CVE-2023-27534: Path Traversal

A path traversal vulnerability exists in curl <8.0.0 SFTP implementation causes the tilde (~) character to be wrongly replaced when used as a prefix in the first path element, in addition to its intended use as the first element to indicate a path relative to the user's home directory. Attackers can exploit this flaw to bypass filtering or execute arbitrary code by crafting a path like /~2/foo while accessing a server with a specific user.

cURL libcurl could allow a remote attacker to obtain sensitive information, caused by a SFTP path ~ resolving discrepancy flaw. By sending a specially crafted request using a tilde (~) character, an attacker could exploit this vulnerability to obtain sensitive information from other directory, and use this information to launch further attacks against the affected system.

— IBM

curl supports SFTP transfers. curl's SFTP implementation offers a special feature in the path component of URLs: a tilde (~) character as the first path element in the path to denotes a path relative to the user's home directory. This is supported because of wording in the once proposed to-become RFC draft that was to dictate how SFTP URLs work. Due to a bug, the handling of the tilde in SFTP path did however not only replace it when it is used stand-alone as the first path element but also wrongly when used as a mere prefix in the first element. Using a path like /~2/foo when accessing a server using the user dan (with home directory /home/dan) would then quite suprisingly access the file /home/dan2/foo. This can be taken advantage of to circumvent filtering or worse.

— Red Hat