CVE-2026-86410 in curl
Summary
by MITRE • 10/20/2026
A flaw in the curl command line tool's logic makes it only set SSH host verification options when interacting directly using SFTP or SCP, and not as a result of following a redirect.
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Analysis
by VulDB Data Team • 10/20/2026
The vulnerability identified in the curl utility stems from an inconsistency in how Secure Shell (SSH) based protocols handle authentication and host key verification during URL redirection sequences. Specifically, when curl is invoked with options to enforce strict SSH host key checking for direct SFTP or SCP transfers, these security settings are not propagated if the initial request triggers a redirect to another server using one of these protocols. This behavior creates a scenario where an attacker who can influence the target URL or exploit a misconfigured web service that issues redirects could force curl to connect to an arbitrary remote host without verifying its identity against known trusted keys. The core technical flaw lies in the internal logic flow of the protocol handler, which resets or fails to carry over specific security flags when transitioning from one request context to another via HTTP redirections such as 301 Moved Permanently or 302 Found.
From a technical perspective, this issue represents a failure in state preservation across transaction boundaries within the application logic. When curl processes an SFTP or SCP URL, it initializes a session that includes strict host key verification by default if configured to do so via command-line flags like --sslv or specific SSH options. However, upon encountering a redirect response from an intermediate server or even the target server itself, curl initiates a new connection context for the redirected location. In this new context, the previously established security constraints regarding SSH host key verification are inadvertently discarded. Consequently, the tool proceeds to establish a connection with the destination server without validating its public host key against the user's known_hosts file or other trust anchors. This effectively downgrades the security posture from authenticated and verified communication to an unverified state, exposing the client to potential man-in-the-middle attacks where an adversary could intercept credentials or data by impersonating the target SSH server.
The operational impact of this vulnerability is significant for environments relying on curl for automated file transfers over SFTP or SCP, particularly in CI/CD pipelines, backup scripts, and enterprise automation tools that utilize URL redirections to manage load balancing or failover scenarios. An attacker with network access could exploit this by crafting a malicious redirect response that points the client to an infrastructure under their control. Since curl would not verify the host key of this new destination, any sensitive data transmitted during the subsequent SFTP upload or SCP transfer would be exposed in plaintext to the attacker. Furthermore, if authentication credentials are sent as part of the initial request context and reused or re-sent in a way that allows interception, those credentials could also be compromised. This undermines the fundamental security guarantees provided by SSH protocols, which rely heavily on host key verification to prevent impersonation attacks.
This vulnerability aligns with CWE-20 Improper Input Validation, as it involves the failure to properly validate and maintain security context when processing external inputs such as redirect headers. It also maps to MITRE ATT&CK technique T1583 Acquire Infrastructure, specifically sub-techniques related to setting up malicious servers for phishing or credential harvesting, although in this case, the exploitation relies on existing infrastructure misconfigurations rather than active setup by a threat actor during an attack. The flaw essentially allows for session hijacking-like behavior where the trust established with one entity is silently transferred to another unverified entity without user intervention or warning.
To mitigate this risk, users should avoid relying solely on curl's default behaviors when dealing with potentially redirecting URLs in security-sensitive contexts. It is recommended to implement additional validation layers outside of curl, such as using wrapper scripts that verify the final destination IP address against a whitelist before executing the transfer. Alternatively, disabling automatic redirects by using the --no-location option can prevent the tool from following paths that might lead to unverified hosts. Organizations should also ensure their internal documentation and security policies explicitly warn against using curl for critical SFTP or SCP operations where redirectability is possible without additional safeguards. Keeping curl updated to versions where this logic has been corrected, if available in newer releases, is essential, though immediate procedural controls remain the most effective defense until patches are widely deployed.