CVE-2025-5916: Mitigating libarchive WARC Overflow in Azure Linux

A recently disclosed vulnerability in the libarchive library — tracked as CVE‑2025‑5916 — exposes an integer overflow in the WARC reader that can be triggered by a crafted Web ARChive (WARC) file, and Microsoft’s public advisory explicitly says Azure Linux includes the affected open‑source library and is therefore potentially affected, while also promising to update the CVE attestation if additional Microsoft products are found to ship the same component. This article explains the vulnerability in clear technical terms, verifies the public remediation status across vendors, analyzes what Microsoft’s statement actually means for customers, and provides an operational checklist to discover, confirm, and remediate exposure across Microsoft images, services, and the broader enterprise estate.

Background / Overview​

The core defect is an integer overflow in libarchive when parsing WARC records that claim an extraordinarily large content length (specifically values around INT64_MAX). If a parser accepts such a size without proper bounds checking and then uses it to compute allocations or offsets, arithmetic wraparound can occur, producing incorrect sizes and leading to unpredictable behavior — from crashes and denial‑of‑service to memory corruption in downstream consumers that rely on libarchive for archive extraction. This is the class of bug commonly categorized as CWE‑190: Integer Overflow or Wraparound. Multiple distribution and vendor trackers ingested the report and mapped fixes into downstream packages. Upstream maintainers produced a fix in the libarchive codebase and vendors (Debian, Ubuntu, SUSE, Red Hat derivatives, Amazon Linux) have published or scheduled patched packages. Those distribution advisories identify the libarchive commit(s) and the release versions that contain the repair, and many distros have pushed fixes into their stable trees or provided guidance on upgrade paths. For example, Debian and Ubuntu list fixed package revisions and point to the upstream commit integrated into libarchive v3.8.0. Microsoft’s public statement about CVE‑2025‑5916 follows its newer CSAF/VEX attestation model: Microsoft has published a machine‑readable attestation for the Azure Linux distribution indicating that Azure Linux includes the open‑source library and is therefore potentially affected, and the company has committed to update the CVE/VEX record if impact to additional Microsoft products is identified. That wording is procedural: it describes the validated scope Microsoft has completed so far (Azure Linux) rather than asserting a guarantee that no other Microsoft product ships the vulnerable component.

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What CVE‑2025‑5916 is — technical summary​

How the bug manifests​

• The WARC reader in libarchive reads header fields that specify content length.
• If a WARC record claims a content length near or above the signed 64‑bit limit (INT64_MAX − small constant), subsequent arithmetic (length + header offset, allocation size calculation) can overflow the 64‑bit signed integer used by the library.
• An overflow may produce a small or negative effective allocation size after wraparound. That value can then be used for memory allocation, buffer indexing, or copy loops.
• The downstream effect can be memory corruption, out‑of‑bounds reads/writes, or application crashes — all depending on how the embedding application uses libarchive for WARC processing. Public trackers classify likely impacts as denial‑of‑service and potential memory corruption.

Affected surface and exploit model​

• The vulnerable code is in libarchive’s WARC format support (archive_read_support_format_warc.c).
• Exploitation requires the ability to supply or upload a malicious WARC archive that the target application will parse with libarchive.
• Typical environments at risk include web crawl ingestion pipelines, archive processors, research platforms, and any tooling that accepts WARC files from untrusted sources.
• Remote exploitation is possible only where the application accepts attacker‑controlled WARC input via a network or shared storage interface — many server pipelines fall into this category. Practical exploitability depends on whether an application performs additional sanity checks before passing data to libarchive.

Current severity and vendor scoring​

• Public scores vary slightly between trackers; NVD and distribution entries show a low to medium numeric severity, reflecting the local/limited attack vector (attacker must provide a malicious archive), but the practical impact can be elevated where the library runs in multi‑tenant, exposed ingestion services.

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Microsoft’s statement: precise reading and operational meaning​

Microsoft’s public update-guide entry (the text you quoted) and its CSAF/VEX outputs say that Azure Linux includes the open‑source library and is therefore potentially affected, and that Microsoft began publishing CSAF/VEX attestations in October 2025 and will update the CVE attestation if other Microsoft products are later found to include the vulnerable component. That is a product‑scoped attestation — it documents the inventory and validation Microsoft has completed to date and publishes a machine‑readable status for that product family only. Microsoft has not, at the time of publication, claimed that no other Microsoft product uses the library; it has only attested the product(s) it has checked. Treat the attestation as authoritative for Azure Linux and as a signpost to check other Microsoft artifacts.
Key operational takeaways from the wording:

• Authoritative scope: The attestation is an authoritative statement for Azure Linux. If you run Microsoft‑published Azure Linux images, Microsoft’s VEX/CSAF file is the authoritative, machine‑readable source for whether that product is impacted.
• Not a universal exclusion: The absence of attestations for other Microsoft products does not constitute proof that those products are unaffected. Microsoft explicitly commits to updating the CVE record if it discovers other internal SKUs that include the component. In practice, absence of mapping is a time‑bounded, phased inventory state, not a guarantee.
• Why vendors publish in phases: Large vendors with many images and artifacts commonly publish attestations per product family as their inventory work completes. Microsoft started with Azure Linux as a logical first product to validate and automate attestations; expect the scope to expand as Microsoft completes additional product inventories.

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Is Azure Linux the only Microsoft product that could be affected? — Short answer and nuance​

Short answer: No — Azure Linux is not necessarily the only Microsoft product that could include libarchive and be potentially affected, but it is the only Microsoft product Microsoft has publicly attested as including the library for this CVE at the time of the advisory. That distinction is crucial.
Why that nuance matters:

• Microsoft ships many different images, containers, kernels, and prebuilt binaries across Azure and other product lines (Azure Marketplace images, curated container images for Azure Machine Learning, Azure Container for PyTorch, WSL kernels, agents, appliances).
• Inclusion of an upstream library is a per‑artifact property: one Microsoft image can ship libarchive while another does not, depending on build configuration, packaging choices, and whether prebuilt static binaries exist.
• Go‑style or statically linked artifacts, native binaries embedded in appliances, or third‑party marketplace images can all contain vulnerable code even if the system package manager does not show a vulnerable libarchive package.

In short: treat Microsoft’s attestation as an authoritative, product‑scoped data point for Azure Linux, and do not assume it automatically covers all Microsoft images or services you run.

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How to verify exposure in your environment (practical detection & discovery)​

The only reliable way to determine exposure is to inspect the actual artifact (image, VM, binary) you run. The steps below prioritize high‑risk surfaces and provide specific checks.

1. Inventory the obvious places first​

• Azure Linux images and kernels (the product Microsoft attested).
• Curated container images used in Azure Machine Learning, Azure Container Registry, and curated ACPT images.
• Marketplace VM images and partner images hosted in Azure.
• WSL2 kernels if you deploy WSL images at scale.
• Any Microsoft‑distributed agent or appliance with a Linux rootfs.
• Third‑party or partner images you run on Azure.

Why: these images are the most likely places Microsoft would find a packaged libarchive through its VEX automation.

2. On an image or VM: check the package manager for libarchive​

• Debian/Ubuntu: dpkg‑query -W -f='${Package} ${Version}\n' libarchive*
• RHEL/CentOS/SUSE: rpm -q libarchive
• Alpine: apk info libarchive

If a package is present, display the installed version and compare to vendor advisories (Debian/Ubuntu/SUSE have published fixed package versions).

3. For container images: inspect offline or run ephemeral containers​

1. docker run --rm -it <image> sh -c "dpkg -l | grep libarchive || rpm -qa | grep libarchive || apk info | grep libarchive"
2. OR: docker save <image> | tar -Ox layers/.../layer.tar | tar -tf - | grep -i libarchive

These commands let you check package contents before you deploy. Many curated images include libarchive for tar/bsdtar functionality.

4. Search for libarchive usage in binaries (static linking / bundling)​

• Check dynamic dependencies (ELF): readelf -d /usr/bin/bsdtar |& grep NEEDED ; ldd /usr/bin/bsdtar
• Search strings for embedded version text: strings /usr/bin/* | grep -i "libarchive"
• If ELFs are statically linked, they may contain the libarchive code baked in; use strings or scan with yara or binwalk to find distinctive libarchive markers.

Important: a missing libarchive package does not guarantee absence of libarchive code — many products ship statically linked binaries or custom builds.

5. Use SBOMs and VEX/CSAF where available​

• Consume Microsoft’s published CSAF/VEX attestation for Azure Linux (machine‑readable); if your estate uses Azure Linux images, ingest that attestation into vulnerability management automation.
• Request SBOMs (software bill of materials) from ISVs and marketplace image authors where possible. If an image or appliance vendor cannot provide an SBOM, require a rebuild or signed attestation proving the vulnerable component is removed or patched. Microsoft has indicated it will expand VEX coverage to additional products when its inventory identifies them.

6. Monitor for vendor updates and CVE mapping changes​

• Subscribe to vendor advisories (your chosen distro, Microsoft MSRC update guide, SUSE/Red Hat security notices).
• Microsoft has promised to update the MSRC/CVE attestation when additional product impact is found — treat that as authoritative when it arrives, but do not wait passively if you run un‑attested Microsoft images.

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Remediation & mitigation: prioritized steps​

1. Patch first where packages are present
   • Upgrade libarchive package to the fixed vendor version specified by your distribution (Debian, Ubuntu, SUSE, etc.. Distributions have published fixed package revisions and backports; follow vendor guidance.
2. Rebuild static binaries
   • For any statically linked binaries compiled against a vulnerable libarchive, rebuild with the patched upstream libarchive and redeploy.
3. Harden ingestion pipelines
   • For WARC ingestion paths that accept untrusted archives, introduce pre‑validation gates:
     • Reject or sandbox WARC records that claim extremely large sizes.
     • Run WARC parsing in isolated processes/containers with strict memory and CPU limits.
     • Use chroot/jail, seccomp, or limited privileges for archive parsing.
4. Add detection and logging
   • Instrument WARC ingest endpoints to log unusual header values and failed parses.
   • Add SIEM rules to flag archives that contain suspiciously large content‑length fields or parsing failures tied to the archive reader.
5. Require SBOMs and rebuild attestations for third‑party images
   • For marketplace or partner images you run in production, require vendor SBOMs or signed attestations asserting the absence of vulnerable libarchive versions.
6. Engage Microsoft support if you run non‑attested Microsoft images
   • If you rely on Microsoft images or services not currently mapped by Microsoft’s VEX/CSAF output, open a support ticket requesting a product‑specific attestation or SBOM. Microsoft has committed to expand attestations when additional product impact is found.

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Detection examples & quick commands​

• Identify libarchive on a Debian/Ubuntu host:
  • dpkg-query -W -f='${Package} ${Version}\n' libarchive13 libarchive-dev
• Check a container for libarchive:
  • docker run --rm --entrypoint sh <image> -c "if command -v dpkg-query >/dev/null 2>&1; then dpkg-query -l 'libarchive'; elif command -v rpm >/dev/null 2>&1; then rpm -qa | grep libarchive; fi"
• Scan binaries for static libarchive symbols:
  • strings /usr/bin/ | grep -i 'libarchive' || readelf -Ws /usr/bin/ | grep -i archive

These are practical, quick checks to find obvious package presence and common static linking patterns. They are not exhaustive for complex or heavily obfuscated images.

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Critical analysis: strengths, limitations, and residual risks​

Strengths in Microsoft’s approach​

• Machine‑readable attestation model (CSAF/VEX): publishing VEX/CSAF outputs for Azure Linux provides an automation‑friendly truth source for that product family and helps customers reduce triage time for known‑affected SKUs. The attestation is a tangible, high‑value signal for automation in vulnerability management.
• Commitment to expand mapping: Microsoft’s promise to update the CVE and VEX outputs if additional Microsoft products are found to include the component creates a single channel for authoritative updates and simplifies downstream automation once the mapping is expanded.

Limitations and residual risk​

• Phased inventory means coverage gaps: Microsoft’s phased attestation rollout inherently leaves a window where other Microsoft products might include the vulnerable code but are not yet attested. Absence of mapping is not evidence of absence. Relying solely on Microsoft’s attestation without per‑artifact checks introduces residual risk.
• Static binaries and long tail images: even after system packages are patched, statically linked binaries or prebuilt container images remain vulnerable until rebuilt. These long‑tail artifacts are the most common operational friction in supply‑chain CVE remediation.
• Third‑party marketplace images: Azure Marketplace and partner images hosted by Microsoft but produced by third parties may not be covered by Microsoft’s VEX/CSAF for Azure Linux; these images require separate supplier verification.
• Detection gaps for obscure surfaces: finding embedded copies of libarchive in custom appliances, vendor bundles, or legacy images can be nontrivial; effective remediation requires SBOMs, filesystem inspection, or vendor attestation.

Unverifiable claims to flag​

• It is not possible for an external party to exhaustively verify whether every Microsoft product includes libarchive without Microsoft providing product‑level SBOMs or attestation for those products. Statements asserting that "no other Microsoft product is affected" are unverifiable without Microsoft‑supplied attestations or SBOM data. Microsoft’s wording is explicit: it will update the CVE if additional Microsoft products are identified. Treat unverified “no” claims with caution.

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Executive checklist for WindowsForum readers and enterprise operators​

1. If you run Azure Linux images: ingest Microsoft’s Azure Linux CSAF/VEX and follow the guidance for the specific Azure Linux versions you run. That attestation is authoritative for those SKUs.
2. Inventory all container images and VM images for libarchive presence using the package manager checks and static binary scans described above. Prioritize network‑facing ingestion services and shared processing pipelines.
3. Patch libarchive packages from your distro vendor immediately where packages are listed as fixed (Debian, Ubuntu, SUSE, etc..
4. Rebuild and redeploy any statically linked or embedded binaries that include libarchive after upgrading the library.
5. Require SBOMs, rebuild attestations, or signed statements from ISVs for marketplace or partner images you rely on.
6. Apply runtime mitigations (sandboxing, memory limits, privilege reduction) for WARC ingestion paths until full remediation is deployed.
7. Log and monitor WARC parsing failures and unusual content‑length header values; add SIEM rules to detect repeated or suspicious archive submissions.

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Conclusion​

CVE‑2025‑5916 is a real integer overflow risk in libarchive’s WARC reader that has been fixed upstream and back‑ported by major distributions. Microsoft’s public attestation that Azure Linux includes the affected library is an important and usable automation signal for customers who run those images, but it is not a universal statement that no other Microsoft product could include libarchive. The correct operational posture is pragmatic: treat Microsoft’s Azure Linux attestation as authoritative for that product family, but perform per‑artifact inventory and verification across your estate (images, containers, appliances, and statically linked binaries) — require SBOMs or attestations for third‑party images, patch packages where present, and rebuild any static binaries. Microsoft has committed to update the CVE/VEX mapping if additional products are identified; until then, customers must verify their own artifacts and apply the remediation and hardening steps outlined above.

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Appendix — quick references (what to check first)

• Confirm whether your environment runs Azure Linux images (Microsoft‑attested). If yes, ingest the Azure Linux CSAF/VEX attestation and apply patches for that distribution.
• Run package checks for libarchive on any Linux images and upgrade to the vendor‑supplied patched package where applicable.
• Inspect container images used in ML and ingestion workloads (Azure Machine Learning curated images are common carriers) and validate their libarchive versions or request rebuild attestations.

This is a supply‑chain problem as much as it is a code defect: fix the package where it is shipped, rebuild any static binaries, and verify images with SBOMs and vendor attestations.

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Source: MSRC Security Update Guide - Microsoft Security Response Center