CVE-2026-20244: 
Clam AntiVirus vulnerability analysis and mitigation

CVE-2026-20244 is a memory corruption vulnerability in the DMG (Apple Disk Image) file format parser of ClamAV, classified as a Denial of Service flaw with potential for expanded impacts. It was disclosed on July 1, 2026, as part of a broader Cisco security advisory covering seven ClamAV vulnerabilities. Affected ClamAV versions include 0.98.1 through 1.4.4 and 1.5.0 through 1.5.2; Cisco Secure Endpoint Connector for Windows (before 8.6.2), macOS (before 1.27.2), and Linux (before 1.29.0) are also affected. The vulnerability carries a CVSS v3.1 base score of 7.5 (High) (Cisco Advisory, GitHub Advisory).

The root cause is improper boundary checking during DMG file content parsing, which can trigger an integer overflow (CWE-190) on 32-bit platforms, leading to a classic buffer overflow (CWE-120) and memory corruption. The vulnerability is network-exploitable with no authentication or user interaction required — an attacker simply submits a specially crafted DMG file to a ClamAV scanning service. The integer overflow condition is specific to 32-bit platform deployments; 64-bit systems are not susceptible to the integer overflow but may still be affected by the underlying boundary check flaw. The vulnerability was reported by Stanley John Tobias and credited in the Cisco advisory (Cisco Advisory, Red Hat Bugzilla).

Successful exploitation causes the ClamAV scanning process to terminate, resulting in a Denial of Service condition that interrupts all scanning operations on the affected system. On Cisco Secure Endpoint Connector for Windows, the scanning engine runs in a privileged security context, meaning termination can render the endpoint unresponsive and require manual intervention such as a system reboot to recover. On Linux and macOS platforms, the scanning process runs in a lower-privileged context, so the impact is limited to delayed or prevented scanning operations without affecting overall system stability. Cisco notes that on Windows-based platforms, similar memory corruption vulnerabilities have historically been leveraged for remote code execution, though no evidence of RCE potential has been confirmed for this specific vulnerability (Cisco Advisory).

As of the time of disclosure, Cisco PSIRT is not aware of any public announcements or malicious use of this vulnerability in the wild, and no public proof-of-concept exploit code is known to exist (Cisco Advisory). The EPSS score is approximately 0.389%, indicating a low near-term probability of exploitation (GitHub Advisory). The vulnerability is automatable (no user interaction required), which lowers the barrier for mass exploitation if a weaponized payload were developed. It is not currently listed in the CISA Known Exploited Vulnerabilities (KEV) catalog. Legacy 32-bit Windows deployments face the highest risk due to the integer overflow condition being platform-specific.

1. Reconnaissance: Identify systems running vulnerable ClamAV versions (0.98.1–1.4.4 or 1.5.0–1.5.2) or Cisco Secure Endpoint Connector deployments, particularly those on 32-bit Windows platforms, using network scanning or service fingerprinting.
2. Craft malicious DMG file: Construct a specially crafted Apple Disk Image (DMG) file with malformed content designed to trigger improper boundary checks in ClamAV's DMG parser, causing an integer overflow on 32-bit platforms.
3. Submit file for scanning: Deliver the crafted DMG file to the target system through any vector that causes ClamAV to scan it — such as email attachment, web upload, file share, or direct submission to a scanning API endpoint.
4. Trigger memory corruption: When ClamAV processes the malformed DMG content, the integer overflow causes memory corruption, leading to the scanning process crashing.
5. Achieve DoS: The ClamAV scanning process terminates, disrupting all scanning operations; on Windows endpoints running in a privileged context, the system may become unresponsive and require a reboot (Cisco Advisory).

• Logs: Unexpected ClamAV process crash logs or core dump files; log entries showing the scanning daemon terminating abnormally while processing a DMG file; system event logs on Windows showing the Cisco Secure Endpoint service stopping unexpectedly.
• File System: Presence of crafted or anomalous .dmg files in directories monitored by ClamAV (e.g., mail queues, upload directories, temp folders); core dump files generated by the ClamAV process (clamd or clamscan).
• Process: Repeated unexpected termination of the clamd or clamscan process; on Windows, the Cisco Secure Endpoint Connector service (sfc.exe) stopping and requiring manual restart.
• Network: Unusual inbound file transfers containing DMG content to systems not typically receiving macOS disk images, particularly from external or untrusted sources.

Cisco has released fixed versions addressing this vulnerability: ClamAV 1.4.5 and 1.5.3, Cisco Secure Endpoint Connector for Windows 8.6.2, for macOS 1.27.2, and for Linux 1.29.0. Cisco Secure Endpoint Private Cloud version 4.2.8 and later distributes the fixed connector versions. There are no workarounds available — upgrading to a fixed release is the only remediation. As a temporary risk reduction measure, organizations can implement network-level controls to restrict untrusted sources from submitting files to ClamAV scanning services, and should prioritize patching 32-bit Windows deployments given their elevated risk (Cisco Advisory, ClamAV Blog).

The vulnerability was part of a broader July 2026 ClamAV security release that patched seven legacy vulnerabilities, some dating back nearly two decades, which drew notable coverage from security media outlets including Help Net Security, VPN Central, and Linuxiac (Help Net Security, VPN Central, Linuxiac). The Cisco advisory credited multiple researchers including those working with AI-assisted vulnerability discovery tools (Trail of Bits with Anthropic, Tencent Xuanwu Lab's automated engine), reflecting growing interest in AI-aided security research. The Hacker News included the ClamAV patches in its weekly security recap, indicating moderate community awareness. Linux distribution maintainers (SUSE, Ubuntu, openSUSE) moved quickly to issue updated packages following the disclosure.