CVE-2023-42456 Vulnerability Analysis & Exploit Details

CVE-2023-42456 Vulnerability Summary

CVE-2023-42456: Sudo-rs, a memory safe implementation of sudo and su, allows users to not have to enter authentication at every sudo attempt, but instead only requiring authentication every once in a while in every terminal or process group. Only once a configurable timeout has passed will the user have to re-authenticate themselves. Supporting this functionality is a set of session files (timestamps) for each user, stored in `/var/run/sudo-rs/ts`. These files are named according to the username from which the sudo attempt is made (the origin user). An issue was discovered in versions prior to 0.2.1 where usernames containing the `.` and `/` characters could result in the corruption of specific files on the filesystem. As usernames are generally not limited by the characters they can contain, a username appearing to be a relative path can be constructed. For example we could add a user to the system containing the username `../../../../bin/cp`. When logged in as a user with that name, that user could run `sudo -K` to clear their session record file. The session code then constructs the path to the session file by concatenating the username to the session file storage directory, resulting in a resolved path of `/bin/cp`. The code then clears that file, resulting in the `cp` binary effectively being removed from the system. An attacker needs to be able to login as a user with a constructed username. Given that such a username is unlikely to exist on an existing system, they will also need to be able to create the users with the constructed usernames. The issue is patched in version 0.2.1 of sudo-rs. Sudo-rs now uses the uid for the user instead of their username for determining the filename. Note that an upgrade to this version will result in existing session files being ignored and users will be forced to re-authenticate. It also fully eliminates any possibility of path traversal, given that uids are always integer values. The `sudo -K` and `sudo -k` commands can run, even if a user has no sudo access. As a workaround, make sure that one's system does not contain any users with a specially crafted username. While this is the case and while untrusted users do not have the ability to create arbitrary users on the system, one should not be able to exploit this issue.

Exploit Prediction Scoring System (EPSS)

The EPSS score estimates the probability that this vulnerability will be exploited in the near future.

EPSS Score: 0.117% (probability of exploit)

EPSS Percentile: 47.33% (lower percentile = lower relative risk)
This vulnerability is less risky than approximately 52.67% of others.

CVE-2023-42456 References

External References

• http://www.openwall.com/lists/oss-security/2023/11/02/1
• https://ferrous-systems.com/blog/sudo-rs-audit/
• https://github.com/memorysafety/sudo-rs/commit/bfdbda22968e3de43fa8246cab1681cfd5d5493d
• https://github.com/memorysafety/sudo-rs/security/advisories/GHSA-2r3c-m6v7-9354
• http://www.openwall.com/lists/oss-security/2023/11/02/1
• https://ferrous-systems.com/blog/sudo-rs-audit/
• https://github.com/memorysafety/sudo-rs/commit/bfdbda22968e3de43fa8246cab1681cfd5d5493d
• https://github.com/memorysafety/sudo-rs/security/advisories/GHSA-2r3c-m6v7-9354

CWE Common Weakness Enumeration

CAPEC Common Attack Pattern Enumeration and Classification

• Path Traversal CAPEC-126 An adversary uses path manipulation methods to exploit insufficient input validation of a target to obtain access to data that should be not be retrievable by ordinary well-formed requests. A typical variety of this attack involves specifying a path to a desired file together with dot-dot-slash characters, resulting in the file access API or function traversing out of the intended directory structure and into the root file system. By replacing or modifying the expected path information the access function or API retrieves the file desired by the attacker. These attacks either involve the attacker providing a complete path to a targeted file or using control characters (e.g. path separators (/ or \) and/or dots (.)) to reach desired directories or files.
• Using Slashes and URL Encoding Combined to Bypass Validation Logic CAPEC-64 This attack targets the encoding of the URL combined with the encoding of the slash characters. An attacker can take advantage of the multiple ways of encoding a URL and abuse the interpretation of the URL. A URL may contain special character that need special syntax handling in order to be interpreted. Special characters are represented using a percentage character followed by two digits representing the octet code of the original character (%HEX-CODE). For instance US-ASCII space character would be represented with %20. This is often referred as escaped ending or percent-encoding. Since the server decodes the URL from the requests, it may restrict the access to some URL paths by validating and filtering out the URL requests it received. An attacker will try to craft an URL with a sequence of special characters which once interpreted by the server will be equivalent to a forbidden URL. It can be difficult to protect against this attack since the URL can contain other format of encoding such as UTF-8 encoding, Unicode-encoding, etc.
• Manipulating Web Input to File System Calls CAPEC-76 An attacker manipulates inputs to the target software which the target software passes to file system calls in the OS. The goal is to gain access to, and perhaps modify, areas of the file system that the target software did not intend to be accessible.
• Using Escaped Slashes in Alternate Encoding CAPEC-78 This attack targets the use of the backslash in alternate encoding. An adversary can provide a backslash as a leading character and causes a parser to believe that the next character is special. This is called an escape. By using that trick, the adversary tries to exploit alternate ways to encode the same character which leads to filter problems and opens avenues to attack.
• Using Slashes in Alternate Encoding CAPEC-79 This attack targets the encoding of the Slash characters. An adversary would try to exploit common filtering problems related to the use of the slashes characters to gain access to resources on the target host. Directory-driven systems, such as file systems and databases, typically use the slash character to indicate traversal between directories or other container components. For murky historical reasons, PCs (and, as a result, Microsoft OSs) choose to use a backslash, whereas the UNIX world typically makes use of the forward slash. The schizophrenic result is that many MS-based systems are required to understand both forms of the slash. This gives the adversary many opportunities to discover and abuse a number of common filtering problems. The goal of this pattern is to discover server software that only applies filters to one version, but not the other.

Vulnerable Configurations

• cpe:2.3:a:memorysafety:sudo:-:*:*:*:*:rust:*:*
  cpe:2.3:a:memorysafety:sudo:-:*:*:*:*:rust:*:*
• cpe:2.3:a:memorysafety:sudo:0.1.0:*:*:*:*:rust:*:*
  cpe:2.3:a:memorysafety:sudo:0.1.0:*:*:*:*:rust:*:*
• cpe:2.3:a:memorysafety:sudo:0.2.0:*:*:*:*:rust:*:*
  cpe:2.3:a:memorysafety:sudo:0.2.0:*:*:*:*:rust:*:*

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