curl libcurl CVE-2026-87865

Exploitable only when an application uses libcurl for SCP/SFTP transfers, configures a known-hosts store (CURLOPT_SSH_KNOWNHOSTS), and registers a host-key verification callback (CURLOPT_SSH_KEYFUNCTION) whose decision depends on distinguishing a MISSING key from a MISMATCHED key. An active man-in-the-middle must substitute the server's host key, and exploitation further requires that the callback treats an unknown/missing key more permissively than a mismatched one. Limiting factors: applications that reject both missing and mismatched keys, or that do not use SSH-based protocols at all, are unaffected; the attacker needs network interception capability; and the flaw grants no direct code execution, only weakened host authentication. Command-line curl without a custom accept-new-but-not-changed policy is largely unaffected.

No CVSS vector, EPSS score, KEV listing, or public POC is present in the supplied data, so quantitative exploitation signals are entirely absent and the only authoritative severity signal is the curl project's own rating of LOW. This is a SSH host-key verification weakness: when a known-hosts key mismatch occurs, libcurl's knownhost callback is reportedly handed a 'missing' status instead of a 'mismatch' status, which can cause an application's key-verification logic to treat a changed/attacker-substituted key as merely unknown rather than as a dangerous mismatch. Real-world risk is conditional and narrow: it only matters to applications that (a) use libcurl for SCP/SFTP and (b) implement a custom CURLOPT_SSH_KEYFUNCTION callback that distinguishes MISSING from MISMATCH, and it additionally requires an active man-in-the-middle to substitute a host key - so despite curl's near-universal deployment, the exploitable surface is small. This is a genuine but low-priority defense-in-depth/verification-bypass issue, not an urgent remote-code-execution threat; the LOW rating aligns with the high preconditions. Note explicitly: no independent CVSS or exploitation telemetry was provided, so the risk framing relies solely on the vendor advisory text - verify affected version range against the curl security page before scoping.

An attacker in a man-in-the-middle position on the network path to an SCP/SFTP server presents a substituted SSH host key during connection setup. An application using libcurl with a known-hosts callback receives a 'key missing' indication instead of 'key mismatch', and if its callback logic auto-accepts or prompts leniently for unknown (but not mismatched) keys, the forged host is accepted, enabling interception of the SSH session. No public exploit code is identified at time of analysis.