CVE-2026-27898: Vaultwarden: Unauthorized Access via Partial Update API on Another User’s Cipher

Summary

In the test environment, it was confirmed that an authenticated regular user can specify another user’s cipherid and call:

PUT /api/ciphers/{id}/partial

Even though the standard retrieval API correctly denies access to that cipher, the partial update endpoint returns 200 OK and exposes cipherDetails (including name, notes, data, secureNote, etc.).

Description

putcipherpartial retrieves the target Cipher but does not perform ownership or access control checks before returning tojson. Authorization checks present in the normal update API are missing here. src/api/core/ciphers.rs:717

rust let Some(cipher) = Cipher::findbyuuid(&cipherid, &conn).await else { err!("Cipher doesn't exist") };

if let Some(ref folderid) = data.folderid { if Folder::findbyuuidanduser(folderid, &headers.user.uuid, &conn).await.isnone() { err!("Invalid folder", "Folder does not exist or belongs to another user"); } }

// Move cipher cipher.movetofolder(data.folderid.clone(), &headers.user.uuid, &conn).await?;

// Update favorite cipher.setfavorite(Some(data.favorite), &headers.user.uuid, &conn).await?;

Ok(Json(cipher.tojson(&headers.host, &headers.user.uuid, None, CipherSyncType::User, &conn).await?))

By comparison, the standard update API includes an explicit authorization check: src/api/core/ciphers.rs:688

rust if !cipher.iswriteaccessibletouser(&headers.user.uuid, &conn).await { err!("Cipher is not write accessible") }

The tojson method does not abort processing when access restrictions are not met; instead, it proceeds to construct and return a detailed response. src/db/models/cipher.rs:175

rust let (readonly, hidepasswords, ) = if synctype == CipherSyncType::User { match self.getaccessrestrictions(useruuid, ciphersyncdata, conn).await { Some((ro, hp, mn)) => (ro, hp, mn), None => { error!("Cipher ownership assertion failure"); (true, true, false) } } } else { (false, false, false) }; src/db/models/cipher.rs:335

rust let mut jsonobject = json!({ "object": "cipherDetails", "id": self.uuid, "type": self.atype, ... "name": self.name, "notes": self.notes, "fields": fieldsjson, "data": datajson, ... });

Preconditions

The attacker possesses a valid regular-user JWT (Bearer token). The attacker knows the target (victim) cipherid.

Steps to Reproduce

1. Prepare the attacker JWT and victim cipherid (preconditions). 2. Baseline check: confirm that standard retrieval is denied. <img width="2014" height="855" alt="image" src="https://github.com/user-attachments/assets/32b12cc9-3672-4a88-afd0-ef7715474662" />

3. Execute the vulnerable API. Confirm that 200 OK is returned and that cipherDetails includes fields such as id, name, notes, secureNote, etc. <img width="2018" height="1113" alt="image" src="https://github.com/user-attachments/assets/341b330c-8d55-4f06-a622-0d7da28f62fd" />

Potential Impact

Unauthorized disclosure of other users’ cipher information (confidentiality breach). Creation of unauthorized associations within the attacker’s user context (e.g., favorite or folder operations). The response from /api/ciphers/<cipherid>/partial includes attachments[].url.

In filesystem (FS) deployments, this returns a tokenized endpoint such as:

/attachments/<cipher>/<file>?token=...

In object storage deployments, it returns a short-lived pre-signed URL.

As a result, an attacker can use these URLs to directly download attachment data that they are not authorized to access.

This can lead to disclosure of sensitive information stored in the Vault, including personal data and authentication credentials. Such exposure may further result in account compromise, lateral movement, and other secondary impacts.