diff --git a/lib/getFilterInfosAndTargetContentTypeFromQueryString.js b/lib/getFilterInfosAndTargetContentTypeFromQueryString.js
index 3822847..c512fdf 100644
--- a/lib/getFilterInfosAndTargetContentTypeFromQueryString.js
+++ b/lib/getFilterInfosAndTargetContentTypeFromQueryString.js
@@ -67,6 +67,299 @@ Object.keys(isOperationByEngineNameAndName).forEach(function (engineName) {
     });
 });
 
+function isNumberWithin(num, min, max) {
+    return typeof num === 'number' && num >= min && num <= max;
+}
+
+function isValidOperation(name, args) {
+    switch (name) {
+    case 'crop':
+        return args.length === 1 && /^(?:east|west|center|north(?:|west|east)|south(?:|west|east))/.test(args[0]);
+    case 'rotate':
+        return args.length === 0 || (args.length === 1 && (args[0] === 0 || args[0] === 90 || args[0] === 180 || args[0] === 270));
+    case 'resize':
+        return args.length === 2 && isNumberWithin(args[0], 1, 16384) && isNumberWithin(args[1], 1, 16384);
+    case 'extract':
+        return args.length === 4 && isNumberWithin(args[0], 1, 16384) && isNumberWithin(args[1], 1, 16384) && isNumberWithin(args[2], 1, 16384) && isNumberWithin(args[3], 1, 16384);
+    case 'interpolateWith':
+        return args.length === 1 && /^(?:nearest|bilinear|vertexSplitQuadraticBasisSpline|bicubic|locallyBoundedBicubic|nohalo)$/.test(args[0]);
+    case 'background':
+        return args.length === 1 && /^#[0-9a-f]{6}$/.test(args[0]);
+    case 'blur':
+        return args.length === 0 || (args.length === 1 && isNumberWithin(args[0], 0.3, 1000));
+    case 'sharpen':
+        return args.length <= 3 &&
+            (typeof args[0] === 'undefined' || typeof args[0] === 'number') &&
+            (typeof args[1] === 'undefined' || typeof args[1] === 'number') &&
+            (typeof args[2] === 'undefined' || typeof args[2] === 'number');
+    case 'threshold':
+        return args.length === 0 || (args.length === 1 && isNumberWithin(args[0], 0, 255));
+    case 'gamma':
+        return args.length === 0 || (args.length === 1 && isNumberWithin(args[0], 1, 3));
+    case 'quality':
+        return args.length === 1 && isNumberWithin(args[0], 1, 100);
+    case 'tile':
+        return args.length <= 2 &&
+            (typeof args[0] === 'undefined' || isNumberWithin(args[0], 1, 8192)) &&
+            (typeof args[1] === 'undefined' || isNumberWithin(args[0], 0, 8192));
+    case 'compressionLevel':
+        return args.length === 1 && isNumberWithin(args[0], 0, 9);
+    case 'png':
+    case 'jpeg':
+    case 'gif':
+    case 'webp':
+    case 'withoutEnlargement':
+    case 'progressive':
+    case 'metadata':
+    case 'ignoreAspectRatio':
+    case 'embed':
+    case 'max':
+    case 'min':
+    case 'negate':
+    case 'flatten':
+    case 'flip':
+    case 'flop':
+    case 'grayscale':
+    case 'greyscale':
+    case 'normalize':
+    case 'withMetadata':
+    case 'withoutChromaSubsampling':
+    case 'withoutAdaptiveFiltering':
+    case 'trellisQuantization':
+    case 'trellisQuantisation':
+    case 'overshootDeringing':
+    case 'optimizeScans':
+    case 'optimiseScans':
+        return args.length === 0;
+    // Not supported: overlayWith
+
+    // Engines:
+    case 'sharp':
+    case 'gm':
+        return args.length === 0;
+
+    // FIXME: Add validation code for all the below.
+    // https://github.com/papandreou/express-processimage/issues/4
+    // Other engines:
+    case 'pngcrush':
+    case 'pngquant':
+    case 'jpegtran':
+    case 'optipng':
+    case 'svgfilter':
+    case 'inkscape':
+        return true;
+
+    // Graphicsmagick specific operations:
+    // FIXME: Add validation code for all the below.
+    // https://github.com/papandreou/express-processimage/issues/4
+    case 'setFormat':
+    case 'identify':
+    case 'selectFrame':
+    case 'subCommand':
+    case 'adjoin':
+    case 'affine':
+    case 'alpha':
+    case 'append':
+    case 'authenticate':
+    case 'average':
+    case 'backdrop':
+    case 'blackThreshold':
+    case 'bluePrimary':
+    case 'border':
+    case 'borderColor':
+    case 'box':
+    case 'channel':
+    case 'chop':
+    case 'clip':
+    case 'coalesce':
+    case 'colorize':
+    case 'colorMap':
+    case 'compose':
+    case 'compress':
+    case 'convolve':
+    case 'createDirectories':
+    case 'deconstruct':
+    case 'define':
+    case 'delay':
+    case 'displace':
+    case 'display':
+    case 'dispose':
+    case 'dissolve':
+    case 'encoding':
+    case 'endian':
+    case 'file':
+    case 'flatten':
+    case 'foreground':
+    case 'frame':
+    case 'fuzz':
+    case 'gaussian':
+    case 'geometry':
+    case 'greenPrimary':
+    case 'highlightColor':
+    case 'highlightStyle':
+    case 'iconGeometry':
+    case 'intent':
+    case 'lat':
+    case 'level':
+    case 'list':
+    case 'log':
+    case 'loop':
+    case 'map':
+    case 'mask':
+    case 'matte':
+    case 'matteColor':
+    case 'maximumError':
+    case 'mode':
+    case 'monitor':
+    case 'mosaic':
+    case 'motionBlur':
+    case 'name':
+    case 'noop':
+    case 'normalize':
+    case 'opaque':
+    case 'operator':
+    case 'orderedDither':
+    case 'outputDirectory':
+    case 'page':
+    case 'pause':
+    case 'pen':
+    case 'ping':
+    case 'pointSize':
+    case 'preview':
+    case 'process':
+    case 'profile':
+    case 'progress':
+    case 'randomThreshold':
+    case 'recolor':
+    case 'redPrimary':
+    case 'remote':
+    case 'render':
+    case 'repage':
+    case 'sample':
+    case 'samplingFactor':
+    case 'scene':
+    case 'scenes':
+    case 'screen':
+    case 'set':
+    case 'segment':
+    case 'shade':
+    case 'shadow':
+    case 'sharedMemory':
+    case 'shave':
+    case 'shear':
+    case 'silent':
+    case 'rawSize':
+    case 'snaps':
+    case 'stegano':
+    case 'stereo':
+    case 'textFont':
+    case 'texture':
+    case 'threshold':
+    case 'thumbnail':
+    case 'tile':
+    case 'title':
+    case 'transform':
+    case 'transparent':
+    case 'treeDepth':
+    case 'update':
+    case 'units':
+    case 'unsharp':
+    case 'usePixmap':
+    case 'view':
+    case 'virtualPixel':
+    case 'visual':
+    case 'watermark':
+    case 'wave':
+    case 'whitePoint':
+    case 'whiteThreshold':
+    case 'window':
+    case 'windowGroup':
+    case 'strip':
+    case 'interlace':
+    case 'setFormat':
+    case 'resizeExact':
+    case 'scale':
+    case 'filter':
+    case 'density':
+    case 'noProfile':
+    case 'resample':
+    case 'rotate':
+    case 'magnify':
+    case 'minify':
+    case 'quality':
+    case 'charcoal':
+    case 'modulate':
+    case 'antialias':
+    case 'bitdepth':
+    case 'colors':
+    case 'colorspace':
+    case 'comment':
+    case 'contrast':
+    case 'cycle':
+    case 'despeckle':
+    case 'dither':
+    case 'monochrome':
+    case 'edge':
+    case 'emboss':
+    case 'enhance':
+    case 'equalize':
+    case 'gamma':
+    case 'implode':
+    case 'label':
+    case 'limit':
+    case 'median':
+    case 'negative':
+    case 'noise':
+    case 'paint':
+    case 'raise':
+    case 'lower':
+    case 'region':
+    case 'roll':
+    case 'sharpen':
+    case 'solarize':
+    case 'spread':
+    case 'swirl':
+    case 'type':
+    case 'trim':
+    case 'extent':
+    case 'gravity':
+    case 'background':
+    case 'fill':
+    case 'stroke':
+    case 'strokeWidth':
+    case 'font':
+    case 'fontSize':
+    case 'draw':
+    case 'drawPoint':
+    case 'drawLine':
+    case 'drawRectangle':
+    case 'drawArc':
+    case 'drawEllipse':
+    case 'drawCircle':
+    case 'drawPolyline':
+    case 'drawPolygon':
+    case 'drawBezier':
+    case 'drawText':
+    case 'setDraw':
+    case 'thumb':
+    case 'thumbExact':
+    case 'morph':
+    case 'sepia':
+    case 'autoOrient':
+    case 'in':
+    case 'out':
+    case 'preprocessor':
+    case 'addSrcFormatter':
+    case 'inputIs':
+    case 'compare':
+    case 'composite':
+    case 'montage':
+        return true;
+    default:
+        return false;
+    }
+}
+
 module.exports = function getFilterInfosAndTargetContentTypeFromQueryString(queryString, options) {
     options = options || {};
     var filters = options.filters || {},
@@ -299,7 +592,7 @@ module.exports = function getFilterInfosAndTargetContentTypeFromQueryString(quer
                     }
                 }) : [];
 
-            if (typeof options.allowOperation === 'function' && !options.allowOperation(operationName, operationArgs)) {
+            if (!isValidOperation(operationName, operationArgs) || (typeof options.allowOperation === 'function' && !options.allowOperation(operationName, operationArgs))) {
                 leftOverQueryStringFragments.push(keyValuePair);
             } else {
                 var filterInfo;
diff --git a/test/processImage.js b/test/processImage.js
index 9022089..b7f4ba9 100644
--- a/test/processImage.js
+++ b/test/processImage.js
@@ -311,14 +311,14 @@ describe('express-processimage', function () {
         });
 
         it('should allow an operation for which allowOperation returns true', function () {
-            return expect('GET /turtle.jpg?resize=87', 'to yield response', {
+            return expect('GET /turtle.jpg?resize=87,100', 'to yield response', {
                 headers: {
                     'Content-Type': 'image/jpeg'
                 },
                 body: expect.it('to have metadata satisfying', { size: { width: 87 } })
             }).then(function () {
                 expect(config.allowOperation, 'to have calls satisfying', function () {
-                    config.allowOperation('resize', [87]);
+                    config.allowOperation('resize', [ 87, 100 ]);
                 });
             });
         });
@@ -357,7 +357,7 @@ describe('express-processimage', function () {
         });
 
         it('should allow retrieving the image metadata for the result of an operation', function () {
-            return expect('GET /turtle.jpg?png&greyscale&resize=10&metadata', 'to yield response', {
+            return expect('GET /turtle.jpg?png&greyscale&resize=10,9&metadata', 'to yield response', {
                 body: {
                     width: 10,
                     height: 9,
@@ -459,7 +459,7 @@ describe('express-processimage', function () {
 
         it('should use sharp when a gif is converted to png', function () {
             config.debug = true;
-            return expect('GET /animated.gif?resize=40&png', 'to yield response', {
+            return expect('GET /animated.gif?resize=40,100&png', 'to yield response', {
                 headers: {
                     'X-Express-Processimage': 'sharp'
                 },
@@ -485,9 +485,14 @@ describe('express-processimage', function () {
             });
         });
 
-        it('should return an error when an invalid syntax is used for a parameter value', function () {
-            return expect('GET /turtle.jpg?resize=100%22', 'to yield response', {
-                errorPassedToNext: { statusCode: 400 }
+        it('should ignore invalid operations', function () {
+            return expect('GET /turtle.jpg?resize=10%22', 'to yield response', {
+                body: expect.it('to have metadata satisfying', {
+                    size: {
+                        width: 481,
+                        height: 424
+                    }
+                })
             });
         });
     });
@@ -663,4 +668,23 @@ describe('express-processimage', function () {
             });
         });
     });
+
+    describe('with invalid parameters', function () {
+        [
+            'resize=foo,100', 'resize=', 'resize=100,200,300', 'resize=0,0', 'resize=-1,-1', 'resize=32000,32000',
+            'crop=foo', 'crop=', 'crop=north,south',
+            'extract=', 'extract=1,2,3,4,5', 'extract=32000,32000,32000,32000',
+            'rotate=95', 'rotate=90,270',
+            'png=hey',
+            'interpolateWith=something'
+        ].forEach(function (invalidOperation) {
+            it('disallows an operation of ' + invalidOperation, function () {
+                return expect('GET /testImage.png?' + invalidOperation, 'to yield response', {
+                    body: expect.it('to have metadata satisfying', {
+                        size: { width: 12, height: 5 }
+                    })
+                });
+            });
+        });
+    });
 });
diff --git a/testdata/testImage.png b/testdata/testImage.png
new file mode 100644
index 0000000..7586ff3
Binary files /dev/null and b/testdata/testImage.png differ