Protection mechanism failure in ModSecurity - #VU159004
Published: November 4, 2026
Vulnerability identifier: #VU159004
CSH Severity: Medium
CVSS v4: 6.9 [CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N]
CVE-ID: N/A
CWE-ID: CWE-693
Exploitation vector: Remote access
Exploit availability:
No public exploit available
Vulnerability details
The vulnerability allows a remote attacker to bypass web application firewall rules that inspect base64-decoded input.
The vulnerability exists due to improper handling of malformed base64 padding in ModSecurity's t:base64DecodeExt transformation when decoding attacker-controlled input for rule matching. A remote attacker can insert a malformed padding group such as A== into a base64-encoded value to cause the entire decoded output to be silently discarded, bypassing rules that inspect the transformed content.
Affected software
ModSecurity
Remediation
Install security update from vendor's website.
ModSecurity - addressed in versions 2.9.15, 3.0.17