Top 15 Vulnerabilities Attackers Exploited Millions of Times to Hack Linux Systems

In Apache Struts 2.0.0 through 2.3.33 and 2.5 through 2.5.10.1, using an unintentional expression in a Freemarker tag instead of string literals can lead to a RCE attack.

CVE-2017-5638 is an improper input validation flaw (CWE-20) in the Jakarta Multipart parser of Apache Struts, in which the parser mishandles the Content-Type value of a file upload and allows malicious upload leading to remote code execution. It is triggered remotely without authentication by sending a crafted Content-Type header in a multipart request to a Struts endpoint; no valid upload or credentials are required. A successful attacker gains code execution in the security context of the application server, which typically enables host compromise, data theft, or ransomware deployment. Any organization running Apache Struts applications that use the Jakarta Multipart parser is affected; the provided data specifies only "Apache Struts" and gives no version ranges. Exploitation is confirmed in the wild: the flaw is listed in CISA KEV (added 2021-11-03) with known ransomware use, EPSS assigns it roughly a 100% exploitation probability (100th percentile), and no public PoC is catalogued in the source data.

Do: Apply updates per vendor instructions: upgrade Apache Struts to the releases that fix this flaw (2.3.32 / 2.5.10.1 or later, per Apache advisory S2-045), and check for Struts jars bundled inside application packages and vendor appliances. Prioritize internet-facing apps, and as an interim mitigation validate or filter the Content-Type header on multipart requests. Because exploitation is in the wild and ransomware use is known, also review web and application server logs for evidence of successful compromise.

In Eclipse Jetty, versions 9.2.x and older, 9.3.x (all configurations), and 9.4.x (non-default configuration with RFC2616 compliance enabled), transfer-encoding chunks are handled poorly. The chunk length parsing was vulnerable to an integer overflow. Thus a large chunk size could be interpreted as a smaller chunk size and content sent as chunk body could be interpreted as a pipelined request. If Jetty was deployed behind an intermediary that imposed some authorization and that intermediary allowed arbitrarily large chunks to be passed on unchanged, then this flaw could be used to bypass the authorization imposed by the intermediary as the fake pipelined request would not be interpreted by the intermediary as a request.

CVE-2017-9805 is an unsafe deserialization vulnerability in the REST plugin of Apache Struts 2, which uses the XStream library to deserialize XML request payloads without any type filtering. An attacker triggers it by sending a crafted XML payload to an endpoint handled by the REST plugin, causing the payload to be deserialized into arbitrary objects. Successful exploitation yields remote code execution on the server with the privileges of the web application, typically enough to fully take over the host. Struts 2.1.1 through 2.3.x before 2.3.34 and 2.5.x before 2.5.13 are affected, including products from Cisco and NetApp that bundle vulnerable Struts versions. The flaw has a public proof-of-concept exploit, is on CISA's Known Exploited Vulnerabilities catalog, and was reported as under active exploitation after its 2017 disclosure, although the Apache Foundation disputed suggestions that this specific flaw was used in the Equifax breach.

Do: Upgrade Apache Struts to 2.3.34, 2.5.13, or later, or apply the fixed updates for any Cisco or NetApp product that embeds Struts per their advisories. If patching is not immediate, remove or disable the REST plugin, or restrict the XML content types it accepts. Inventory internet-facing applications for Struts REST endpoints and hunt for suspicious XML POSTs, since the CVE is on the CISA KEV list and was reported as actively exploited.

Apache Struts 2 contains an improper input validation flaw (CWE-20) that allows unauthenticated remote code execution when an attacker-controlled namespace (for example, part of the request URL path) is evaluated as an OGNL expression on the server. It is triggered under two circumstances: (1) the alwaysSelectFullNamespace option is true and a result defined in the configuration has no namespace while its parent package's namespace is unset or a wildcard, or (2) a URL tag in a template has no namespace under the same parent-package conditions. An attacker who can reach an application in one of these configurations gains arbitrary code execution with the privileges of the application server, so any organization running affected Struts 2 releases (2.3.7-2.3.34 and 2.5-2.5.16) in such configurations is exposed. CISA added the flaw to its Known Exploited Vulnerabilities catalog on 2021-11-03, confirming exploitation in the wild (ransomware use unknown), and EPSS currently assigns a 100% probability of exploitation within 30 days. CVSS is not yet scored in this feed, but the KEV listing and EPSS signal indicate urgent patching priority.

Do: Upgrade Apache Struts to 2.3.35 or 2.5.17 (or later) on all applications, prioritizing internet-facing ones, per vendor instructions. If upgrading is not immediately possible, set alwaysSelectFullNamespace to false and ensure every result and URL tag specifies an explicit, non-wildcard namespace. Audit configurations, especially applications using the Convention Plugin or wildcard namespaces, for the vulnerable conditions and review web server logs for signs of OGNL injection in namespace values.

CVE-2018-7600, widely known as 'Drupalgeddon 2', is an unauthenticated remote code execution flaw in Drupal Core caused by insufficient input validation (CWE-20) in how the CMS processes certain structured request data. It can be triggered through multiple attack vectors, such as crafted parameters submitted to commonly used form and rendering features that are reachable by anonymous users with a single HTTP request. Successful exploitation lets an attacker run arbitrary code under the web application, typically resulting in complete site compromise, and CISA notes the flaw has been used in ransomware operations. Any site running unpatched Drupal 7.x or 8.x core is affected; Drupal's installed base at the time of disclosure was on the order of one million sites. Exploitation is confirmed in the wild (CISA KEV, added 2021-11-03), EPSS assigns a 100% probability of exploitation within 30 days, and no public PoC is recorded in the supplied data.

Do: Upgrade immediately per vendor instructions: Drupal 7.58, 8.5.1, or the corresponding 8.4.6/8.3.9 updates if you remain on older 8.x branches, prioritizing internet-facing sites. Because this flaw has been exploited in the wild and used in ransomware operations, also check patched sites for backdoors, unexpected administrator accounts, modified core files, and rotate credentials.

Apache Struts 2.0.0 to 2.5.20 forced double OGNL evaluation, when evaluated on raw user input in tag attributes, may lead to remote code execution.

CVE-2020-11651 is a critical (CVSS 9.8) authentication bypass in SaltStack Salt's salt-master process: the ClearFuncs class fails to validate method calls, so a remote attacker can invoke privileged methods without authenticating. By sending crafted messages to the salt-master's network interface, an unauthenticated user can retrieve the master's root tokens/eauth credentials and use them to execute arbitrary commands on salt minions, yielding full remote code execution across the entire Salt deployment. Anyone running affected Salt versions before 2019.2.4 or 3000.x before 3000.2 — including Salt shipped in Debian, Ubuntu, openSUSE Leap, and VMware's Application Remote Collector — is exposed, especially masters reachable on their ZeroMQ ports. Exploitation is confirmed in the wild: the flaw is on CISA's KEV catalog, EPSS puts exploitation probability at 96.6%, attackers have been mass-scanning for vulnerable Salt installs, and breaches using this bug hit Cisco (six servers), LineageOS, Ghost, and DigiCert.

Do: Upgrade salt-master to Salt 2019.2.4 or 3000.2 (or later), or apply the patched packages issued by Debian, Ubuntu, and openSUSE; VMware vROps customers should apply VMware's Application Remote Collector fix. Restrict access to the salt-master ZeroMQ ports (4505/4506) to trusted networks and review masters for signs of compromise, such as unexpected root tokens or unauthorized jobs run on minions.

Affected versions of Atlassian Jira Server and Data Center allow remote, unauthenticated attackers to view custom field names and custom SLA names via an Information Disclosure vulnerability in the /secure/QueryComponent!Default.jspa endpoint. The affected versions are before version 8.5.8, and from version 8.6.0 before 8.11.1.

CVE-2020-14750 is a critical vulnerability in the Console component of Oracle WebLogic Server (Oracle Fusion Middleware), functioning as a bypass of the earlier fix for the widely exploited CVE-2020-14882. An unauthenticated attacker with network access over HTTP can trigger it simply by sending crafted requests to the web administration console, with no credentials or user interaction required. Successful exploitation results in full takeover of the WebLogic Server, yielding high impact to confidentiality, integrity and availability (CVSS 3.1 score of 9.8). All supported WebLogic Server versions are affected — 10.3.6.0.0, 12.1.3.0.0, 12.2.1.3.0, 12.2.1.4.0 and 14.1.1.0.0 — particularly deployments whose admin console is reachable from the internet. Exploitation is confirmed in the wild: the flaw is in CISA's KEV catalog (added 2021-11-03), carries a 99.3% EPSS, and multiple botnets have been observed exploiting it, with reports of access brokers selling compromised-server access to ransomware operators.

Do: Apply Oracle's emergency/out-of-cycle patch for CVE-2020-14750 or any later Critical Patch Update on all affected WebLogic versions (10.3.6.0.0, 12.1.3.0.0, 12.2.1.3.0, 12.2.1.4.0, 14.1.1.0.0), per vendor instructions. Until patched, restrict HTTP access to the WebLogic admin console to trusted networks and prioritize remediating internet-facing consoles. Hunt for signs of exploitation, since multiple botnets are actively scanning for and exploiting this flaw.

CVE-2020-17496 is an unauthenticated remote command execution flaw in vBulletin 5 (CWE-74, improper command neutralization) that exists because the vendor's fix for the earlier CVE-2019-16759 was incomplete. An attacker triggers it by sending a request to the ajax/render/widget_tabbedcontainer_tab_panel route with crafted subWidgets data, which the application processes without adequate sanitization. Successful exploitation allows a remote, unauthenticated attacker to execute arbitrary commands on the web server with the privileges of the web application, enabling site compromise, data theft, or deployment of webshells or botnet malware. All vBulletin deployments running versions 5.5.4 through 5.6.2 are affected. Exploitation is confirmed: the flaw is in CISA's Known Exploited Vulnerabilities catalog (added 2021-11-03), public proof-of-concept exploits and a detailed public analysis exist, and EPSS estimates an 87.7% probability of exploitation within 30 days; ransomware association is listed as unknown.

Do: Upgrade vBulletin to a release newer than 5.6.2 following the vendor's update instructions, as required by CISA's KEV listing. As an interim mitigation, restrict or block access to the ajax/render route (e.g., via WAF rules). Given confirmed in-the-wild exploitation, review web server logs for requests to ajax/render/widget_tabbedcontainer_tab_panel with crafted subWidgets parameters and check for signs of compromise such as webshells or unexpected processes running as the web server user.

The File Manager (wp-file-manager) WordPress plugin before version 6.9 ships an example elFinder connector file that the plugin renames to have a .php extension, exposing it directly to unauthenticated web requests. An attacker can invoke the elFinder upload (or mkfile and put) commands on that connector to write arbitrary PHP code into the wp-content/plugins/wp-file-manager/lib/files/ directory, which the web server then executes. Successful exploitation yields full unauthenticated remote code execution on the hosting server, compromising the WordPress site and potentially the broader host. Any WordPress site running an affected version of the File Manager plugin is exposed, and the flaw was heavily exploited in the wild in August and September 2020, including mass backdooring campaigns (WP-SHELLSTORM) and chaining with other vulnerabilities such as Zerologon. It carries a CVSS 3.1 score of 9.8, sits in CISA's Known Exploited Vulnerabilities catalog, and has a 97.3% EPSS probability of exploitation.

Do: Update the File Manager (wp-file-manager) plugin to version 6.9 or later immediately, per vendor and CISA instructions. If updating is not immediately possible, deactivate the plugin or remove/restrict the exposed example elFinder connector file, and check wp-content/plugins/wp-file-manager/lib/files/ for unexpected PHP files (e.g., webshells) plus unfamiliar admin users, since exploitation was widespread in August–September 2020.

Liferay Portal contains a deserialization of untrusted data flaw (CWE-502) that allows remote attackers to execute arbitrary code through the product's JSON web services. An attacker triggers it by sending crafted, untrusted serialized data to the portal's JSON web services endpoints; the available data does not specify an affected version range or authentication requirements. Successful exploitation yields remote code execution on the portal server, a foothold attackers can use to pivot into the wider environment. Any organization running Liferay Portal is affected, with internet-facing deployments at highest risk; public internet-wide scans have counted tens of thousands of exposed instances. The vulnerability is on CISA's Known Exploited Vulnerabilities catalog (added 2021-11-03) with a 99.9% EPSS (100th percentile), indicating active or near-certain exploitation, though no public PoC is documented, ransomware use is unknown, and CVSS has not been scored.

Do: Apply Liferay's updates per vendor instructions as required by CISA KEV, prioritizing internet-facing portals, and use the Liferay advisory to identify affected instances since no version range is given here. If patching must wait, restrict external access to the JSON web services endpoints and monitor for exploitation, given the in-the-wild status and 99.9% EPSS.

Nacos is a platform designed for dynamic service discovery and configuration and service management. In Nacos before version 1.4.1, when configured to use authentication (-Dnacos.core.auth.enabled=true) Nacos uses the AuthFilter servlet filter to enforce authentication. This filter has a backdoor that enables Nacos servers to bypass this filter and therefore skip authentication checks. This mechanism relies on the user-agent HTTP header so it can be easily spoofed. This issue may allow any user to carry out any administrative tasks on the Nacos server.