diff --git a/modules/yieldmoBidAdapter.js b/modules/yieldmoBidAdapter.js
index f606194e132..425f1d7af11 100644
--- a/modules/yieldmoBidAdapter.js
+++ b/modules/yieldmoBidAdapter.js
@@ -12,6 +12,7 @@ import {
   isNumber,
   isStr,
   logError,
+  logWarn,
   parseQueryStringParameters,
   parseUrl
 } from '../src/utils.js';
@@ -59,7 +60,7 @@ export const spec = {
    */
   isBidRequestValid: function (bid) {
     return !!(bid && bid.adUnitCode && bid.bidId && (hasBannerMediaType(bid) || hasVideoMediaType(bid)) &&
-      validateVideoParams(bid));
+      validateVideoParams(bid) && validateBlocklistParams(bid));
   },
 
   /**
@@ -151,6 +152,18 @@ export const spec = {
       if (eids.length) {
         serverRequest.eids = JSON.stringify(eids);
       };
+
+      // Blocklists (request-level): merge ortb2 + params, send as comma-delimited
+      // params per the ad server's prebid-js endpoint (AS-5349). Omitted when empty.
+      const bcat = getBlocklist(bidderRequest, bannerBidRequests[0], 'bcat');
+      if (bcat.length) {
+        serverRequest.bcat = bcat.join(',');
+      }
+      const badv = getBlocklist(bidderRequest, bannerBidRequests[0], 'badv');
+      if (badv.length) {
+        serverRequest.badv = badv.join(',');
+      }
+
       // check if url exceeded max length
       const fullUrl = `${bannerUrl}?${parseQueryStringParameters(serverRequest)}`;
       let extraCharacters = fullUrl.length - MAX_BANNER_REQUEST_URL_LENGTH;
@@ -398,6 +411,40 @@ function getId(request, idType) {
   return (typeof deepAccess(request, 'userId') === 'object') ? request.userId[idType] : undefined;
 }
 
+/**
+ * Resolve a request-level blocklist field (bcat/badv) from its two sources — the
+ * standardized ORTB global (`bidderRequest.ortb2.<field>`) and the Yieldmo-specific
+ * param (`bid.params.<field>`) — into a single deduped array of strings. Neither
+ * source is allowed to silently win (union, not precedence). Invalid values are
+ * ignored and logged rather than dropping the bid: a source that isn't an array, and
+ * any non-string/empty element, are filtered out (with a warning) so a
+ * misconfiguration is surfaced without losing the impression.
+ * @param {BidderRequest} bidderRequest bidder request (source of ortb2.<field>)
+ * @param {BidRequest} bid bid request (source of params.<field>)
+ * @param {string} field blocklist field name — 'bcat' or 'badv'
+ * @return {string[]} deduped, trimmed, non-empty string entries (possibly empty)
+ */
+function getBlocklist(bidderRequest, bid, field) {
+  const normalize = (value, source) => {
+    if (value === undefined || value === null) {
+      return [];
+    }
+    if (!isArray(value)) {
+      logWarn(`yieldmo: ignoring ${source} blocklist value; expected an array of strings, got ${JSON.stringify(value)}`);
+      return [];
+    }
+    const dropped = value.filter(item => !isStr(item) || !item.trim());
+    if (dropped.length) {
+      logWarn(`yieldmo: ignoring invalid ${source} blocklist entries (expected non-empty strings): ${JSON.stringify(dropped)}`);
+    }
+    return value.filter(item => isStr(item) && item.trim()).map(item => item.trim());
+  };
+  return [...new Set([
+    ...normalize(deepAccess(bidderRequest, `ortb2.${field}`), 'ortb2'),
+    ...normalize(deepAccess(bid, `params.${field}`), 'params'),
+  ])];
+}
+
 /**
  * @param {BidRequest[]} bidRequests bid request object
  * @param {BidderRequest} bidderRequest bidder request object
@@ -412,8 +459,8 @@ function openRtbRequest(bidRequests, bidderRequest) {
     imp: bidRequests.map(bidRequest => openRtbImpression(bidRequest)),
     site: openRtbSite(bidRequests[0], bidderRequest),
     device: deepAccess(bidderRequest, 'ortb2.device'),
-    badv: bidRequests[0].params.badv || [],
-    bcat: deepAccess(bidderRequest, 'bcat') || bidRequests[0].params.bcat || [],
+    badv: getBlocklist(bidderRequest, bidRequests[0], 'badv'),
+    bcat: getBlocklist(bidderRequest, bidRequests[0], 'bcat'),
     ext: {
       prebid: '$prebid.version$',
     },
@@ -602,6 +649,54 @@ function populateOpenRtbGdpr(openRtbRequest, bidderRequest) {
   }
 }
 
+const isDefined = val => typeof val !== 'undefined';
+
+const paramRequired = (paramStr, value, conditionStr) => {
+  let error = `"${paramStr}" is required`;
+  if (conditionStr) {
+    error += ' when ' + conditionStr;
+  }
+  throw new Error(error);
+};
+
+const paramInvalid = (paramStr, value, expectedStr) => {
+  expectedStr = expectedStr ? ', expected: ' + expectedStr : '';
+  value = JSON.stringify(value);
+  throw new Error(`"${paramStr}"=${value} is invalid${expectedStr}`);
+};
+
+/**
+ * Build a field validator bound to a bid. `video.*` paths are checked against both
+ * `params.video.*` and `mediaTypes.video.*`; any other path is read directly.
+ * The error callback (paramRequired/paramInvalid) throws, so callers wrap in try/catch.
+ * @param {BidRequest} bid bid request
+ * @return {(fieldPath: string, validateCb: Function, errorCb: Function, errorCbParam?: string) => *}
+ */
+const createParamValidator = (bid) => (fieldPath, validateCb, errorCb, errorCbParam) => {
+  if (fieldPath.indexOf('video') === 0) {
+    const valueFieldPath = 'params.' + fieldPath;
+    const mediaFieldPath = 'mediaTypes.' + fieldPath;
+    const valueParams = deepAccess(bid, valueFieldPath);
+    const mediaTypesParams = deepAccess(bid, mediaFieldPath);
+    const hasValidValueParams = validateCb(valueParams);
+    const hasValidMediaTypesParams = validateCb(mediaTypesParams);
+
+    if (hasValidValueParams) return valueParams;
+    else if (hasValidMediaTypesParams) return hasValidMediaTypesParams;
+    else {
+      if (!hasValidValueParams) errorCb(valueFieldPath, valueParams, errorCbParam);
+      else if (!hasValidMediaTypesParams) errorCb(mediaFieldPath, mediaTypesParams, errorCbParam);
+    }
+    return valueParams || mediaTypesParams;
+  } else {
+    const value = deepAccess(bid, fieldPath);
+    if (!validateCb(value)) {
+      errorCb(fieldPath, value, errorCbParam);
+    }
+    return value;
+  }
+};
+
 /**
  * Determines whether or not the given video bid request is valid. If it's not a video bid, returns true.
  * @param {object} bid bid to validate
@@ -611,46 +706,7 @@ function validateVideoParams(bid) {
   if (!hasVideoMediaType(bid)) {
     return true;
   }
-
-  const paramRequired = (paramStr, value, conditionStr) => {
-    let error = `"${paramStr}" is required`;
-    if (conditionStr) {
-      error += ' when ' + conditionStr;
-    }
-    throw new Error(error);
-  };
-
-  const paramInvalid = (paramStr, value, expectedStr) => {
-    expectedStr = expectedStr ? ', expected: ' + expectedStr : '';
-    value = JSON.stringify(value);
-    throw new Error(`"${paramStr}"=${value} is invalid${expectedStr}`);
-  };
-
-  const isDefined = val => typeof val !== 'undefined';
-  const validate = (fieldPath, validateCb, errorCb, errorCbParam) => {
-    if (fieldPath.indexOf('video') === 0) {
-      const valueFieldPath = 'params.' + fieldPath;
-      const mediaFieldPath = 'mediaTypes.' + fieldPath;
-      const valueParams = deepAccess(bid, valueFieldPath);
-      const mediaTypesParams = deepAccess(bid, mediaFieldPath);
-      const hasValidValueParams = validateCb(valueParams);
-      const hasValidMediaTypesParams = validateCb(mediaTypesParams);
-
-      if (hasValidValueParams) return valueParams;
-      else if (hasValidMediaTypesParams) return hasValidMediaTypesParams;
-      else {
-        if (!hasValidValueParams) errorCb(valueFieldPath, valueParams, errorCbParam);
-        else if (!hasValidMediaTypesParams) errorCb(mediaFieldPath, mediaTypesParams, errorCbParam);
-      }
-      return valueParams || mediaTypesParams;
-    } else {
-      const value = deepAccess(bid, fieldPath);
-      if (!validateCb(value)) {
-        errorCb(fieldPath, value, errorCbParam);
-      }
-      return value;
-    }
-  };
+  const validate = createParamValidator(bid);
 
   try {
     validate('video.context', val => !isEmpty(val), paramRequired);
@@ -680,10 +736,28 @@ function validateVideoParams(bid) {
     validate('video.skippable', val => !isDefined(val) || isBoolean(val), paramInvalid);
     validate('video.skipafter', val => !isDefined(val) || isNumber(val), paramInvalid);
     validate('video.pos', val => !isDefined(val) || isNumber(val), paramInvalid);
-    validate('params.badv', val => !isDefined(val) || isArray(val), paramInvalid,
-      'array of strings, ex: ["ford.com","pepsi.com"]');
+    return true;
+  } catch (e) {
+    logError(e.message);
+    return false;
+  }
+}
+
+/**
+ * Validate the publisher-set blocklist params (`bcat`/`badv`) for all media types
+ * (banner and video). A missing value is allowed; a value that is present but is
+ * not an array is rejected (drops the bid) — the agreed middle ground between
+ * dropping nothing and dropping over an absent optional field. See FS-12403.
+ * @param {BidRequest} bid bid request
+ * @return {boolean} true if valid (or absent), false if present but malformed
+ */
+function validateBlocklistParams(bid) {
+  const validate = createParamValidator(bid);
+  try {
     validate('params.bcat', val => !isDefined(val) || isArray(val), paramInvalid,
       'array of strings, ex: ["IAB1-5","IAB1-6"]');
+    validate('params.badv', val => !isDefined(val) || isArray(val), paramInvalid,
+      'array of strings, ex: ["ford.com","pepsi.com"]');
     return true;
   } catch (e) {
     logError(e.message);
diff --git a/modules/yieldmoBidAdapter.md b/modules/yieldmoBidAdapter.md
index c98e2ab5c74..135cd89c54f 100644
--- a/modules/yieldmoBidAdapter.md
+++ b/modules/yieldmoBidAdapter.md
@@ -31,7 +31,9 @@ var adUnits = [{ // Banner adUnit
     params: {
       placementId: '1779781193098233305', // string with at most 19 characters (may include numbers only)
       bidFloor: .28, // optional param
-      lr_env: '***' // Optional. Live Ramp ATS envelope
+      lr_env: '***', // Optional. Live Ramp ATS envelope
+      bcat: ['IAB1-5', 'IAB1-6'], // optional, array of blocked IAB content categories (strings)
+      badv: ['ford.com', 'pepsi.com'] // optional, array of blocked advertiser domains (strings)
     }
   }]
 }];
@@ -66,7 +68,9 @@ var adUnits = [{ // Video adUnit
         skippable: true,    // optional, boolean
         skipafter: 10       // optional, integer
       },
-      lr_env: '***' // Optional. Live Ramp ATS envelope
+      lr_env: '***', // Optional. Live Ramp ATS envelope
+      bcat: ['IAB1-5', 'IAB1-6'], // optional, array of blocked IAB content categories (strings)
+      badv: ['ford.com', 'pepsi.com'] // optional, array of blocked advertiser domains (strings)
     }
   }]
 }];
@@ -104,3 +108,20 @@ Please also note, that we support the following OpenRTB params:
 'mimes', 'startdelay', 'placement', 'startdelay', 'skipafter', 'protocols', 'api',
 'playbackmethod', 'maxduration', 'minduration', 'pos', 'skip', 'skippable'.
 They can be specified in `mediaTypes.video` or in `bids[].params.video`.
+
+# Blocklists (bcat / badv)
+
+`bcat` (blocked IAB content categories) and `badv` (blocked advertiser domains) are
+supported for **both banner and video**. Each is an optional array of strings and can
+be supplied from either source:
+
+* the standardized first-party-data global — `ortb2.bcat` / `ortb2.badv`, or
+* the Yieldmo bid params — `bids[].params.bcat` / `bids[].params.badv`.
+
+When both sources are present they are **merged** (union) and de-duplicated — neither
+source overrides the other.
+
+Validation: a **missing** `bcat`/`badv` is always allowed, but if `params.bcat` /
+`params.badv` is **present and not an array** the bid is rejected (`isBidRequestValid`
+returns false). Non-string / empty elements inside an otherwise-valid array, and a
+malformed `ortb2` value, are dropped with a console warning rather than failing the bid.
diff --git a/test/spec/modules/yieldmoBidAdapter_spec.js b/test/spec/modules/yieldmoBidAdapter_spec.js
index 4f4454c17ae..24dc7c4721b 100644
--- a/test/spec/modules/yieldmoBidAdapter_spec.js
+++ b/test/spec/modules/yieldmoBidAdapter_spec.js
@@ -148,6 +148,28 @@ describe('YieldmoAdapter', function () {
         expect(spec.isBidRequestValid(getBidAndExclude('api'))).to.be.false;
       });
     });
+
+    describe('Blocklist params (bcat / badv):', function () {
+      it('allows a bid when bcat/badv are absent (missing is fine)', function () {
+        expect(spec.isBidRequestValid(mockBannerBid())).to.be.true;
+        expect(spec.isBidRequestValid(mockVideoBid())).to.be.true;
+      });
+
+      it('allows a bid when bcat/badv are arrays', function () {
+        expect(spec.isBidRequestValid(mockBannerBid({}, { bcat: ['IAB1-1'], badv: ['x.com'] }))).to.be.true;
+        expect(spec.isBidRequestValid(mockVideoBid({}, { bcat: ['IAB1-1'], badv: ['x.com'] }))).to.be.true;
+      });
+
+      it('drops a bid when bcat is present but not an array', function () {
+        expect(spec.isBidRequestValid(mockBannerBid({}, { bcat: 'IAB1-1' }))).to.be.false;
+        expect(spec.isBidRequestValid(mockVideoBid({}, { bcat: 'IAB1-1' }))).to.be.false;
+      });
+
+      it('drops a bid when badv is present but not an array', function () {
+        expect(spec.isBidRequestValid(mockBannerBid({}, { badv: 'ford.com' }))).to.be.false;
+        expect(spec.isBidRequestValid(mockVideoBid({}, { badv: 'ford.com' }))).to.be.false;
+      });
+    });
   });
 
   describe('buildRequests', function () {
@@ -829,6 +851,89 @@ describe('YieldmoAdapter', function () {
         expect(payload.device.language).to.exist;
       });
     });
+
+    describe('bcat / badv blocklists (FS-12403)', function () {
+      it('banner: sends merged bcat/badv as comma-delimited GET params', function () {
+        const bidderReq = mockBidderRequest({ ortb2: { bcat: ['IAB1-1'], badv: ['ortb.com'] } });
+        const data = buildAndGetData([mockBannerBid({}, { bcat: ['IAB2-2'], badv: ['param.com'] })], 0, bidderReq);
+        expect(data.bcat).to.equal('IAB1-1,IAB2-2');
+        expect(data.badv).to.equal('ortb.com,param.com');
+      });
+
+      it('banner: unions ortb2 + params (neither source silently wins)', function () {
+        const bidderReq = mockBidderRequest({ ortb2: { bcat: ['A'] } });
+        const data = buildAndGetData([mockBannerBid({}, { bcat: ['B'] })], 0, bidderReq);
+        expect(data.bcat.split(',')).to.have.members(['A', 'B']);
+      });
+
+      it('banner: dedupes values across the two sources, preserving order', function () {
+        const bidderReq = mockBidderRequest({ ortb2: { bcat: ['DUP', 'A'] } });
+        const data = buildAndGetData([mockBannerBid({}, { bcat: ['DUP', 'B'] })], 0, bidderReq);
+        expect(data.bcat).to.equal('DUP,A,B');
+      });
+
+      it('banner: reads from ortb2 alone', function () {
+        const bidderReq = mockBidderRequest({ ortb2: { bcat: ['IAB1-1'], badv: ['x.com'] } });
+        const data = buildAndGetData([mockBannerBid()], 0, bidderReq);
+        expect(data.bcat).to.equal('IAB1-1');
+        expect(data.badv).to.equal('x.com');
+      });
+
+      it('banner: reads from params alone', function () {
+        const data = buildAndGetData([mockBannerBid({}, { bcat: ['IAB1-1'], badv: ['x.com'] })], 0, mockBidderRequest());
+        expect(data.bcat).to.equal('IAB1-1');
+        expect(data.badv).to.equal('x.com');
+      });
+
+      it('banner: omits bcat/badv entirely when empty', function () {
+        const data = buildAndGetData([mockBannerBid()], 0, mockBidderRequest());
+        expect(data).to.not.have.property('bcat');
+        expect(data).to.not.have.property('badv');
+      });
+
+      it('video: sends merged bcat/badv as deduped arrays', function () {
+        const bidderReq = mockBidderRequest({ ortb2: { bcat: ['IAB1-1'], badv: ['ortb.com'] } }, [mockVideoBid()]);
+        const payload = buildAndGetData([mockVideoBid({}, { bcat: ['IAB2-2'], badv: ['param.com'] })], 0, bidderReq);
+        expect(payload.bcat).to.deep.equal(['IAB1-1', 'IAB2-2']);
+        expect(payload.badv).to.deep.equal(['ortb.com', 'param.com']);
+      });
+
+      it('video: reads ortb2.bcat (not the legacy bidderRequest.bcat path)', function () {
+        const bidderReq = mockBidderRequest({ bcat: ['WRONG'], ortb2: { bcat: ['RIGHT'] } }, [mockVideoBid()]);
+        const payload = buildAndGetData([mockVideoBid()], 0, bidderReq);
+        expect(payload.bcat).to.deep.equal(['RIGHT']);
+      });
+
+      it('video: defaults to empty arrays when no blocklists are set', function () {
+        const payload = buildAndGetData([mockVideoBid()], 0, mockBidderRequest({}, [mockVideoBid()]));
+        expect(payload.bcat).to.deep.equal([]);
+        expect(payload.badv).to.deep.equal([]);
+      });
+
+      describe('blocklist normalization in mergeBlocklist', function () {
+        let logWarnStub;
+        beforeEach(function () { logWarnStub = sinon.stub(utils, 'logWarn'); });
+        afterEach(function () { logWarnStub.restore(); });
+
+        it('ignores a non-array ortb2 source and warns (ortb2 is not bid-validated)', function () {
+          const bidderReq = mockBidderRequest({ ortb2: { bcat: 'IAB1-1' } });
+          const data = buildAndGetData([mockBannerBid()], 0, bidderReq);
+          expect(data).to.not.have.property('bcat');
+          expect(logWarnStub.called).to.be.true;
+        });
+
+        it('filters non-string / empty elements out of a valid array and warns', function () {
+          const data = buildAndGetData([mockBannerBid({}, { bcat: ['IAB1-1', '', 5, '  '] })], 0, mockBidderRequest());
+          expect(data.bcat).to.equal('IAB1-1');
+          expect(logWarnStub.called).to.be.true;
+        });
+
+        it('trims whitespace around entries', function () {
+          const data = buildAndGetData([mockBannerBid({}, { bcat: [' IAB1-1 '] })], 0, mockBidderRequest());
+          expect(data.bcat).to.equal('IAB1-1');
+        });
+      });
+    });
   });
 
   describe('interpretResponse', function () {