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TFTP Path Traversal

Low
RaimoNiskanen published GHSA-hmrc-prh3-rpvp Feb 20, 2026

Package

OTP

Affected versions

>= 17.0

Patched versions

26.2.5.17, 27.3.4.8, 28.3.2
inets (OTP)
>= 5.10
7.0
tftp (OTP)
>= 1.0
1.1.1.1, 1.2.2.1, 1.2.4

Description

Impact

If a system designer has deployed the Erlang/OTP TFTP server and assumed that the callback state value {root_dir,RootDir} mentioned in a type signature in the documentation, firstly, can be used as a server option for the undocumented tftp_file module, and secondly, would protect against relative path traversal above RootDir; then the file systems on the machine are open for remote read and writes without authentication, as the user and group running the Erlang VM.

File names in remote requests are, by the Erlang/OTP TFTP server, simply concatenated with RootDir, so if they contain "../" components the request can reach above RootDir and thereby any position in the file system hierarchy.

Note that Erlang/OTP TFTP server configurations that do not use root_dir allows remote file access anywhere on the system, with absolute or relative file paths, and this is a known property of the TFTP protocol as stated in RFC 1350 under Security Considerations, so it cannot be considered a vulnerability.

For a system to be vulnerable, the system designer must have used the undescribed {root_dir,RootDir} state as an option under incorrect assumptions. The state value/type is present in the documentation, in a function signature specification, but it is never described. It is only the option's name that may suggest that it could protect against relative path traversal.

To figure out how to use the root_dir state as an option, the system designer would have to consult the source code for the Erlang/OTP TFTP application and then it should be obvious that the option implements a file name concatenation that lack path traversal protection.

The system must also be reachable from untrusted hosts and contain confidential or precious data readable or writable by the OS user running the TFTP server, in this case the Erlang VM. For a TFTP installation without the root_dir option these points would be design errors, as the TFTP RFC 1359 indicates under Security Considerations.

Workarounds

  • Ensure that no sensitive or precious data is readable or writable by the OS user running the Erlang VM
  • Ensure that the TFTP server port on the machine running the Erlang/OTP TFTP server is not reachable from untrusted machines.
  • Use the connection_option() {reject,write} to tftp:start(ConnectionOptions) when starting the TFTP Server. This prevents write access.

Affected/Unaffected Versions

A version larger than or equal to one of the listed patched versions is unaffected; otherwise, a version that satisfies an expression listed under affected versions is affected, and if it does not, it is unaffected.

The documentation of the new OTP version scheme describes how versions should be compared. Note that versions used prior to OTP 17.0, when the new OTP version scheme was introduced, are never listed since it is not well defined how to compare those versions.

In the case of this vulnerability, versions prior to OTP 17.0 are also affected.

Credits

Thanks to Luigino Camastra at Aisle Research for finding and responsibly disclosing this vulnerability to the Erlang/OTP project.

Severity

Low

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v4 base metrics

Exploitability Metrics
Attack Vector Network
Attack Complexity Low
Attack Requirements Present
Privileges Required Low
User interaction None
Vulnerable System Impact Metrics
Confidentiality Low
Integrity Low
Availability None
Subsequent System Impact Metrics
Confidentiality None
Integrity None
Availability None

CVSS v4 base metrics

Exploitability Metrics
Attack Vector: This metric reflects the context by which vulnerability exploitation is possible. This metric value (and consequently the resulting severity) will be larger the more remote (logically, and physically) an attacker can be in order to exploit the vulnerable system. The assumption is that the number of potential attackers for a vulnerability that could be exploited from across a network is larger than the number of potential attackers that could exploit a vulnerability requiring physical access to a device, and therefore warrants a greater severity.
Attack Complexity: This metric captures measurable actions that must be taken by the attacker to actively evade or circumvent existing built-in security-enhancing conditions in order to obtain a working exploit. These are conditions whose primary purpose is to increase security and/or increase exploit engineering complexity. A vulnerability exploitable without a target-specific variable has a lower complexity than a vulnerability that would require non-trivial customization. This metric is meant to capture security mechanisms utilized by the vulnerable system.
Attack Requirements: This metric captures the prerequisite deployment and execution conditions or variables of the vulnerable system that enable the attack. These differ from security-enhancing techniques/technologies (ref Attack Complexity) as the primary purpose of these conditions is not to explicitly mitigate attacks, but rather, emerge naturally as a consequence of the deployment and execution of the vulnerable system.
Privileges Required: This metric describes the level of privileges an attacker must possess prior to successfully exploiting the vulnerability. The method by which the attacker obtains privileged credentials prior to the attack (e.g., free trial accounts), is outside the scope of this metric. Generally, self-service provisioned accounts do not constitute a privilege requirement if the attacker can grant themselves privileges as part of the attack.
User interaction: This metric captures the requirement for a human user, other than the attacker, to participate in the successful compromise of the vulnerable system. This metric determines whether the vulnerability can be exploited solely at the will of the attacker, or whether a separate user (or user-initiated process) must participate in some manner.
Vulnerable System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the VULNERABLE SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the VULNERABLE SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the VULNERABLE SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
Subsequent System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the SUBSEQUENT SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the SUBSEQUENT SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the SUBSEQUENT SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
CVSS:4.0/AV:N/AC:L/AT:P/PR:L/UI:N/VC:L/VI:L/VA:N/SC:N/SI:N/SA:N

CVE ID

CVE-2026-21620

Weaknesses

Relative Path Traversal

The product uses external input to construct a pathname that should be within a restricted directory, but it does not properly neutralize sequences such as .. that can resolve to a location that is outside of that directory. Learn more on MITRE.

Credits