CVE-2026-20215: ClamAV 7z Parser DoS Vulnerability

CVE-2026-20215 Overview

CVE-2026-20215 is an out-of-bounds write vulnerability in the 7z file format parser of ClamAV. The flaw stems from improper boundary checks when scanning content inside 7z archives. An unauthenticated, remote attacker can exploit it by submitting a crafted 7z file for scanning. Successful exploitation terminates the ClamAV scanning process and causes a denial-of-service (DoS) condition. The vendor also notes possible expanded impacts from memory corruption on affected devices. The vulnerability is classified under [CWE-120] (Buffer Copy without Checking Size of Input).

Critical Impact
A single crafted 7z file scanned by ClamAV can crash the scanning engine, disrupting antivirus coverage and email or file-gateway pipelines that rely on ClamAV.

Affected Products

ClamAV (7z file format parser component)
Cisco products bundling vulnerable ClamAV versions — see the Cisco Security Advisory
Third-party security appliances and email gateways embedding ClamAV

Discovery Timeline

2026-07-01 - CVE-2026-20215 published to NVD
2026-07-01 - Last updated in NVD database

Technical Details for CVE-2026-20215

Vulnerability Analysis

The vulnerability resides in the ClamAV parser responsible for decoding 7z archive content during malware scanning. When ClamAV processes 7z files, it fails to properly validate size constraints on embedded content. This validation gap enables an out-of-bounds write into an adjacent memory region. The immediate consequence is process termination of the ClamAV scanner. Depending on runtime layout and allocator behavior, memory corruption may enable additional impacts beyond DoS. The attack requires no authentication, no user interaction, and can be delivered through any channel that routes files to ClamAV for inspection, such as email scanning, web proxies, or on-demand file scanning.

Root Cause

The root cause is missing or insufficient boundary checks on attacker-controlled length or offset fields within 7z archive structures. The parser copies content into a fixed-size buffer without verifying that the source size fits the destination. This produces a classic buffer overflow condition, tracked under [CWE-120].

Attack Vector

Exploitation is remote and network-reachable. An attacker delivers a crafted 7z archive to any workflow that hands files to ClamAV, including mail transfer agents using clamd, web content filters, and file-upload scanners. When ClamAV parses the malicious 7z structure, the out-of-bounds write occurs and the scanning process crashes.

No verified public exploit code was available at the time of publication. Refer to the Cisco Security Advisory for authoritative technical detail.

Detection Methods for CVE-2026-20215

Indicators of Compromise

Unexpected termination or crash of the clamd or clamscan process during file scanning
Core dumps or segmentation fault entries in system logs referencing ClamAV
Repeated scan failures tied to inbound 7z attachments or uploads
Sudden gaps in antivirus scan telemetry from mail or web gateways

Detection Strategies

Monitor ClamAV logs for parser errors, abnormal exits, and repeated scan aborts on 7z content
Alert on SIGSEGV or SIGABRT signals delivered to ClamAV processes
Correlate mail or upload gateway telemetry with ClamAV scan-failure events to spot targeted delivery
Inspect archive-heavy traffic for malformed 7z headers using network or gateway DPI tooling

Monitoring Recommendations

Track process uptime and restart counts for clamd across all hosts running the service
Ingest ClamAV and host syslog into a centralized SIEM for cross-host correlation
Establish a baseline for scan volume and failure rates; alert on deviations
Watch for 7z file submissions from untrusted senders combined with ClamAV service disruptions

How to Mitigate CVE-2026-20215

Immediate Actions Required

Inventory all systems running ClamAV directly and all appliances that embed ClamAV
Apply the fixed ClamAV release as soon as the vendor publishes it, per the Cisco Security Advisory
Restrict inbound 7z archives at mail and web gateways until patches are deployed
Ensure clamd runs under a least-privilege service account with resource limits

Patch Information

Refer to the Cisco Security Advisory for the list of fixed ClamAV versions and vendor product updates. Apply patches to ClamAV and any downstream product that bundles the vulnerable parser. Restart clamd after upgrade to load the fixed engine.

Workarounds

Disable 7z archive scanning in the ClamAV configuration until a patched version is deployed
Block or quarantine 7z attachments at the email gateway during the exposure window
Run ClamAV inside a sandboxed or containerized environment with automatic restart to limit DoS duration
Rate-limit and log archive submissions from external sources to detect exploitation attempts

bash

# Configuration example: disable 7z scanning in clamd.conf until patched
# Edit /etc/clamav/clamd.conf and set:
ScanArchive yes
ArchiveBlockEncrypted no
# Add a mail/gateway filter rule to reject or strip .7z attachments
# Example postfix header_check rule:
# /name=".*\.7z"/ REJECT 7z archives temporarily blocked (CVE-2026-20215)
sudo systemctl restart clamav-daemon