CVE-2026-40296 — Cross-Site Scripting (XSS)
CVE-2026-40296 — PhpSpreadsheet is a pure PHP library for reading and writing spreadsheet files. The HTML writer skips htmlspecialchars escaping when a cell's formatted value differs from the original value. When a cell has a custom number format containing the text placeholder @ along with any addi
What This Means For You
- If your environment is affected by CWE-79, review your exposure and prioritize patching based on your environment. Monitor vendor advisories for CVE-2026-40296 updates and patches.
Related ATT&CK Techniques
🛡️ Detection Rules
3 rules · 6 SIEM formats3 detection rules auto-generated for this incident, mapped to MITRE ATT&CK. Sigma YAML is free — export to any SIEM format via the Intel Bot.
PhpSpreadsheet HTML Writer XSS via Custom Number Format — CVE-2026-40296
title: PhpSpreadsheet HTML Writer XSS via Custom Number Format — CVE-2026-40296
id: scw-2026-05-06-ai-1
status: experimental
level: high
description: |
Detects potential exploitation of CVE-2026-40296 by looking for specific custom number format patterns ('. @', '@ ', 'x@') within the web server's query string. These patterns are indicative of an attacker attempting to craft a spreadsheet that, when converted to HTML by PhpSpreadsheet, bypasses HTML escaping and injects malicious script.
author: SCW Feed Engine (AI-generated)
date: 2026-05-06
references:
- https://shimiscyberworld.com/posts/nvd-CVE-2026-40296/
tags:
- attack.initial_access
- attack.t1190
logsource:
category: webserver
detection:
selection:
cs-uri-query|contains:
- '. @'
- '@ '
- 'x@'
condition: selection
falsepositives:
- Legitimate administrative activity
Source: Shimi's Cyber World · License & reuse
Indicators of Compromise
| ID | Type | Indicator |
|---|---|---|
| CVE-2026-40296 | vulnerability | CVE-2026-40296 |
| CWE-79 | weakness | CWE-79 |
Written by SCW Vulnerability Desk
Source & Attribution
| Source Platform | NVD |
| Channel | National Vulnerability Database |
| Published | May 07, 2026 at 01:16 UTC |
This content was curated and summarized by Shimi's Cyber World for informational purposes. It is not copied or republished in full. All intellectual property rights remain with the original author and source.
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