CVE-2024-4603 OpenSSL DoS: Azure Linux Attestation and Microsoft Artifacts

The recent CVE-2024-4603 disclosure — an OpenSSL weakness that allows excessive CPU time when validating specially crafted DSA keys or parameters — is important for any team that consumes OpenSSL libraries or that performs explicit key/parameter checks. Microsoft’s public guidance correctly calls out Azure Linux as a product that “includes this open‑source library and is therefore potentially affected,” but that product-level attestation is an inventory snapshot, not a blanket guarantee that other Microsoft products do or do not contain the same vulnerable code. Security teams must therefore treat Azure Linux as a confirmed carrier while also verifying other Microsoft artifacts in their environment rather than assuming they are safe.

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Background / Overview​

CVE-2024-4603 was published in May 2024 and affects OpenSSL’s DSA parameter and key checking routines. The functions implicated — principally EVP_PKEY_param_check() and EVP_PKEY_public_check() — perform mathematical validation steps that, when fed DSA parameters with an excessively large modulus (p), can take an outsized amount of time to complete. If those checks are performed on untrusted inputs supplied by a remote user, an attacker can cause prolonged CPU consumption and a practical denial-of-service (DoS) condition. The vulnerability is classified as low/medium severity in vendor advisories because exploitation requires the victim application to call those specific check functions on untrusted data.
OpenSSL and distribution advisories list the affected library versions and the upstream commits that fixed the issue; affected branches included earlier releases of the OpenSSL 3.x line (patched in subsequent micro releases). Major distribution advisories (Debian, SUSE, Oracle, etc.) and security scanners flagged the issue and issued vendor-specific package updates. Administrators should therefore treat the issue as a library-level defect that is remediated by upgrading OpenSSL packages to patched releases.

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What exactly goes wrong (technical summary)​

• The DSA parameter checks perform iterative or expensive mathematical tests that scale poorly if the modulus (p) is unusually large.
• OpenSSL’s general runtime enforces a separate upper bound (public-key usage is limited to ~10,000 bits for verification), but the internal check functions did not apply the same size guard when validating parameters, which allowed pathological inputs to force long-running computation.
• The attack model is: an attacker supplies a DSA parameter set or public key (for example, uploaded or submitted to an API) and this untrusted data is passed into the check routines. The calling thread or process then spends a long time performing the checks.
• Normal TLS client/server paths are not typically impacted because OpenSSL doesn’t call these parameter-check functions on untrusted DSA parameters during ordinary TLS handshakes; the real risk is for applications or utilities that intentionally validate keys, or for command-line tools that expose a -check option (for example, openssl pkeyparam -check).

This implies two practical realities for defenders:

• If your application never calls these specific check functions on untrusted input, your exposure is effectively zero for this CVE.
• If your application does validate arbitrary public keys or parameters supplied by users, you should treat the function calls as potentially exploitable until you update OpenSSL or add defensive controls.

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Microsoft’s public statement and the CSAF/VEX rollout​

Microsoft’s Security Response Center (MSRC) has adopted a machine-readable disclosure model (CSAF and VEX) and began publishing VEX attestations for Microsoft products starting with Azure Linux in October 2025. In the context of CVE‑2024‑4603, MSRC’s published guidance stated that Azure Linux includes the open‑source library (OpenSSL) and is therefore potentially affected, and that MSRC will update the CVE/VEX entries if additional Microsoft products are later identified as carriers of the same upstream component. That disclosure model and the wording are deliberate: the VEX/CSAF output is a per‑product inventory attestation and not an exhaustive statement about every Microsoft artifact.
Multiple independent writeups and community analyses have emphasized the same operational point: Microsoft’s attestation is product‑scoped. Azure Linux being listed as “potentially aoft catalogued the presence of the upstream library in that product family and prioritized it for patching and VEX publication — it does not prove that other Microsoft SKUs (WSL2 kernel images bundled with Windows, linux‑azure builds used for certain VM families, curated Marketplace images, or standalone Microsoft applications that ship their own libssl/libcrypto) are free of the same vulnerable code. Treat the Azure Linux VEX as authoritative for Azure Linux and treat other Microsoft artifacts as “not yet attested” until the vendor explicitly says otherwise or until you verify locally.

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Is Azure Linux the only Microsoft product that includes OpenSSL (and therefore potentially affected)?​

Short answer: No — but Azure Linux is the only Microsoft product Microsoft has publicly attested (via its VEX/CSAF outputs) as including the implicated OpenSSL component for this CVE at the time of the advisory. That attestation is an authoritative indicator for Azure Linux but should not be interpreted as an exclusivity guarantee for all Microsoft artifacts.
Why this matters technically and operationally:

• Microsoft produces many artifacts that can embed OpenSSL or other third‑party libraries: container/VM images, the linux-azure kernel packages used for some VM SKUs, Azure Marketplace images, AKS node images, vendor appliances, and even some Microsoft client applications that ship native binaries with libcrypto/libssl. A VEX/CSAF attestation for one product is focused on that product’s build artifacts — different build configuratices determine whether a specific upstream source file or library version is present.
• There is also precedent: community reports and Microsoft Q&A discussion show that Microsoft applications have at times bundled OpenSSL DLLs (for example, OneDrive shipping libssl/libcrypto binaries) and subsequently rolled updates to newer OpenSSL releases. That demonstrates Microsoft does ship OpenSSL in other contexts beyond Azure Linux, so a blanket exclusion would be unsafe to assume.

In short: Azure Linux has been inspected and confirmed to include the library; other Microsoft-distributed artifacts may or may not include it depending on build choices and packaging. Until Microsoft publishes an attestations map that covers those artifacts, they remain “unverified” rather than explicitly “not affected.”

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Evidence and cross-checks (what we verified)​

• OpenSSL vendor advisory and vulnerability listings describe CVE‑2024‑4603 and the involved functions and fixes. Those advisories list the affected OpenSSL releases and the upstream commits that implemented the fix. This is the primary technical source for the vulnerability description and the recommended mitigation (upgrade OpenSSL).
• Multiple distribution security trackers (Debian, SUSE, Oracle) and vulnerability aggregators (NVD, Tenable, Chainguard) catalogued the CVE, published distribution-specific status, and pointed to fixed package versions — confirming both the presence of the issue in OpenSSL 3.x lines and the appropriate remedial action (package update). These are independent confirmations from multiple vendor ecosystems.
• Microsoft’s MSRC program publicly announced the phased CSAF/VEX rollout starting with Azure Linux in October 2025 and used that model to assert Azure Linux contains certain upstream libraries and therefore is potentially affected; MSRC also declared it will update CVE/VEX records if additional Microsoft products are found to ship the same upstream code. That corporate policy decision explains why Azure Linux appears first in Microsoft’s VEX catalogue.
• Community analysis and forum posts — including independent WindowsForum writeups — consistently interpret Microsoft’s attestation language as product‑scoped and recommend artifact‑level verification for other Microsoft components. Those community signals align with best practice: do not assume absence of an attestation equals absence of risk.

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Practical guidance — what defenders should do now​

If you maintain Azure Linux systems, Microsoft has already called those images out; prioritize patching and validation there. If you operate Microsoft artifacts beyond Azure Linux, follow these steps immediately:

• Inventory
• Enumerate all systems and images where OpenSSL may be present (server images, containers, Marketplace images, WSL2 kernel usage, AKS node images, appliances, and native Windows apps that bundle libssl/libcrypto).
• Use package management tooling and binary scanning to detect OpenSSL versions and the presence of libssl/libcrypto.
• Identify usage patterns
• Determine whether your applications call EVP_PKEY_param_check(), EVP_PKEY_public_check(), or other OpenSSL check routines on untrusted inputs.
• Also identify any scripts or operational procedures that invoke openssl pkey/pkeyparam -check or similar utilities on externally provided key material.
• Patch / Upgrade
• Upgrade OpenSSL to a vendor-provided fixed release. For the OpenSSL 3.x line, patched micro releases were published after the advisory. Use your vendor distro’s patched packages where available.
• If a vendor patch is not available for a packaged product, follow vendor guidance (workarounds, mitigations or backported patches) until an official fix is supplied. Distribution advisories (SUSE, Debian, Oracle, Microsoft’s Azure Linux updates) show release-level fixes and dates.
• Add runtime defenses
• For services that must validate arbitrary keys, implement input size checks before invoking expensive library checks. Reject or sandbox keys with unreasonably large modulus sizes (for practical purposes, restrict modulus sizes to reasonable cryptographic limits for your environment).
• Consider performing validation in an isolated process with a CPU or wall-time budget; if a check exceeds that budget, kill and log the offending input rather than allowing a single worker to be consumed indefinitely.
• Detection and monitoring
• Add telemetry for long-running OpenSSL-related calls, and monitor process CPU spikes and prolonged openssl invocations.
• Use EDR/XDR rules to flag processes invoking OpenSSL parameter checks from externally sourced inputs.
• Artifact verification for Microsoft products
• Don’t assume that because Azure Linux is attested, other Microsoft products are unaffected. Where you depend on Microsoft-delivered images or packages, perform artifact-level checks (scan installed binaries, inspect embedded libraries) or reach out to Microsoft support channels for clarification; Microsoft’s VEX/CSAF outputs will expand over time.

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Special considerations for Windows environments and Microsoft-distributed software​

• Native Windows applications can and have bundled OpenSSL binaries (for example, third-party components used by OneDrive and other Microsoft applications have been reported to include libssl/libcrypto). Evidence in Microsoft support threads shows Microsoft shipping updates that replaced or upgraded bundled OpenSSL libraries in Windows client apps. This proves Microsoft artifacts beyond Azure Linux can include OpenSSL — reinforcing that a single Azure Linux attestation does not capture the full Microsoft supply chain. Verify shipped DLLs and package manifests in Windows install directories when scanners flag OpenSSL-related CVEs.
• For WSL2 users: the WSL2 kernel and its supplied Linux userspace are distinct build artifacts and may or may not include upstream OpenSSL at the version(s) implicated. If you use WSL2 images that you or your org deploy, scan them for OpenSSL version and apply vendor guidance where necessary.

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Why a product-level attestation matters — and why it’s limited​

Microsoft’s VEX/CSAF initiative is a big step forward for supply‑chain transparency because it allows vendors to publish machine-readable, per-product attestations that security automation can ingest. When MSRC lists Azure Linux as a product that “includes the open-source library and is therefore potentially affected,” that gives security teams a high-confidence, actionable signal: prioritize Azure Linux patching now.
However, the limitation is operational: scanning and inventorying every build artifact, kernel configuration, package variant and trim of a global vendor’s ecosystem is a multi-stage effort. Microsoft’s phased rollout, starting with Azure Linux, reflects a pragmatic approach: start with the controlled, curated distribution and expand coverage. In practice, that means:

• Attestations are authoritative for the named product families.
• Absence of an attestation is not a proof of safety for other products; it is merely an absence of published inventory for those products at the time of the advisory.
• Vendors should (and Microsoft has committed to) update their VEX/CSAF artifacts as more inventory work is completed; customers must treat the absence of data as “unknown” rather than assume “not affected.”

Community analysis and multiple WindowsForum writeups underline this distinction and recommend artifact-level verification for systems that rely on Microsoft-delivered components. Those community posts are consistent with vendor guidance and practical security engineering practice.

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Risks, caveats, and unverifiable areas​

• Vendor attestations are only as current as the inventory work behind them. Where Microsoft (or any vendor) states it will update a CVE if additional products are affected, that promise is meaningful, but until the update appears the state for other artifacts is unverified.
• It is difficult for third parties to prove a negative (that a vendor does not ship a vulnerable library in every possible artifact). Publicly available attestations are the best practical signal we have; local scanning completes the picture for a specific environment.
• If a Microsoft product’s support documentation or package manifest is not publicly visible (for example, proprietary appliance images or partner-managed Marketplace images), external verification may be limited; in those cases, escalate to vendor support for definitive attestations or request CSAF/VEX outputs for the specific product. If Microsoft has not yet attested a product, you should treat that artifact as “not yet attested” and perform your own checks.

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Takeaway: how to prioritize your response​

• Treat Azure Linux as a confirmed priority: patch those systems first. Microsoft has explicitly attested Azure Linux and distribution patches are available via normal package channels.
• Simultaneously, inventory and scan all other Microsoft artifacts and third‑party images you run. Look for bundled libssl/libcrypto, check OpenSSL versions, and identify any code paths that intentionally validate external DSA parameters or keys.
• Harden applications that accept external keys: add size limits, sandbox checks, and timeouts for expensive cryptographic validation operations.
• Consume vendor-supplied CSAF/VEX outputs as they appear; incorporate them into automation that maps CVEs to the set of in‑scope products and images in your environment. But do not rely solely on vendor attestations to replace your own artifact scanning — they’re complementary, not substitutive.

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Conclusion​

CVE‑2024‑4603 is an OpenSSL implementation weakness that enables denial-of-service-style impacts when DSA parameter or key checks are performed on untrusted, artificially large inputs. Microsoft’s public attestation that Azure Linux includes the library and is potentially affected is an important, actionable signal that Azure Linux customers must prioritize. However, that attestation is product‑scoped and does not prove that every other Microsoft product is unaffected — nor does the converse hold. Security teams must therefore combine vendor VEX/CSAF feeds with their own binary and package inventories to establish a complete picture of exposure and to prioritize patching, hardening, and detection accordingly. The practical route to safety is straightforward: patch OpenSSL where it is present, add pre-checks and timeouts for any code that validates externally supplied keys, and maintain artifact-level verification for all Microsoft-based images and applications in your estate.

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Source: MSRC Security Update Guide - Microsoft Security Response Center