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An out-of-bounds write vulnerability exists in sslvpnd (the SSL-VPN daemon) of Fortinet FortiOS and Fortinet FortiProxy products. It can be triggered remotely via specifically crafted requests. The product can be triggered into writing data past the end, or before the beginning, of the intended buffer. The product may modify an index or perform pointer arithmetic that references a memory location that is outside of the boundaries of the buffer. A subsequent write operation then produces undefined or unexpected results.
Exploit allows attacker to execute unauthorized code or commands remotely. Arbitrary code execution grants attackers the ability to run any commands or code of the attacker’s choice on the target system, allow its complete takeover and sequestration.
NOTE: This vulnerability has been reported by the CISA (America’s Cyber Defense Agency) to be known to be currently actively exploited in the wild as of 2024-02-10. VPN appliances are often internet-facing by design and frequent targets for exploitation by cyber threat groups. Previous Fortinet SSL-VPN vulnerabilities have been exploited in the wild within days of official disclosure. Yesterday, Fortinet disclosed that Chinese state-sponsored threat actors known as Volt Typhoon targeted FortiOS vulnerabilities to deploy custom malware known as COATHANGER – a custom remote access trojan (RAT) designed to infect FortiGate network security appliances and was recently found used in attacks on the Dutch Ministry of Defence. Prioritisation should be given to remediation in any impacted environment.
At the time this blog post was published on February 10th 2024, no public proof-of-concept (PoC) exploit or indicators of compromise (IoC) for CVE-2024-21762 have been made available. by the vendor.
Fortinet has released a security update to address. Customers are advised to upgrade to the latest version of the impacted product. Minimum fixed versions are as below:
NOTE: Remediation of this vulnerability by patching to a specific version indicated may not be sufficient to secure the product against further vulnerabilities discovered in later versions, subsequent to the publication of this guidance. Unless contra-indicated, customers are therefore advised to always upgrade to the latest version of the product available.
For those unable to apply patches, you can mitigate the flaw by disabling SSL VPN on your FortiOS devices. Fortinet’s advisory warns that simply disabling webmode is “NOT a valid workaround.”
NOTE: Caution should always be taken in applying any temporary mitigations listed. Mitigations are only recommended in cases where patches to remediate the vulnerability are not available, or cannot safely be applied to a given environment immediately. A given mitigation may not in all cases be recommended officially by the application vendor. The viability of any given temporary mitigation measure may vary, depending on server platform and existing configuration. Mitigations listed may incompletely remediate any given vulnerability. Configuration changes to implement listed mitigations may impact/disrupt required functionality within a given customer application. Care should therefore be taken to carefully analyse any listed mitigations for appropriateness to a given environment. Customers are advised to test any configuration changes prior to their being introduced into a production environment.
References:
Category: Memory Access Violation
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AppCheck is a software security vendor based in the UK, offering a leading security scanning platform that automates the discovery of security flaws within organisations websites, applications, network and cloud infrastructure. AppCheck are authorised by the Common Vulnerabilities and Exposures (CVE) Program as a CVE Numbering Authority (CNA)
