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CVE-2026-33691 is a file upload validation bypass in the OWASP Core Rule Set (CRS) for compatible web application firewalls. In affected versions prior to 3.3.9 and 4.25.0, CRS file upload rules evaluate filename extensions with regular expressions without first normalizing embedded or trailing whitespace in the supplied filename. As a result, an attacker can craft multipart upload requests whose filenames contain whitespace padding adjacent to a dangerous extension, causing the extension-matching logic to fail while downstream components may later normalize or trim the filename. This can allow prohibited server-executable file types, including common script and archive formats used for web shell deployment, to pass WAF inspection. The issue is especially relevant where backend platforms, applications, or web servers canonicalize or trim whitespace before storing or executing the uploaded file.
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No public exploit code observed for this vulnerability.
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25 sources tracked across advisories, community write-ups, and news. New activity surfaces here as Mallory finds it.
A high-severity WAF bypass vulnerability in Progress Kemp LoadMaster where whitespace padding in filenames can circumvent file upload extension checks.
A high-severity validation and normalization flaw in Progress Kemp LoadMaster's built-in defense modules that can allow specially crafted multipart requests to bypass firewall filtering.
A vulnerability addressed by Progress security updates; no further details are provided in the content.
A vulnerability addressed by Progress in Kemp LoadMaster in the June 2026 critical security bulletin.
Query your assets running an affected version, and investigate the blast radius.
Every observed campaign linking this CVE to a named adversary.
Malware families riding this exploit, with evidence and IOCs.
YARA, Sigma, Snort, and vendor rules, auto-deployed to your SIEM.
Cross-references every affected SKU, including bundled OEM variants.
Community discussion across Reddit, Mastodon, and other social sources.