CISA published 2021 Top 15 most exploited software vulnerabilities

CVE-2018-13379 is a path traversal flaw (CWE-22) in the Fortinet FortiOS SSL VPN web portal that allows an unauthenticated attacker to download FortiOS system files via specially crafted HTTP resource requests. By traversing directories through crafted requests to the exposed web portal, the attacker can retrieve sensitive files, a technique publicly documented as yielding the SSL VPN session file containing usernames and passwords in plaintext. Any organization running the SSL VPN web portal on a FortiGate appliance is affected, and risk is highest where the portal is directly reachable from the internet. The flaw is confirmed in the wild: it was added to the CISA KEV catalog on 2021-11-03 with known ransomware use, and EPSS assigns it a 100% probability of exploitation within 30 days. No public PoC is listed in the provided data, but credential theft tied to this bug has been widely reused by threat actors.

Do: Apply the patched FortiOS release per Fortinet's vendor advisory immediately, as this is a CISA KEV required action; if the fixed version is not known from this data, follow Fortinet's FG-IR-18-384 advisory for the correct upgrade path. Rotate SSL VPN credentials and review VPN access logs for path-traversal requests, since successful exploitation exposes plaintext session credentials, and restrict SSL VPN portal exposure to trusted sources where possible.

Ivanti Pulse Connect Secure, an enterprise SSL VPN appliance, contains an arbitrary file read vulnerability (CWE-22, path traversal) that requires no authentication. An unauthenticated remote attacker with network access to the appliance over HTTPS can send a specially crafted URI containing traversal sequences to read arbitrary files from the device. The attacker gains access to sensitive appliance files, potentially including configuration or credential material useful for further compromise, and CISA records known ransomware use of this flaw. Any organization running Pulse Connect Secure, especially gateways exposed to the internet for remote access, is affected. Exploitation is established: CISA added the flaw to its Known Exploited Vulnerabilities catalog on 2021-11-03 with known ransomware use, EPSS assigns a 100% probability of exploitation in the next 30 days, and no public PoC is known.

Do: Apply updates per Ivanti's instructions, the required action in the CISA KEV entry, prioritizing internet-facing Pulse Connect Secure gateways, and consult the vendor advisory for the applicable fixed release since no version range is provided here. Where patching is not immediate, restrict HTTPS access to the appliance and hunt for signs of exploitation (unexpected file reads, anomalous VPN logins or sessions, and follow-on ransomware activity), as CISA reports known ransomware use.

CVE-2021-21972 is a remote code execution vulnerability in a plugin of the vSphere Client in VMware vCenter Server, underpinned by a path traversal flaw (CWE-23) in the plugin's file-upload functionality. It is triggered over the network through port 443: an attacker who can reach the vCenter web interface can submit crafted file-upload requests, traverse to arbitrary filesystem paths, and plant executable files on the underlying operating system. Successful exploitation yields unrestricted privileges on the vCenter host OS, giving attackers control of the central management platform for an organization's entire VMware vSphere virtualized estate. Any organization running an affected vCenter Server release whose port 443 is reachable from untrusted networks is exposed. The flaw is listed in CISA's Known Exploited Vulnerabilities catalog (added 2021-11-03) with known ransomware use and a 99.9% EPSS score (100th percentile), indicating active, widespread exploitation in the wild, though no public PoC is recorded in this data.

Do: Apply the vCenter Server updates published in VMware's advisory for CVE-2021-21972 as soon as possible, prioritizing internet-facing or partner-reachable vCenter instances. Until patched, restrict access to vCenter on port 443 to trusted management networks and review access logs for unauthenticated file-upload activity against the vSphere Client upload endpoint. Because ransomware operators have actively exploited this bug, hunt for signs of compromise such as webshells or unexpected new local accounts on vCenter appliances.

CVE-2021-26084 is an OGNL injection vulnerability (CWE-917) in Atlassian Confluence Server and Data Center that may allow an unauthenticated attacker to execute arbitrary code. Any organization running Confluence Server or Data Center is potentially affected. It is significant because it is listed in CISA's Known Exploited Vulnerabilities catalog with known ransomware use, and EPSS assigns a 100.0% probability of exploitation within 30 days (100th percentile).

Do: Apply updates to Confluence Server and Data Center per Atlassian's instructions, which is the required action in the CISA KEV listing. Prioritize remediation given confirmed in-the-wild exploitation and known ransomware use.

CVE-2021-40539 is a critical (CVSS 9.8) authentication bypass in the REST API of Zoho ManageEngine ADSelfService Plus, caused by use of an incorrectly resolved name or reference (CWE-706). An unauthenticated, network-adjacent or internet-reachable attacker sends specially crafted requests to the product's REST API, bypassing authentication, and can chain the bypass to full remote code execution with no privileges or user interaction required. Successful exploitation yields complete compromise of the self-service portal server (high impact to confidentiality, integrity and availability); public reporting and vendor notes document attackers dropping malicious code and web shells onto vulnerable servers. Any organization running ManageEngine ADSelfService Plus build 6113 or earlier is affected, which typically means enterprise Microsoft Active Directory environments running this widely deployed self-service password/SSO portal. Exploitation is confirmed in the wild: the flaw was added to CISA KEV on 2021-11-03 with known ransomware use, EPSS puts the 30-day exploitation probability at 99% (100th percentile), Microsoft warned that Chinese threat actors were actively exploiting it, a public proof-of-concept is available, and it ranked among CISA's most routinely exploited vulnerabilities.

Do: Immediately upgrade ManageEngine ADSelfService Plus to a fixed build newer than 6113 per the vendor's update instructions, as required by CISA. Because exploitation predates patching and the flaw has been used to drop malicious code, check ADSelfService Plus servers for web shells, unexpected scheduled tasks, and unexplained accounts/processes, and hunt for indicators from the published analyses. Where possible, restrict internet exposure of the ADSelfService Plus REST API while patching, prioritized for externally reachable instances.