CVE-2020-3561: Cisco Adaptive Security Appliance Software and Firepower Threat Defense Software WebVPN CRLF Injection Vulnerability

Severity: medium (CVSS 4.7 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 1.3% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2020-10-21 · updated 2026-08-11
Weakness: CWE-93

Description

A vulnerability in the Clientless SSL VPN (WebVPN) of Cisco Adaptive Security Appliance (ASA) Software and Cisco Firepower Threat Defense (FTD) Software could allow an unauthenticated, remote attacker to inject arbitrary HTTP headers in the responses of the affected system. The vulnerability is due to improper input sanitization. An attacker could exploit this vulnerability by persuading a user of the interface to click a crafted link. A successful exploit could allow the attacker to conduct a CRLF injection attack, adding arbitrary HTTP headers in the responses of the system and redirecting the user to arbitrary websites.

Affected products

Cisco Adaptive Security Appliance: before 9.6.4.35 — fixed in 9.6.4.35
Cisco ASA Software: from 9.8.0 before 9.8.4.20; from 9.9.0 before 9.9.2.80; from 9.10.0 before 9.10.1.43; from 9.12.0 before 9.12.3.9; from 9.13.0 before 9.13.1.10; from 9.14.0 before 9.14.1.10 — fixed in 9.10.1.43, 9.12.3.9, 9.13.1.10, 9.14.1.10, 9.8.4.20, 9.9.2.80
Cisco Firepower Threat Defense Software: before 6.3.0.6; from 6.4.0 before 6.4.0.10; from 6.5.0 before 6.5.0.5; from 6.6.0 before 6.6.1 — fixed in 6.3.0.6, 6.4.0.10, 6.5.0.5, 6.6.1

How to fix

Upgrade to a fixed version: Cisco Adaptive Security Appliance 9.6.4.35; Cisco ASA Software 9.10.1.43, 9.12.3.9, 9.13.1.10, 9.14.1.10; Cisco Firepower Threat Defense Software 6.3.0.6, 6.4.0.10, 6.5.0.5, 6.6.1. Check the vendor advisory for mitigations.

References

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