promises to update the advisory if other products are identified — but they have not (at publication) published a broader product mapping for this CVE. That leaves a window of ambiguity for defenders who rely only on MSRC CVE entries to infer what to patch.

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Evidence and independent confirmations​

• Apache’s official security bulletin shows CVE‑2024‑42516 fixed in 2.4.64 and lists 2.4.0 through 2.4.63 as affected. This is the upstream authoritative source for technical details and the canonical fix.
• NVD and other vulnerability aggregators (Openwall/oss‑security, CVE databases) mirror Apache’s description and its remediation recommendation — independent confirmation of both the vulnerability’s nature and the fix release.
• Microsoft’s Security Update Guide entries include the standard FAQ that names Azure Linux as “including the open-source library and therefore potentially affected” and point to Microsoft’s CSAF/VEX publishing program that began in October 2025; MSRC also says it will update CVE product mappings if additional product impact is identified. That is Microsoft’s official position and promise of transparency — but it is product-specific, not a blanket statement of exclusivity.
• Third‑party vulnerability scanners and advisories (for example, Tenable/Nessus plugins) specifically call out Azure Linux httpd packages for this CVE and ship checks targeted at Azure Linux images, reinforcing the fact that Azure Linux images include the affected httpd versions in many environments prior to vendor updates. This underscores the operational reality: Microsoft‑provided images can and do contain well-known server packages such as httpd.
• Distribution package updates (Debian, Amazon Linux advisories, etc.) show multiple vendors issuing fixes or packaging upgraded httpd releases, which illustrates that patched httpd binaries get distributed differently across distros and vendor images — an important reminder that a single upstream fix does not instantaneously translate into patched binaries in every downstream product.

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Practical implications for defenders and IT operations​

Assume compromise risk exists anywhere a vulnerable httpd binary or package could be present. Do not rely on a single-vendor attestation to cover your entire Microsoft-supplied estate.
Recommended steps — prioritized:

• Confirm inventory and exposure for any environment you operate:
• Search your image catalog (Azure Marketplace images, VM templates, marketplace appliances) for installed httpd/apache2 packages.
• Inspect container base images (Azure Container Registry, ACR, and any Marketplace container offerings you use) for httpd layers.
• Query host OS package managers in Microsoft‑supplied images (e.g., apt/dpkg, yum/dnf, zypper, etc.) to find apache/httpd versions.
• Include hosted CI/CD runners or build images in this inventory, since hosted runner images can include common packages and are used to build/deploy services.
• Patch or rebuild:
• If you control the httpd instance, upgrade to Apache httpd 2.4.64 or a vendor-patched package (the canonical upstream fix). For distribution-packaged installs, apply the vendor-supplied update that moves to 2.4.64+ (Debian/Ubuntu, Amazon Linux, Red Hat, SUSE, etc.).
• For container images that you don’t control but use in production, rebuild the images with updated packages or replace them with upstream images that contain the patched httpd.
• Mitigate pending updates:
• If you cannot immediately upgrade a particular image or service, apply intermediate mitigation: block or strip CRLF sequences from untrusted content and headers, and prevent upstream backends from injecting arbitrary response header values. Use a WAF (mod_security or commercial appliance) to detect and block suspicious header payloads containing CRLF sequences. Also consider reverse-proxy rules to sanitize or unset dangerous headers emitted by backends. Note: these are mitigation strategies, not substitutes for the patch.
• Monitor and validate:
• Use vulnerability scanning (Nessus/Tenable/Qualys) to detect vulnerable httpd packages across VMs and container images.
• Subscribe to MSRC’s CSAF/VEX feeds and vendor advisories for explicit attestations about additional Microsoft products; when Microsoft publishes a VEX mapping for a product you consume, treat that mapping as authoritative inventory for that product.
• Add policy controls:
• Enforce image-scanning gates in CI/CD pipelines to ensure base images do not contain vulnerable system packages before production deployment.
• Add automated inventory and drift detection to find instances where an unpatched Apache binary persists in a managed Microsoft artifact.

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Why relying solely on a vendor’s single-product attestation is risky​

• Vendors typically publish attestations only after they can confirm inventory for the product in question. That means an absence of an attestation for product X does not prove absence of the component in X — it might simply reflect that the vendor has not completed the inventory or remediation work for X yet. Microsoft’s MSRC language explicitly embraces that approach: it confirms Azure Linux and promises further updates if other Microsoft products are found to be impacted. Relying on the attestation as a negative claim creates a blind spot.
• Microsoft’s cloud and developer ecosystem is large and heterogeneous: Marketplace images, third‑party appliances, and even managed services may embed Linux packages packaged by third parties. Inferring that a single confirmed product is the entire scope of impact is an operationally dangerous assumption. Practical security requires explicit verification, not inference.

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Concrete checklist you can use today​

• Inventory:
• List all Microsoft-published VM images and container images you run. Search them for httpd, apache2, or for the apache binary (/usr/sbin/httpd, /usr/sbin/apache2).
• Patch:
• For package-managed systems: run your distro’s package update mechanism to move to patched httpd packages (Debian/Ubuntu, Red Hat, Amazon Linux updates reflect the 2.4.64 upgrade).
• For containers: update Dockerfiles to use base images with patched httpd or rebuild images after apt/yum upgrade.
• Mitigate:
• Deploy mod_security or your WAF to block header values containing CR or LF characters.
• Configure reverse proxies to sanitize backend-supplied headers or to explicitly set Content-Type at the proxy layer rather than trusting upstream backends.
• Verify:
• Rerun your vulnerability scans after patching/rebuilding.
• Maintain automated detection rules in your image registry and CI pipeline to prevent reintroduction of vulnerable binaries.

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Critical analysis: strengths, gaps, and residual risk​

Strengths

• Microsoft’s transparency model — the decision to publish CSAF/VEX attestations and to include a specific inventory line for Azure Linux — is a net positive. It gives customers a clear, machine-readable signal about at least one Microsoft product that includes the impacted library, and commits Microsoft to update CVE entries if more products are found affected. This reduces ambiguity for customers relying on Azure Linux images.
• Apache’s clear upstream fix (2.4.64) and public acknowledgement of the CVE, plus wide vendor confirmation and packaging updates from distributions, provide a clear technical remediation path. Multiple independent trackers (NVD, oss‑security) confirm the fix and version range, which helps defenders prioritize action.

Gaps and risks

• Microsoft’s inventory statement can be misread as a negative assertion about other products. That semantic gap is operationally meaningful: defenders who interpret MSRC text as “only Azure Linux is affected” may fail to search other Microsoft-supplied artifacts that could contain httpd. The wording should be read as product‑specific confirmation, not a comprehensive list across Microsoft’s entire product portfolio.
• Even with upstream fixes, downstream propagation is uneven. A patched httpd release does not automatically appear in every distro or third‑party image at the same time. Distributors and appliance vendors must rebuild and re‑release their packages; until they do, vulnerable binaries may persist in Marketplace images, container layers, or appliances. That lag is the practical window during which exploitation remains feasible.
• Vendor or marketplace images may be published by third parties (ISVs) and still appear in Microsoft’s Marketplace. Those images often bundle application stacks where the packaging vendor controls the set of binaries included. The presence of third‑party marketplace artifacts complicates any claim of “only product X is affected.”

Unverifiable or caveated claims

• It is not possible to categorically assert every Microsoft product that could conceivably contain Apache httpd without full, product-by-product inventory from Microsoft. Microsoft has promised to publish CSAF/VEX attestations to reduce this opacity; until those attestations are available for more products, defenders must use targeted scanning and risk assumptions. Where Microsoft has not explicitly attested one way or the other, treat that product as unverified (meaning you must verify it yourself or wait for an official VEX).

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Bottom line and recommended operating posture​

• Treat Apache httpd CVE‑2024‑42516 as a real and actionable vulnerability if you run any Apache HTTP Server or host proxied applications that could cause upstream header injection. The upstream fix is httpd 2.4.64; apply it where possible.
• Microsoft’s MSRC entry confirming Azure Linux is useful and authoritative for that specific product, but it is not a universal statement that no other Microsoft artifact could contain the vulnerable library. Operators must therefore audit Microsoft‑supplied images, containers, and managed runtime images they consume and patch or rebuild any instances containing vulnerable httpd packages.
• Operational best practice: perform inventory → patch/rebuild → mitigate (WAF/header sanitation) → verify with scanning and continuous image gating. This approach closes the practical window of exposure while you await vendor attestations or broader upstream packaging rollouts.

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CVE-2024-42516 is a textbook example of why software supply‑chain and image hygiene matter in cloud environments: the upstream fix was straightforward, but downstream distribution patterns and vendor attestations determine how quickly your fleet becomes safe. Don’t let a single vendor’s product-level attestation be the only source of truth for your environment — confirm by inspection, patch promptly, and apply defense‑in‑depth mitigations where immediate patching isn’t possible.

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Source: MSRC Security Update Guide - Microsoft Security Response Center