CVE-2026-20213: ClamAV PE Parser DoS Vulnerability

CVE-2026-20213 Overview

CVE-2026-20213 is an out-of-bounds write vulnerability in the Portable Executable (PE) file format parser of ClamAV. An unauthenticated, remote attacker can trigger memory corruption by submitting a crafted PE file for scanning. Successful exploitation terminates the ClamAV scanning process, producing a denial-of-service (DoS) condition. Cisco's advisory notes that expanded impacts beyond DoS are possible due to the nature of the memory corruption. The flaw is tracked under [CWE-120] (Buffer Copy without Checking Size of Input) and affects deployments that rely on ClamAV for automated file scanning, including mail gateways and file transfer inspection pipelines.

Critical Impact
A single crafted PE file submitted to an affected ClamAV instance can crash the scanning engine and disrupt malware inspection across dependent services.

Affected Products

ClamAV (PE file format parser component)
Cisco products embedding vulnerable ClamAV versions — refer to the Cisco Security Advisory for the authoritative product list
Third-party mail and web security appliances bundling ClamAV

Discovery Timeline

2026-07-01 - CVE-2026-20213 published to NVD
2026-07-01 - Last updated in NVD database

Technical Details for CVE-2026-20213

Vulnerability Analysis

The vulnerability resides in the routine responsible for parsing PE file structures during antivirus scanning. ClamAV performs improper boundary checks on content within PE headers and sections. When a crafted PE file is processed, the parser writes past the bounds of an allocated buffer. The resulting memory corruption terminates the clamd scanning process. Because ClamAV runs as an inline scanning component in many environments, a crash disrupts email delivery, file uploads, and web content inspection. Cisco's advisory indicates that additional impacts beyond process termination may be possible depending on memory layout and platform.

Root Cause

The root cause is missing or insufficient size validation on attacker-controlled fields within the PE file structure. The parser trusts length or offset values embedded in the file and copies data into a fixed-size buffer without verifying that the destination can accommodate the source content. This pattern maps directly to [CWE-120], a classic buffer overflow condition.

Attack Vector

Exploitation requires no authentication and no user interaction. An attacker delivers a malicious PE file to any channel monitored by ClamAV. Common vectors include email attachments processed by a mail gateway, files uploaded to web applications scanned via clamd, and objects placed in shared storage inspected by on-access scanning. The scanning engine parses the file automatically, triggering the out-of-bounds write.

No verified public exploit code is available. See the Cisco Security Advisory for authoritative technical details.

Detection Methods for CVE-2026-20213

Indicators of Compromise

Unexpected termination or repeated restarts of the clamd or clamscan process
Segmentation faults or abort signals recorded in system logs referencing ClamAV binaries
Scan queue backlog or timeout errors in mail transfer agents and file gateways integrated with ClamAV

Detection Strategies

Monitor process exit codes and crash dumps for ClamAV binaries, correlating with the file being scanned at the time of failure
Alert on abnormal rates of scan failures returned by clamd to upstream services such as Amavis, Rspamd, or MTA milter integrations
Inspect quarantined and inbound PE files for malformed headers using an independent parser to identify probing activity

Monitoring Recommendations

Enable and centralize clamd logging with verbose output to capture parser errors preceding a crash
Ship system journal events and core dump metadata to a SIEM for correlation with mail and web traffic logs
Track ClamAV version and engine build across the fleet to confirm patched deployments

How to Mitigate CVE-2026-20213

Immediate Actions Required

Inventory all systems running ClamAV, including embedded use inside third-party appliances and container images
Apply the fixed ClamAV release referenced in the Cisco Security Advisory as soon as it is available for your platform
Restart dependent services after patching to ensure the updated engine is loaded

Patch Information

Cisco Talos maintains ClamAV and publishes fixed versions through the ClamAV project release channel. Consult the Cisco Security Advisory for the specific fixed version numbers, distribution package updates, and any bundled-product hotfixes.

Workarounds

Disable PE file scanning in clamd.conf by setting ScanPE no if operationally acceptable, understanding this reduces malware coverage for Windows executables
Restrict which upstream services can submit files to clamd by binding the socket to localhost and applying strict file-size and rate limits
Deploy pre-filtering to reject or sandbox unknown PE files at the network perimeter until patching is complete

bash

# Configuration example: disable PE parsing as a temporary workaround
# Edit /etc/clamav/clamd.conf
ScanPE no

# Restart the ClamAV daemon to apply
sudo systemctl restart clamav-daemon

# Verify the running configuration
clamconf | grep -i ScanPE