diff --git a/deploy/docker-compose.yml b/deploy/docker-compose.yml
index 679ace5..903119d 100644
--- a/deploy/docker-compose.yml
+++ b/deploy/docker-compose.yml
@@ -32,6 +32,16 @@ services:
       PROMETHEUS_PAYMENT_ADDRESS: ${PROMETHEUS_PAYMENT_ADDRESS:-}
       PROMETHEUS_LISTING_FEE_USD: ${PROMETHEUS_LISTING_FEE_USD:-1}
       BNS_GATEWAY: ${BNS_GATEWAY:-}
+      # Sia-backed storage: when set, scan-clean uploads are pushed to the
+      # hosted s3d and the submit UI pre-fills the download URL. All four
+      # must be set for storage to turn on; otherwise publishers host
+      # their own binaries. See service/storage.js for defaults.
+      PROMETHEUS_S3_ENDPOINT: ${PROMETHEUS_S3_ENDPOINT:-}
+      PROMETHEUS_S3_ACCESS_KEY: ${PROMETHEUS_S3_ACCESS_KEY:-}
+      PROMETHEUS_S3_SECRET: ${PROMETHEUS_S3_SECRET:-}
+      PROMETHEUS_S3_BUCKET: ${PROMETHEUS_S3_BUCKET:-bns}
+      PROMETHEUS_S3_PREFIX: ${PROMETHEUS_S3_PREFIX:-prometheus-storage/}
+      PROMETHEUS_PUBLIC_BASE: ${PROMETHEUS_PUBLIC_BASE:-}
     volumes:
       - data:/data
     networks:
diff --git a/service/pages.js b/service/pages.js
index 6292c29..a40e797 100644
--- a/service/pages.js
+++ b/service/pages.js
@@ -506,7 +506,7 @@ export function submit() {
     <label>Download URL
       <input name="download_url" required type="url" placeholder="https://example.com/myapp-1.0.0.exe"
         style="width:100%;padding:10px;background:var(--panel);border:1px solid var(--line);border-radius:8px;color:var(--ink);font-family:ui-monospace,monospace;margin-top:.3rem">
-      <small style="color:var(--dim)">Where readers' clients fetch the binary. The SHA-256 (from step 1) must match what the URL serves.</small>
+      <small style="color:var(--dim)" id="download-url-hint">Where readers' clients fetch the binary. The SHA-256 (from step 1) must match what the URL serves.</small>
     </label>
 
     <label>Homepage (optional)
@@ -730,6 +730,18 @@ export function submit() {
       return;
     }
     W.upload = json;
+    // If the service hosted the file for us (Sia-backed storage is wired
+    // up), pre-fill the download_url input in step 2 so the publisher
+    // doesn't need external hosting. They can still override by typing
+    // their own URL — pre-fill is a convenience, not a lock.
+    if (json.hosted_url) {
+      const dlInput = document.querySelector('input[name="download_url"]');
+      if (dlInput && !dlInput.value) dlInput.value = json.hosted_url;
+      const hint = $('download-url-hint');
+      if (hint) hint.innerHTML = 'Pre-filled with the URL we hosted this file at. ' +
+        'Overwrite if you prefer to host it elsewhere — the SHA-256 (from step 1) ' +
+        'must match whatever the URL serves.';
+    }
     $('u-name').textContent = json.upload.name;
     $('u-size').textContent = fmtBytes(json.upload.size);
     $('u-hash').textContent = json.upload.hash.slice(0,8) + '…' + json.upload.hash.slice(-6);
diff --git a/service/s3.js b/service/s3.js
new file mode 100644
index 0000000..ebba004
--- /dev/null
+++ b/service/s3.js
@@ -0,0 +1,259 @@
+// A small S3 client for s3d: AWS SigV4 (path-style, us-east-1, UNSIGNED-PAYLOAD
+// for bodies so uploads stream straight through) plus the handful of bucket
+// and object calls the console needs.
+
+import crypto from 'node:crypto';
+import http from 'node:http';
+import https from 'node:https';
+import { Readable } from 'node:stream';
+
+export const REGION = 'us-east-1';
+export const SERVICE = 's3';
+
+const sha256 = (s) => crypto.createHash('sha256').update(s).digest('hex');
+const hmac = (k, s) => crypto.createHmac('sha256', k).update(s).digest();
+
+// RFC 3986 encoding as SigV4 wants it; '/' kept for object keys in paths.
+export function uriEncode(str, keepSlash) {
+  const out = encodeURIComponent(str)
+    .replace(/[!'()*]/g, (c) => '%' + c.charCodeAt(0).toString(16).toUpperCase());
+  // (A "never matches" regex like /$^/ is not one: it matches the empty
+  // string, which turned every empty query value into "/".)
+  return keepSlash ? out.replace(/%2F/g, '/') : out;
+}
+
+export function canonicalQuery(query) {
+  return Object.keys(query).sort()
+    .map((k) => `${uriEncode(k)}=${uriEncode(String(query[k] ?? ''))}`)
+    .join('&');
+}
+
+export function amzDate(d = new Date()) {
+  return d.toISOString().replace(/[-:]/g, '').replace(/\.\d{3}/, '');
+}
+
+export function signingKey(secret, date) {
+  return hmac(hmac(hmac(hmac('AWS4' + secret, date), REGION), SERVICE), 'aws4_request');
+}
+
+export function objectPath(bucket, key) {
+  return '/' + uriEncode(bucket) + (key != null ? '/' + uriEncode(key, true) : '');
+}
+
+export class S3Client {
+  // endpoint: 'http://127.0.0.1:8000'
+  constructor({ endpoint, accessKeyId, secretKey }) {
+    this.endpoint = new URL(endpoint);
+    this.accessKeyId = accessKeyId;
+    this.secretKey = secretKey;
+  }
+
+  sign(method, path, query, headers, { now = amzDate(), payloadHash = 'UNSIGNED-PAYLOAD' } = {}) {
+    const date = now.slice(0, 8);
+    const h = { ...headers, host: this.endpoint.host, 'x-amz-date': now, 'x-amz-content-sha256': payloadHash };
+    const names = Object.keys(h).map((k) => k.toLowerCase()).sort();
+    const lower = Object.fromEntries(Object.entries(h).map(([k, v]) => [k.toLowerCase(), String(v).trim()]));
+    const signed = names.join(';');
+    const canonical = [method, path, canonicalQuery(query), names.map((n) => `${n}:${lower[n]}\n`).join(''), signed, payloadHash].join('\n');
+    const scope = `${date}/${REGION}/${SERVICE}/aws4_request`;
+    const toSign = ['AWS4-HMAC-SHA256', now, scope, sha256(canonical)].join('\n');
+    const sig = crypto.createHmac('sha256', signingKey(this.secretKey, date)).update(toSign).digest('hex');
+    lower.authorization = `AWS4-HMAC-SHA256 Credential=${this.accessKeyId}/${scope}, SignedHeaders=${signed}, Signature=${sig}`;
+    delete lower.host;
+    return lower;
+  }
+
+  // Presigned GET for share links. publicEndpoint lets the link name the
+  // address readers will use, which may differ from the loopback the GUI uses.
+  presign(bucket, key, expiresSec = 3600, publicEndpoint, now = amzDate()) {
+    const base = publicEndpoint ? new URL(publicEndpoint) : this.endpoint;
+    const date = now.slice(0, 8);
+    const scope = `${date}/${REGION}/${SERVICE}/aws4_request`;
+    const path = objectPath(bucket, key);
+    const query = {
+      'X-Amz-Algorithm': 'AWS4-HMAC-SHA256',
+      'X-Amz-Credential': `${this.accessKeyId}/${scope}`,
+      'X-Amz-Date': now,
+      'X-Amz-Expires': String(Math.min(Math.max(1, expiresSec | 0), 604800)),
+      'X-Amz-SignedHeaders': 'host',
+    };
+    const canonical = ['GET', path, canonicalQuery(query), `host:${base.host}\n`, 'host', 'UNSIGNED-PAYLOAD'].join('\n');
+    const toSign = ['AWS4-HMAC-SHA256', now, scope, sha256(canonical)].join('\n');
+    query['X-Amz-Signature'] = crypto.createHmac('sha256', signingKey(this.secretKey, date)).update(toSign).digest('hex');
+    return `${base.origin}${path}?${canonicalQuery(query)}`;
+  }
+
+  // Low-level request. body: Buffer | string | Readable | undefined.
+  // Resolves the raw http.IncomingMessage so callers can stream it.
+  request(method, path, { query = {}, headers = {}, body, payloadHash } = {}) {
+    const signedHeaders = this.sign(method, path, query, headers, payloadHash ? { payloadHash } : undefined);
+    const qs = canonicalQuery(query);
+    // The hosted gateway is https; a local s3d is plain http on loopback.
+    const transport = this.endpoint.protocol === 'https:' ? https : http;
+    return new Promise((resolve, reject) => {
+      const req = transport.request({
+        protocol: this.endpoint.protocol,
+        hostname: this.endpoint.hostname,
+        port: this.endpoint.port,
+        method,
+        path: path + (qs ? '?' + qs : ''),
+        headers: signedHeaders,
+      }, resolve);
+      req.on('error', reject);
+      if (body instanceof Readable) body.pipe(req);
+      else req.end(body);
+    });
+  }
+
+  async call(method, path, opts) {
+    const res = await this.request(method, path, opts);
+    const text = await readBody(res);
+    if (res.statusCode >= 300) throw s3Error(res.statusCode, text);
+    return { status: res.statusCode, headers: res.headers, text };
+  }
+
+  async listBuckets() {
+    const { text } = await this.call('GET', '/');
+    return blocks(text, 'Bucket').map((b) => ({ name: tag(b, 'Name'), created: tag(b, 'CreationDate') }));
+  }
+
+  createBucket(bucket) { return this.call('PUT', objectPath(bucket)); }
+  deleteBucket(bucket) { return this.call('DELETE', objectPath(bucket)); }
+
+  async listObjects(bucket, { prefix = '', delimiter = '/', token, max = 1000 } = {}) {
+    const query = { 'list-type': '2', prefix, delimiter, 'max-keys': String(max) };
+    if (token) query['continuation-token'] = token;
+    const { text } = await this.call('GET', objectPath(bucket), { query });
+    return {
+      folders: blocks(text, 'CommonPrefixes').map((b) => tag(b, 'Prefix')),
+      objects: blocks(text, 'Contents').map((b) => ({
+        key: tag(b, 'Key'), size: Number(tag(b, 'Size')), modified: tag(b, 'LastModified'), etag: tag(b, 'ETag'),
+      })),
+      truncated: tag(text, 'IsTruncated') === 'true',
+      nextToken: tag(text, 'NextContinuationToken'),
+    };
+  }
+
+  async deleteObject(bucket, key) { return this.call('DELETE', objectPath(bucket, key)); }
+
+  // Server-side copy. s3d copies the entry in its own database: the copy
+  // points at the same data on Sia, so nothing is uploaded again.
+  copyObject(srcBucket, srcKey, dstBucket, dstKey) {
+    return this.call('PUT', objectPath(dstBucket, dstKey), { headers: { 'x-amz-copy-source': objectPath(srcBucket, srcKey) } });
+  }
+
+  // Every key under prefix (no folder grouping), all pages.
+  async allKeys(bucket, prefix = '') {
+    const keys = [];
+    let token;
+    do {
+      const page = await this.listObjects(bucket, { prefix, delimiter: '', token });
+      keys.push(...page.objects.map((o) => o.key));
+      token = page.truncated ? page.nextToken : null;
+    } while (token);
+    return keys;
+  }
+
+  // S3 has no rename: copy to the new name, then delete the old one.
+  async renameObject(bucket, from, to, dstBucket = bucket) {
+    await this.copyObject(bucket, from, dstBucket, to);
+    await this.deleteObject(bucket, from);
+  }
+
+  // Rename a folder (or move a whole drive's contents): every key under
+  // `from` moves under `to`. Keys are listed first, so the loop never
+  // walks into the copies it makes. Returns the number moved.
+  async renamePrefix(bucket, from, to, dstBucket = bucket) {
+    const keys = await this.allKeys(bucket, from);
+    for (const k of keys) await this.renameObject(bucket, k, to + k.slice(from.length), dstBucket);
+    return keys.length;
+  }
+
+  // Deletes every key under prefix. Returns the number deleted.
+  async deletePrefix(bucket, prefix) {
+    let n = 0;
+    let token;
+    do {
+      const page = await this.listObjects(bucket, { prefix, delimiter: '', token });
+      for (const o of page.objects) { await this.deleteObject(bucket, o.key); n++; }
+      token = page.truncated ? page.nextToken : null;
+    } while (token);
+    return n;
+  }
+
+  putObject(bucket, key, body, { contentType, contentLength } = {}) {
+    const headers = {};
+    if (contentType) headers['content-type'] = contentType;
+    if (contentLength != null) headers['content-length'] = String(contentLength);
+    return this.call('PUT', objectPath(bucket, key), { headers, body });
+  }
+
+  getObject(bucket, key, { range } = {}) {
+    return this.request('GET', objectPath(bucket, key), { headers: range ? { range } : {} });
+  }
+
+  async getPolicy(bucket) {
+    const res = await this.request('GET', objectPath(bucket), { query: { policy: '' } });
+    const text = await readBody(res);
+    if (res.statusCode === 404) return null;
+    if (res.statusCode >= 300) throw s3Error(res.statusCode, text);
+    return JSON.parse(text);
+  }
+
+  putPolicy(bucket, policy) {
+    const body = JSON.stringify(policy);
+    return this.call('PUT', objectPath(bucket), { query: { policy: '' }, body, headers: { 'content-type': 'application/json', 'content-length': Buffer.byteLength(body) } });
+  }
+
+  deletePolicy(bucket) { return this.call('DELETE', objectPath(bucket), { query: { policy: '' } }); }
+}
+
+// The only policy shape s3d accepts: anonymous read of objects and listing.
+export function publicReadPolicy(bucket, { list = false } = {}) {
+  const statements = [{
+    Effect: 'Allow', Principal: '*', Action: ['s3:GetObject'], Resource: [`arn:aws:s3:::${bucket}/*`],
+  }];
+  if (list) statements.push({ Effect: 'Allow', Principal: '*', Action: ['s3:ListBucket'], Resource: [`arn:aws:s3:::${bucket}`] });
+  return { Version: '2012-10-17', Statement: statements };
+}
+
+export function readBody(res) {
+  return new Promise((resolve, reject) => {
+    const chunks = [];
+    res.on('data', (c) => chunks.push(c));
+    res.on('end', () => resolve(Buffer.concat(chunks).toString('utf8')));
+    res.on('error', reject);
+  });
+}
+
+export class S3Error extends Error {
+  constructor(status, code, message) { super(message || code); this.status = status; this.code = code; }
+}
+
+function s3Error(status, text) {
+  return new S3Error(status, tag(text, 'Code') || `HTTP ${status}`, tag(text, 'Message') || text.slice(0, 200) || `HTTP ${status}`);
+}
+
+// S3 responses are flat enough that tag extraction is all we need.
+function blocks(xml, name) {
+  const re = new RegExp(`<${name}>([\\s\\S]*?)</${name}>`, 'g');
+  const out = [];
+  let m;
+  while ((m = re.exec(xml))) out.push(m[1]);
+  return out;
+}
+
+function tag(xml, name) {
+  const m = xml.match(new RegExp(`<${name}>([\\s\\S]*?)</${name}>`));
+  return m ? unescapeXml(m[1]) : null;
+}
+
+function unescapeXml(s) {
+  return s.replace(/&(lt|gt|amp|quot|apos|#\d+|#x[0-9a-f]+);/gi, (_, e) => {
+    switch (e.toLowerCase()) {
+      case 'lt': return '<'; case 'gt': return '>'; case 'amp': return '&';
+      case 'quot': return '"'; case 'apos': return "'";
+      default: return String.fromCodePoint(e[1].toLowerCase() === 'x' ? parseInt(e.slice(2), 16) : parseInt(e.slice(1), 10));
+    }
+  });
+}
diff --git a/service/storage.js b/service/storage.js
new file mode 100644
index 0000000..4677ca8
--- /dev/null
+++ b/service/storage.js
@@ -0,0 +1,127 @@
+/**
+ * Sia-backed storage for scan-clean uploads.
+ *
+ * When the service is configured with S3 credentials pointing at the Silent
+ * Mode hosted s3d (s3.silentmode.st), every clean upload also goes to Sia and
+ * the public download URL is handed back to the submit UI. The publisher no
+ * longer has to self-host — their binary gets a stable https URL right out
+ * of step 1.
+ *
+ * Design notes
+ * ------------
+ * - Writes go to `s3://${bucket}/${prefix}<sha256>.bin` by content hash.
+ *   Collisions on hash are a no-op: the same bytes always land at the same
+ *   key, so re-uploads do not duplicate storage.
+ * - Downloads are served through the s3 gateway's public URL
+ *   (`${publicBase}/${bucket}/${prefix}<sha256>.bin`). The bns bucket on
+ *   s3.silentmode.st is already public-read so site mirrors and Pithos
+ *   share-links work from any client — nothing extra to configure here.
+ * - If storage is not configured, isConfigured() returns false and the
+ *   service falls back to its old behavior (publisher provides their own
+ *   download_url). That keeps dev loops working without creds.
+ * - Content-Type is passed through so the gateway serves apk/exe with the
+ *   right MIME, which matters for mobile download prompts.
+ *
+ * Env
+ * ---
+ *   PROMETHEUS_S3_ENDPOINT     e.g. https://s3.silentmode.st
+ *   PROMETHEUS_S3_ACCESS_KEY   access key for a sia-storage user
+ *   PROMETHEUS_S3_SECRET       matching secret
+ *   PROMETHEUS_S3_BUCKET       default: bns
+ *   PROMETHEUS_S3_PREFIX       default: prometheus-storage/
+ *   PROMETHEUS_PUBLIC_BASE     default: same as PROMETHEUS_S3_ENDPOINT
+ */
+
+import { createReadStream } from 'node:fs';
+import { stat } from 'node:fs/promises';
+import { S3Client } from './s3.js';
+
+const CFG = {
+  endpoint: process.env.PROMETHEUS_S3_ENDPOINT || '',
+  accessKey: process.env.PROMETHEUS_S3_ACCESS_KEY || '',
+  secret: process.env.PROMETHEUS_S3_SECRET || '',
+  bucket: process.env.PROMETHEUS_S3_BUCKET || 'bns',
+  prefix: normalizePrefix(process.env.PROMETHEUS_S3_PREFIX || 'prometheus-storage/'),
+  publicBase: (process.env.PROMETHEUS_PUBLIC_BASE || process.env.PROMETHEUS_S3_ENDPOINT || '').replace(/\/+$/, ''),
+};
+
+let client = null;
+
+export function isConfigured() {
+  return Boolean(CFG.endpoint && CFG.accessKey && CFG.secret);
+}
+
+export function describe() {
+  if (!isConfigured()) return { configured: false };
+  return {
+    configured: true,
+    endpoint: CFG.endpoint,
+    bucket: CFG.bucket,
+    prefix: CFG.prefix,
+    publicBase: CFG.publicBase,
+  };
+}
+
+function getClient() {
+  if (!isConfigured()) return null;
+  if (!client) {
+    client = new S3Client({
+      endpoint: CFG.endpoint,
+      accessKeyId: CFG.accessKey,
+      secretKey: CFG.secret,
+    });
+  }
+  return client;
+}
+
+/**
+ * Upload a scan-clean file to Sia and return the public URL readers can fetch.
+ *
+ * Idempotent: if an object with the same key already exists (same sha256 →
+ * same bytes), the PUT overwrites with identical content and the URL is the
+ * same. The returned URL is stable across reruns, so a listing that already
+ * references it stays valid.
+ *
+ * @param {object} args
+ * @param {string} args.localPath — absolute path to the file on disk
+ * @param {string} args.sha256    — content hash, used as the object key
+ * @param {string} args.name      — original filename (for the display suffix)
+ * @param {string} args.mime      — content-type for the stored object
+ * @returns {Promise<{ downloadUrl: string, key: string, bytes: number }>}
+ */
+export async function uploadScannedFile({ localPath, sha256, name, mime }) {
+  const c = getClient();
+  if (!c) throw new Error('storage not configured — set PROMETHEUS_S3_*');
+
+  const { size } = await stat(localPath);
+  const key = `${CFG.prefix}${sha256}.bin`;
+  const body = createReadStream(localPath);
+  const contentType = mime || 'application/octet-stream';
+
+  // The s3d upstream wants Content-Length for streaming PUTs; without it
+  // s3d falls back to transfer-encoding: chunked which Sia's gateway does
+  // not pass through cleanly on large bodies. Pithos has the same gotcha.
+  await c.putObject(CFG.bucket, key, body, {
+    contentType,
+    contentLength: size,
+  });
+
+  // Content-Disposition is a client decision, not something we persist on
+  // the object; the UI can add it to the download link if needed. We just
+  // hand back the raw public URL — stable, cacheable, hash-addressed.
+  const downloadUrl = publicUrlFor(key);
+  return { downloadUrl, key, bytes: size };
+}
+
+export function publicUrlFor(key) {
+  if (!CFG.publicBase) return null;
+  // Path-style URL: the bucket is a path segment, matching how s3d exposes
+  // the hosted gateway. (Virtual-hosted URLs would need per-bucket DNS,
+  // which the public gateway does not set up.)
+  return `${CFG.publicBase}/${encodeURIComponent(CFG.bucket)}/${encodeURI(key)}`;
+}
+
+function normalizePrefix(p) {
+  const t = String(p || '').replace(/^\/+/, '');
+  return t.endsWith('/') || t === '' ? t : t + '/';
+}
diff --git a/service/uploads.js b/service/uploads.js
index e19332e..deea627 100644
--- a/service/uploads.js
+++ b/service/uploads.js
@@ -32,6 +32,7 @@ import { PassThrough } from 'node:stream';
 import Busboy from 'busboy';
 import { scanStream } from './clamd.js';
 import { signReport } from './signing.js';
+import { isConfigured as storageConfigured, uploadScannedFile } from './storage.js';
 
 const UPLOAD_DIR = process.env.PROMETHEUS_UPLOAD_DIR
   || resolve(dirname(fileURLToPath(import.meta.url)), '..', 'uploads');
@@ -162,6 +163,7 @@ export async function handleUpload(req, res) {
 
   let stored = false;
   let finalPath = null;
+  let hostedUrl = null;
 
   if (scan.verdict === 'clean') {
     finalPath = resolve(UPLOAD_DIR, `${sha256}.bin`);
@@ -180,6 +182,27 @@ export async function handleUpload(req, res) {
       stored = true;
       finalPath = resolve(UPLOAD_DIR, `${sha256}.bin`);
     }
+
+    // Push to Sia-backed storage if the service is configured for it, so
+    // the submit UI can skip the "where will you host this?" question. A
+    // failure here is non-fatal — the local scan report still works and
+    // the publisher can fall back to providing their own download_url.
+    if (storageConfigured()) {
+      try {
+        const result = await uploadScannedFile({
+          localPath: finalPath,
+          sha256,
+          name: originalName,
+          mime: mimeType,
+        });
+        hostedUrl = result.downloadUrl;
+      } catch (err) {
+        // Keep going — the local copy stays on disk and the publisher can
+        // paste their own URL in step 2. Logged for the operator.
+        console.error('[prometheus] sia upload failed:',
+          err?.message || String(err));
+      }
+    }
   } else {
     // Infected or error — the bytes do not stay on our disk.
     await safeUnlink(tempPath);
@@ -222,6 +245,11 @@ export async function handleUpload(req, res) {
       ...(scan.virustotal ? { virustotal: scan.virustotal } : {}),
     },
     stored,
+    // When Sia-backed storage is wired up and the push succeeded, this is
+    // the public download URL the UI pre-fills in step 2 so the publisher
+    // doesn't need external hosting. Null when storage is not configured
+    // or the push failed — publisher pastes their own URL.
+    hosted_url: hostedUrl,
     report,
   });
 }