CVE-2026-120338 Vulnerability Analysis & Exploit Details

CVE-2026-120338 Vulnerability Summary

CVE-2026-120338: Prefab UI's Generative UI sandbox runs caller-supplied Python inside Pyodide, Python compiled to WebAssembly, in a Deno subprocess. The feature is documented as isolation for untrusted code, including LLM-written UI. From prefab-ui 0.14.0 through 0.20.2 the launcher starts Deno with unrestricted --allow-read, --allow-write, and --allow-net, and the runner leaves the Deno API on the JavaScript global object. Pyodide's js module proxies that global, so the Python can call Deno.readTextFileSync and Deno.writeTextFileSync. A caller who can supply the code can read and write files as the server OS user, and can persist code execution by writing a .pth file into the server virtualenv's site-packages. The usual remote path is FastMCP's GenerativeUI provider, which registers the MCP tool generate_prefab_ui and forwards that tool's code argument into this sandbox. On a FastMCP server with no authentication configured, any client who can reach the server can call the tool. With authentication, any client allowed to call tools can. Generative UI is opt-in, and the host must have Deno installed. Confirmed on prefab-ui 0.20.2. Fixed in 0.21.0: after the runtime and Prefab sources load, the runner revokes Deno read, write, and network permission before it accepts code, the process is started with --no-prompt so a permission request cannot grant them again, and bootstrap network access is limited to cdn.jsdelivr.net. The fix is commit 030f074 (pull request 538).

CVE-2026-120338 References

External References

• https://github.com/PrefectHQ/prefab
• https://github.com/PrefectHQ/prefab/commit/030f074f85e0b1be0dd44b0283185aecb26337a2
• https://github.com/PrefectHQ/prefab/pull/538
• https://github.com/PrefectHQ/prefab/releases/tag/v0.14.0
• https://github.com/PrefectHQ/prefab/releases/tag/v0.20.2
• https://github.com/PrefectHQ/prefab/releases/tag/v0.21.0
• https://github.com/PrefectHQ/prefab/security/advisories/GHSA-595q-jv4r-q7j9
• https://pypi.org/project/prefab-ui/
• https://pypi.org/project/prefab-ui/0.21.0/

CWE Common Weakness Enumeration

CAPEC Common Attack Pattern Enumeration and Classification

• Accessing Functionality Not Properly Constrained by ACLs CAPEC-1 In applications, particularly web applications, access to functionality is mitigated by an authorization framework. This framework maps Access Control Lists (ACLs) to elements of the application's functionality; particularly URL's for web apps. In the case that the administrator failed to specify an ACL for a particular element, an attacker may be able to access it with impunity. An attacker with the ability to access functionality not properly constrained by ACLs can obtain sensitive information and possibly compromise the entire application. Such an attacker can access resources that must be available only to users at a higher privilege level, can access management sections of the application, or can run queries for data that they otherwise not supposed to.
• Cross Site Tracing CAPEC-107 Cross Site Tracing (XST) enables an adversary to steal the victim's session cookie and possibly other authentication credentials transmitted in the header of the HTTP request when the victim's browser communicates to a destination system's web server.
• Directory Indexing CAPEC-127 An adversary crafts a request to a target that results in the target listing/indexing the content of a directory as output. One common method of triggering directory contents as output is to construct a request containing a path that terminates in a directory name rather than a file name since many applications are configured to provide a list of the directory's contents when such a request is received. An adversary can use this to explore the directory tree on a target as well as learn the names of files. This can often end up revealing test files, backup files, temporary files, hidden files, configuration files, user accounts, script contents, as well as naming conventions, all of which can be used by an attacker to mount additional attacks.
• Using Malicious Files CAPEC-17 An attack of this type exploits a system's configuration that allows an adversary to either directly access an executable file, for example through shell access; or in a possible worst case allows an adversary to upload a file and then execute it. Web servers, ftp servers, and message oriented middleware systems which have many integration points are particularly vulnerable, because both the programmers and the administrators must be in synch regarding the interfaces and the correct privileges for each interface.
• Encryption Brute Forcing CAPEC-20 An attacker, armed with the cipher text and the encryption algorithm used, performs an exhaustive (brute force) search on the key space to determine the key that decrypts the cipher text to obtain the plaintext.
• Exploiting Trust in Client CAPEC-22 An attack of this type exploits vulnerabilities in client/server communication channel authentication and data integrity. It leverages the implicit trust a server places in the client, or more importantly, that which the server believes is the client. An attacker executes this type of attack by communicating directly with the server where the server believes it is communicating only with a valid client. There are numerous variations of this type of attack.
• Escaping a Sandbox by Calling Code in Another Language CAPEC-237 The attacker may submit malicious code of another language to obtain access to privileges that were not intentionally exposed by the sandbox, thus escaping the sandbox. For instance, Java code cannot perform unsafe operations, such as modifying arbitrary memory locations, due to restrictions placed on it by the Byte code Verifier and the JVM. If allowed, Java code can call directly into native C code, which may perform unsafe operations, such as call system calls and modify arbitrary memory locations on their behalf. To provide isolation, Java does not grant untrusted code with unmediated access to native C code. Instead, the sandboxed code is typically allowed to call some subset of the pre-existing native code that is part of standard libraries.
• Using Unpublished Interfaces or Functionality CAPEC-36 An adversary searches for and invokes interfaces or functionality that the target system designers did not intend to be publicly available. If interfaces fail to authenticate requests, the attacker may be able to invoke functionality they are not authorized for.
• Signature Spoofing by Mixing Signed and Unsigned Content CAPEC-477 An attacker exploits the underlying complexity of a data structure that allows for both signed and unsigned content, to cause unsigned data to be processed as though it were signed data.
• Escaping Virtualization CAPEC-480 An adversary gains access to an application, service, or device with the privileges of an authorized or privileged user by escaping the confines of a virtualized environment. The adversary is then able to access resources or execute unauthorized code within the host environment, generally with the privileges of the user running the virtualized process. Successfully executing an attack of this type is often the first step in executing more complex attacks.
• Poison Web Service Registry CAPEC-51 SOA and Web Services often use a registry to perform look up, get schema information, and metadata about services. A poisoned registry can redirect (think phishing for servers) the service requester to a malicious service provider, provide incorrect information in schema or metadata, and delete information about service provider interfaces.
• Utilizing REST's Trust in the System Resource to Obtain Sensitive Data CAPEC-57 This attack utilizes a REST(REpresentational State Transfer)-style applications' trust in the system resources and environment to obtain sensitive data once SSL is terminated.
• Session Credential Falsification through Prediction CAPEC-59 This attack targets predictable session ID in order to gain privileges. The attacker can predict the session ID used during a transaction to perform spoofing and session hijacking.
• Sniff Application Code CAPEC-65 An adversary passively sniffs network communications and captures application code bound for an authorized client. Once obtained, they can use it as-is, or through reverse-engineering glean sensitive information or exploit the trust relationship between the client and server. Such code may belong to a dynamic update to the client, a patch being applied to a client component or any such interaction where the client is authorized to communicate with the server.
• Key Negotiation of Bluetooth Attack (KNOB) CAPEC-668 An adversary can exploit a flaw in Bluetooth key negotiation allowing them to decrypt information sent between two devices communicating via Bluetooth. The adversary uses an Adversary in the Middle setup to modify packets sent between the two devices during the authentication process, specifically the entropy bits. Knowledge of the number of entropy bits will allow the attacker to easily decrypt information passing over the line of communication.
• Manipulating State CAPEC-74 The adversary modifies state information maintained by the target software or causes a state transition in hardware. If successful, the target will use this tainted state and execute in an unintended manner. State management is an important function within a software application. User state maintained by the application can include usernames, payment information, browsing history as well as application-specific contents such as items in a shopping cart. Manipulating user state can be employed by an adversary to elevate privilege, conduct fraudulent transactions or otherwise modify the flow of the application to derive certain benefits. If there is a hardware logic error in a finite state machine, the adversary can use this to put the system in an undefined state which could cause a denial of service or exposure of secure data.
• Forceful Browsing CAPEC-87 An attacker employs forceful browsing (direct URL entry) to access portions of a website that are otherwise unreachable. Usually, a front controller or similar design pattern is employed to protect access to portions of a web application. Forceful browsing enables an attacker to access information, perform privileged operations and otherwise reach sections of the web application that have been improperly protected.

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