ClamAV CVE-2024-20505 DoS Risk: Patch PDF Parser Now

ClamAV users and defenders should treat the latest PDF-parsing weakness, tracked as CVE-2024-20505, as a production risk: a crafted PDF can trigger an out‑of‑bounds read in the ClamAV PDF parser that reliably crashes the scanner process and produces a denial‑of‑service (DoS) condition unless fixed.

Background / Overview​

ClamAV is a widely deployed open‑source antivirus engine used in mail gateways, file‑scanning pipelines, containerized security stacks, and as a component bundled into third‑party products. On September 4, 2024, the ClamAV project published security patch releases—1.4.1, 1.3.2, 1.0.7 and 0.103.12—that include a fix for a PDF parser out‑of‑bounds read, identified as CVE‑2024‑20505, and credited to OSS‑Fuzz. The project explicitly warns that the flaw could cause scanned processes to terminate when a crafted PDF is present.
Independent vulnerability trackers and national databases corroborate the bug: the U.S. National Vulnerability Database and multiple vendor advisories describe the issue as an out‑of‑bounds read in the PDF parsing module that enables a remote, unauthenticated actor to induce a DoS by submitting a malicious PDF for scanning. The NVD record and several third‑party vulnerability feeds list affected version ranges that match the ClamAV advisory and note remediation via the patched releases.

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What exactly is CVE‑2024‑20505?​

The technical root cause​

At a technical level this is a classic out‑of‑bounds read (CWE‑125) inside ClamAV’s PDF handling code: when the parser processes certain PDF structures crafted to violate expected internal bounds, the code reads memory outside the intended buffer. That out‑of‑bounds read can cause the scanner process to crash or otherwise enter an unstable state—resulting in a denial of service for the scanning function. The public advisories do not disclose a line‑by‑line exploit, but the vulnerability class and method of triggering (a crafted PDF file submitted to the scanner) are clear.

Attack vector and impact​

• Attack vector: Network / file submission — any place where untrusted PDFs can be delivered to ClamAV for scanning (email attachments, upload endpoints, automated archival or ingestion processes, containerized scanning pipelines).
• Privileges required: None; the vulnerability can be triggered by an unauthenticated sender providing a malicious PDF to a scanner instance.
• Likely impact: Denial of service for the scanning service. In integrated environments this can degrade mail handling, file ingestion, malware detection coverage, and automated processing that depends on ClamAV. In some downstream products that do not gracefully handle scan failures, the result can be message delays, dropped files, or reduced detection coverage until the scanner is recovered or patched.

Severity scoring — what to believe​

Different databases sometimes display slightly different severity numbers as they apply different scoring practices or perform subsequent reassessments. At the time of writing:

• The NVD’s public entry describes the vulnerability as an out‑of‑bounds read that enables DoS (CWE‑125).
• Several vulnerability aggregators report a CVSS v3.1 base score of 7.5 (High) with the vector string AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H — reflecting a network‑accessible DoS with no confidentiality or integrity impact but a high availability impact. Other services list a lower or medium score due to alternative interpretations of impact; when in doubt, treat a DoS against a detection engine as operationally important even if confidentiality is not affected.

Because security posture depends on operational context, defenders should factor in how central ClamAV is to availability or defense-in-depth in their specific deployment rather than relying solely on a single numeric score.

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Affected versions and vendor fixes​

Upstream ClamAV releases​

ClamAV issued patch releases on September 4, 2024 that explicitly fix the PDF parser out‑of‑bounds condition. The fixed versions listed by the project and mirrored in downstream advisories are:

• ClamAV 1.4.1
• ClamAV 1.3.2
• ClamAV 1.0.7
• ClamAV 0.103.12

Upgrading to one of these patched releases (or a later version) is the canonical remediation. The ClamAV team patched all actively supported release lines to cover diverse environments.

Distribution and vendor advisories​

Multiple Linux distributors and vendors released updates aligning with the upstream fixes:

• Ubuntu published security notices referencing CVE‑2024‑20505 and providing patched clamav packages for supported releases; Ubuntu’s USN entry tracks the upstream fix and lists the patched packages for Ubuntu channels.
• SUSE / openSUSE published advisories that include the ClamAV 1.4.1 fix in their packaged updates.
• Commercial appliance vendors and security distributors that embed ClamAV have also issued updates or configuration guidance; vendor‑specific advisories vary, but many reference the upstream ClamAV blog post.

If your ClamAV instance is provided by a third‑party product (mail gateway, web filter, SIEM, container image), you must follow the vendor’s update path rather than attempting to upgrade the engine in isolation.

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Practical mitigation and remediation guidance​

Patching is the recommended path, but real environments often require interim controls. Below are prioritized actions operators should take now.

Immediate (0–48 hours)​

• Inventory and identify affected scanners. Use your configuration management database, package manager, or container images to find instances running ClamAV. Verify versions using the engine tools (for example, clamscan or clamd output) and package metadata. Treat any pre‑1.4.1 or pre‑0.103.12 releases as potentially vulnerable until confirmed patched.
• Apply vendor/distribution patches immediately. Use your distro vendor’s security update channels (apt/yum/zypper, container image rebuilds, vendor appliance updates) to move to the patched ClamAV packages that correlate to your support channel. For packaged installs, prefer the distribution’s security update packages rather than manually compiling unless that is part of your standard release process.
• If you cannot patch immediately, implement temporary controls:
• Isolate scanning of untrusted PDFs — route untrusted attachments or uploads to a temporary quarantine or manual triage queue instead of automatic scanning pipelines.
• Rate‑limit or file‑type filter ingress for PDF processing where operationally feasible.
• Restart supervision — ensure systemd or container orchestration will detect and restart failed clamd/clamscan processes to prevent extended downtime after a crash. While restarts don’t fix the root cause, they reduce recovery time following exploitation.
• Monitor logs and detection metrics. Look for unusual increases in clamd crashes, restarted processes, OOM messages, or service outages. Correlate with mail gateway events and upload logs to detect attempted exploit deliveries.

Short term (up to 2 weeks)​

• Apply the official patched releases for all affected systems: upstream ClamAV 1.4.1 / 1.3.2 / 1.0.7 / 0.103.12 or vendor‑provided packages. Validate the update by confirming the running engine reports the new version and by testing scan coverage on benign PDF samples.
• Harden file handling around the scanning pipeline: run the scanner under constrained accounts (least privilege), use container or chroot isolation, and enable OS‑level resource limits (cgroups, ulimits) that prevent a single process crash from compromising host availability.
• Document and communicate the change to downstream teams (mail admins, web teams, DevOps) so they can expect the patch window and post‑patch validation steps.

Long term (ongoing)​

• Adopt multi‑engine scanning for critical paths. For high‑value ingestion (mail gateways for executives, sensitive file stores), consider defense‑in‑depth by running a second, independent scanning engine or detonation sandbox so that a single scanner’s DoS cannot blind detection.
• Track OSS‑Fuzz and other fuzzing findings. This bug was found by OSS‑Fuzz; incorporate fuzzing results and upstream project security notifications into vulnerability management for proactive hardening.

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Operational risk analysis: who should worry most?​

This vulnerability is not a remote code execution (RCE) vector in the information publicly disclosed for CVE‑2024‑20505, but a reliable DoS against a detection engine has outsized operational consequences.

• Mail gateways and MTA clusters that rely on ClamAV for in‑line scanning are high‑risk: an attacker who can deliver crafted PDFs via email can reduce detection coverage, create backlogs, or trigger service disruptions. Administrators of these systems should prioritize patching.
• Multi‑tenant file services and web upload pipelines are also at risk if untrusted PDFs are scanned automatically and the scanner runs with broad privileges or without robust supervision.
• Appliance vendors and integrated security stacks that embed ClamAV must issue coordinated updates; customers should follow vendor guidance and avoid ad‑hoc engine replacements that could void support.

For environments where ClamAV runs as one detection component among many, the immediate confidentiality risk is low (the advisories indicate no direct data exposure from this specific out‑of‑bounds read), but the availability impact—a scanner crash and resultant blind spot—makes it an important operational vulnerability.

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Why this matter beyond a crash: secondary risks​

A crash in an antivirus scanner might appear purely operational, but there are secondary, cascading risks that justify urgency:

• Detection gaps: If ClamAV is the primary or sole signature engine for a pipeline, a persistent crash gives attackers a window to deliver malware or phishing materials that go undetected.
• Automation failures: Many orchestration systems assume scanning is always available; unexpected scanner failures can break downstream flows (automated quarantines, inbound mail pipelines) and create manual work that delays incident response.
• Supply‑chain exposure: Vendors and appliance vendors who package ClamAV must coordinate updates. Delays in vendor rollouts mean enterprise customers may rely on vulnerable versions for longer periods.

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What to validate after patching​

After applying the vendor patches or upgrading upstream, operators should verify:

• The running ClamAV engine reports the patched version (for example, clamd or clamscan version string matches the patched release).
• Service stability under load — reintroduce representative PDF samples (non‑malicious test cases) and measure that scanners no longer crash on malformed but non‑malicious PDFs during fuzz or edge‑case tests.
• That orchestration and monitoring detect and alert on both new crashes and unusual restarts.
• That mail and upload processing latency returns to normal and that no backlogs persist.
• That vendor appliance signatures and integration tests pass when vendor images containing the patched ClamAV are deployed.

If you see lingering crashes after installing the upstream fix, open a support case with the distribution or appliance vendor rather than assuming the patch failed—packaging differences or ABI mismatches in integrated products can produce unexpected results.

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Detection and hunting guidance​

While public exploit code for CVE‑2024‑20505 has not been widely published at the time of the upstream advisory, defenders can proactively hunt for indicators:

• Look for spikes in clamd or clamscan process crashes, segmentation faults, or unexpected termination messages in systemd/unit logs.
• Search mail gateway logs and upload pipelines for repeated PDF submissions from the same source around the time of scanner crashes.
• Watch for automated restarts or process churn that correlates with PDF handling.
• Where feasible, sandbox suspicious PDFs and analyze parsing behavior in an isolated testbed to determine whether a sample triggers the parser failure.

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Developer and maintainer notes (for teams building on ClamAV)​

If you package ClamAV or depend on it inside appliances or product images:

• Do not delay applying upstream security releases to your product image or OS packages. The ClamAV project issued coordinated patch releases across supported branches specifically to assist downstream packagers.
• Rebuild and test images across your supported platforms and publish vendor advisories with explicit package versions and remediation steps.
• Consider packaging additional runtime hardening: run clamd with reduced privileges, chroot or containerize the scanning process, and apply resource limits to contain failure impact.

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Final assessment and recommendations​

CVE‑2024‑20505 is a pragmatic reminder that memory‑safety issues discovered by fuzzing can have immediate operational effects on security infrastructure. While this specific issue is an out‑of‑bounds read that results in DoS rather than RCE in the public reporting, the operational risk is real: mailbox backlogs, blind spots in file scanning, and vendor coordination headaches.

• Immediate action: Patch affected ClamAV instances to the fixed releases (upstream 1.4.1 / 1.3.2 / 1.0.7 / 0.103.12) or apply vendor‑provided updates as your distribution/vendor recommends.
• Short‑term mitigations: Isolate scanning of untrusted PDFs, enable robust process supervision, and apply rate limits where feasible.
• Operational hardening: Use least privilege, containment (containers/chroots), and defense‑in‑depth scanning strategies where availability of scanning is critical.
• Monitoring: Add crash and restart metrics for clamd to your observability dashboards and hunt for correlated upload or mail events around crash windows.

When dealing with scanner vulnerabilities, remember: a crashed scanner is not a minor inconvenience — it is an operational hole in your defenses. Treat ClamAV CI/CD updates and vendor advisories as high priority for any environment that depends on automated scanning for inbound content.

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Conclusion: patch first, validate second, and harden your scanning pipeline to limit the blast radius of future parser bugs.

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Source: MSRC Security Update Guide - Microsoft Security Response Center