CVE-2026-20346: ClamAV PDF Parser DoS Vulnerability

CVE-2026-20346 Overview

CVE-2026-20346 is an out-of-bounds read vulnerability in the PDF file format parser of ClamAV. An unauthenticated, remote attacker can trigger the flaw by submitting a crafted PDF file for scanning. Successful exploitation causes the ClamAV scanning process to terminate, resulting in a denial-of-service (DoS) condition. Cisco notes the flaw may also enable other expanded impacts due to memory corruption on affected devices. The vulnerability is tracked as CWE-125 (Out-of-Bounds Read).

Critical Impact
Remote attackers can crash ClamAV scanning services with a single crafted PDF, disrupting mail gateways, file scanning pipelines, and endpoint anti-malware workflows that depend on ClamAV.

Affected Products

ClamAV (PDF file format parser component)
Cisco products that embed ClamAV for content scanning
Third-party mail and file security gateways using ClamAV as a scan engine

Discovery Timeline

2026-08-07 - CVE-2026-20346 published to NVD
2026-08-11 - Last updated in NVD database

Technical Details for CVE-2026-20346

Vulnerability Analysis

The vulnerability resides in the routine that parses PDF file structures during a ClamAV scan. ClamAV performs deep content inspection on PDF documents to detect embedded malicious streams, JavaScript, and objects. During this parsing, the scanner reads structured data from the PDF without validating that offsets and lengths remain within allocated buffer boundaries.

When the parser encounters a malformed or maliciously crafted PDF, it reads memory beyond the intended buffer. This out-of-bounds read leads to memory corruption or an immediate crash of the scanning process. Because ClamAV is typically invoked as a long-running daemon (clamd) or embedded in scanning pipelines, terminating the process interrupts anti-malware coverage across the environment.

The flaw is network-reachable through any channel that feeds PDF content to a ClamAV instance, including SMTP gateways, web proxies, file upload handlers, and endpoint on-access scans.

Root Cause

The root cause is improper boundary checking on content inside PDF files. The parser does not sufficiently validate that requested read offsets and lengths fall within the mapped buffer before accessing memory. This corresponds to CWE-125: Out-of-Bounds Read.

Attack Vector

Exploitation requires no authentication and no user interaction. An attacker submits a crafted PDF to any workflow that eventually invokes ClamAV. Common delivery paths include email attachments to a mail gateway using clamav-milter, file uploads scanned by clamd, or archives extracted and scanned recursively. Refer to the Cisco Security Advisory for vendor-confirmed technical details.

Detection Methods for CVE-2026-20346

Indicators of Compromise

Unexpected termination or repeated restarts of the clamd or clamscan process, particularly following PDF scan operations.
Scan logs referencing PDF objects immediately before the process exits or emits a signal such as SIGSEGV.
Mail gateway or upload queues showing PDF messages held or bypassed because the scanner failed mid-scan.

Detection Strategies

Monitor process telemetry for abnormal exits of ClamAV binaries and correlate with the input file being scanned at the time of failure.
Inspect PDF submissions that consistently precede scanner crashes and retain samples for offline analysis in an isolated environment.
Enable verbose logging in clamd.conf with LogVerbose yes to capture the last file processed before termination.

Monitoring Recommendations

Alert on repeated restarts of ClamAV services within short intervals, which suggest crash-loop exploitation attempts.
Track scan bypass counters at mail and web gateways; a spike often indicates a scan engine that is failing rather than clean traffic.
Ingest ClamAV, mail gateway, and endpoint scanner logs into a centralized data lake to correlate crash events across hosts.

How to Mitigate CVE-2026-20346

Immediate Actions Required

Inventory all systems running ClamAV directly or embedded in third-party products, including mail gateways and endpoint agents.
Apply the patched ClamAV release referenced in the Cisco Security Advisory as soon as it is available for your platform.
Configure service supervisors such as systemd to automatically restart clamd on failure to reduce DoS window while patching is in progress.

Patch Information

Cisco has published a security advisory for ClamAV covering CVE-2026-20346. Consult the Cisco Security Advisory for the fixed version numbers, download locations, and product-specific guidance for Cisco appliances that embed ClamAV.

Workarounds

Temporarily disable PDF scanning in ClamAV configuration by setting ScanPDF no in clamd.conf where PDF inspection is not required. This reduces detection coverage and should be reverted after patching.
Pre-filter PDF attachments at upstream gateways using a secondary engine to reduce direct exposure of ClamAV to untrusted PDFs.
Restrict network access to clamd sockets so only trusted local processes submit files for scanning.

bash

# Example: disable PDF parsing in clamd.conf as a temporary workaround
# /etc/clamav/clamd.conf
ScanPDF no

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

# Verify the daemon is running after restart
sudo systemctl status clamav-daemon