Tautulli is a Python based monitoring and tracking tool for Plex Media Server. The real_pms_image_proxy endpoint in Tautulli v2.15.3 and prior is vulnerable to path traversal, allowing unauthenticated attackers to read arbitrary files from the application servers filesystem. The real_pms_image_proxy is used to fetch an image directly from the backing Plex Media Server. The image to be fetched is specified through an img URL parameter, which can either be a URL or a file path. There is some validation ensuring that img begins with the prefix interfaces/default/images in order to be served from the local filesystem. However this can be bypassed by passing an img parameter which begins with a valid prefix, and then adjoining path traversal characters in order to reach files outside of intended directories. An attacker can exfiltrate files on the application file system, including the tautulli.db SQLite database containing active JWT tokens, as well as the config.ini file which contains the hashed admin password, the JWT token secret, and the Plex Media Server token and connection details. If the password is cracked, or if a valid JWT token is present in the database, an unauthenticated attacker can escalate their privileges to obtain administrative control over the application. Version 2.16.0 contains a fix for the issue.
Weakness
The product uses external input to construct a pathname that should be within a restricted directory, but it does not properly neutralize multiple internal “../” sequences that can resolve to a location that is outside of that directory.
Extended Description
This allows attackers to traverse the file system to access files or directories that are outside of the restricted directory.
The ‘directory/../../filename’ manipulation is useful for bypassing some path traversal protection schemes. Sometimes a program only removes one “../” sequence, so multiple “../” can bypass that check. Alternately, this manipulation could be used to bypass a check for “../” at the beginning of the pathname, moving up more than one directory level.
Potential Mitigations
- Assume all input is malicious. Use an “accept known good” input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does.
- When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, “boat” may be syntactically valid because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as “red” or “blue.”
- Do not rely exclusively on looking for malicious or malformed inputs. This is likely to miss at least one undesirable input, especially if the code’s environment changes. This can give attackers enough room to bypass the intended validation. However, denylists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright.
- When validating filenames, use stringent allowlists that limit the character set to be used. If feasible, only allow a single “.” character in the filename to avoid weaknesses such as CWE-23, and exclude directory separators such as “/” to avoid CWE-36. Use a list of allowable file extensions, which will help to avoid CWE-434.
- Do not rely exclusively on a filtering mechanism that removes potentially dangerous characters. This is equivalent to a denylist, which may be incomplete (CWE-184). For example, filtering “/” is insufficient protection if the filesystem also supports the use of “" as a directory separator. Another possible error could occur when the filtering is applied in a way that still produces dangerous data (CWE-182). For example, if “../” sequences are removed from the “…/…//” string in a sequential fashion, two instances of “../” would be removed from the original string, but the remaining characters would still form the “../” string.
References