Week in review: Windows crypto flaw, API security risks, exploits for Citrix security hole abound

Broadcom based cable modems across multiple vendors are vulnerable to a buffer overflow, which allows a remote attacker to execute arbitrary code at the kernel level via JavaScript run in a victim's browser. Examples of affected products include Sagemcom F@st 3890 prior to 50.10.21_T4, Sagemcom F@st 3890 prior to 05.76.6.3f, Sagemcom F@st 3686 3.428.0, Sagemcom F@st 3686 4.83.0, NETGEAR CG3700EMR 2.01.05, NETGEAR CG3700EMR 2.01.03, NETGEAR C6250EMR 2.01.05, NETGEAR C6250EMR 2.01.03, Technicolor TC7230 STEB 01.25, COMPAL 7284E 5.510.5.11, and COMPAL 7486E 5.510.5.11.

CVE-2019-19781 is a path-traversal flaw (classified CWE-22, though CISA's description calls it unspecified) in Citrix ADC (formerly NetScaler ADC), Citrix Gateway, and Citrix SD-WAN WANOP appliances that lets an unauthenticated remote attacker traverse directories via crafted requests and execute arbitrary commands on the appliance, typically with root privileges. It is triggered by sending specially crafted directory-traversal requests (crafted URLs/requests to the appliance's management or VPN endpoints), which lets the attacker write files and run commands with no credentials. Successful exploitation yields arbitrary code execution on the appliance, enabling theft of VPN/ADC credentials, lateral movement into the corporate network, and installation of persistent backdoors. Any organization running affected ADC, Gateway, or SD-WAN WANOP firmware is affected, with internet-facing gateways used for remote access at the highest risk. Exploitation is confirmed in the wild: the vulnerability is on CISA's KEV (added 2021-11-03) with known ransomware use, EPSS assigns near-certain (100.0%) probability of exploitation within 30 days, and no public PoC is listed despite confirmed abuse.

Do: Upgrade Citrix ADC, Gateway, and SD-WAN WANOP appliances to the fixed firmware builds listed in Citrix advisory CTX267020; if patching cannot be done immediately, apply Citrix's published interim mitigation and restrict internet exposure to the appliance. Because exploitation grants root code execution and persistence, after patching hunt for indicators of compromise (unexpected nsroot account, modified system files, crontab/scheduled entries), kill all active and inactive sessions, and rotate appliance and VPN credentials. Prioritize internet-facing gateways and comply with CISA's required action to apply vendor updates.

CVE-2020-0601 (known as 'Curveball') is a spoofing flaw in Crypt32.dll, the Windows CryptoAPI component that validates Elliptic Curve Cryptography (ECC) certificates, which mishandles ECC certificate parameters when checking signatures. An attacker can trigger it by signing malicious content, most notably an executable, with a spoofed, attacker-controlled ECC certificate that Windows accepts as if it were issued by a trusted certificate authority; the attack vector is network-based but requires user interaction (CVSS 3.1: 8.1, AV:N/AC:L/PR:N/UI:R). Successful exploitation makes attacker-supplied malware appear to come from a legitimate, trusted software vendor, defeating code-signing trust prompts and signature-based trust decisions, with high impact on confidentiality and integrity. Affected systems are the Microsoft Windows 10 builds 1507 through 1909 and Windows Server versions 1803, 1903, 1909, 2016 and 2019 in the CISA data, with Go also listed as an affected vendor via its ECC certificate validation, effectively covering the mainstream Windows install base at the time. The flaw was discovered by the NSA and patched in the January 2020 Patch Tuesday; it is now CISA KEV-listed (added 2021-11-03, ransomware use unknown) with very high predicted exploitation risk (EPSS 89.4%, 100th percentile), and no public PoC is tracked in this data.

Do: Apply Microsoft's January 2020 Patch Tuesday cumulative security updates for all affected Windows 10 and Windows Server builds, per CISA's required action (apply updates per vendor instructions). Prioritize user workstations and servers that validate signed binaries, Authenticode signatures, or TLS certificates, since exploitation hinges on a user or application trusting a spoofed ECC-signed artifact. If Go is deployed in your environment, update Go toolchains to a patched January 2020 or later release, as Go was also acknowledged as an affected vendor.