FortiOS CVE-2026-59837

Exploitation requires an authenticated attacker holding high-privilege (administrative) credentials on the targeted FortiOS, FortiPAM, FortiProxy, or FortiSASE instance - confirmed by CVSS PR:H. Beyond authentication, the attacker must additionally bypass two independent memory protection mechanisms: stack canaries and ASLR, as explicitly stated in the CVE description. This secondary bypass requirement is what drives AC:H and substantially differentiates this from a straightforward buffer overflow. The attack is delivered via crafted HTTP requests over the network (AV:N), requiring network reachability to the management interface. No user interaction is required (UI:N). Exploitation is limited by: (1) requirement for existing privileged credentials, ruling out unauthenticated external attackers; (2) requirement to defeat ASLR and stack canaries, demanding advanced exploitation capability; (3) management interface typically accessible only from restricted administrative networks in hardened deployments. Default configurations that expose the admin interface to broad networks increase risk materially.

The CVSS base score of 5.9 (Medium) is consistent with the vector AV:N/AC:H/PR:H, where high attack complexity and high privilege requirements each significantly discount the otherwise maximum CIA impact. Real-world risk is further limited by the requirement to bypass both ASLR and stack canaries - a non-trivial secondary exploitation step that demands attacker skill beyond a simple memory corruption trigger. EPSS data was not provided, so probabilistic exploitation likelihood cannot be quantified from available data. The vulnerability is not listed in CISA KEV, meaning active in-the-wild exploitation has not been confirmed by government intelligence at time of analysis. However, E:P in the temporal vector confirms proof-of-concept exploit code exists, raising the ceiling for potential exploitation. The combination of PR:H (admin access required) and AC:H (memory protection bypass required) means exploitation is realistically constrained to well-resourced or insider-threat scenarios. Organizations with FortiOS/FortiPAM/FortiProxy management interfaces exposed beyond trusted management networks face meaningfully higher risk than those with properly segmented admin access.

An attacker with privileged administrative credentials to a FortiOS or FortiProxy management interface sends a sequence of crafted HTTP requests designed to overflow a stack-allocated buffer in the HTTP parsing subsystem. The attacker first defeats ASLR through a separate information-disclosure technique or heap-shaping primitive to predict stack addresses, then overwrites the return address to redirect execution to attacker-controlled shellcode or a ROP chain. A proof-of-concept demonstrating this technique has been confirmed by the CVSS E:P temporal metric.