1"use strict";(self.webpackChunkblog_folder=self.webpackChunkblog_folder||[]).push([[993],{13081:(e,i,n)=>{n.r(i),n.d(i,{assets:()=>r,contentTitle:()=>o,default:()=>h,frontMatter:()=>s,metadata:()=>t,toc:()=>d});const t=JSON.parse('{"id":"troubleshooting/troubleshooting_twilio_request_validation_aws_lambda","title":"How to Fix Twilio Signature Validation Failures in AWS Lambda","description":"Struggling with Twilio\'s X-Twilio-Signature in AWS Lambda? Learn how a single argument, keep_blank_values=True, can make or break request validation when building serverless apps with AWS Lambda and AWS SAM.","source":"@site/docs/troubleshooting/troubleshooting_twilio_request_validation_aws_lambda.md","sourceDirName":"troubleshooting","slug":"/troubleshooting/troubleshooting-hub-twilio-request-validation-aws-sam-lambda-keep-blank-values","permalink":"/tutorials/troubleshooting/troubleshooting-hub-twilio-request-validation-aws-sam-lambda-keep-blank-values","draft":false,"unlisted":false,"tags":[{"inline":true,"label":"twilio","permalink":"/tutorials/tags/twilio"},{"inline":true,"label":"aws sam","permalink":"/tutorials/tags/aws-sam"},{"inline":true,"label":"aws lambda","permalink":"/tutorials/tags/aws-lambda"},{"inline":true,"label":"serverless","permalink":"/tutorials/tags/serverless"},{"inline":true,"label":"signature validation","permalink":"/tutorials/tags/signature-validation"},{"inline":true,"label":"parse_qs keep_blank_values","permalink":"/tutorials/tags/parse-qs-keep-blank-values"},{"inline":true,"label":"AWS API gateway","permalink":"/tutorials/tags/aws-api-gateway"},{"inline":true,"label":"twilio request validation","permalink":"/tutorials/tags/twilio-request-validation"},{"inline":true,"label":"debugging","permalink":"/tutorials/tags/debugging"}],"version":"current","sidebarPosition":1,"frontMatter":{"title":"How to Fix Twilio Signature Validation Failures in AWS Lambda","subtitle":"A step-by-step troubleshooting guide to handling empty form fields when validating Twilio webhooks in AWS Lambda","slug":"troubleshooting-hub-twilio-request-validation-aws-sam-lambda-keep-blank-values","description":"Struggling with Twilio\'s X-Twilio-Signature in AWS Lambda? 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Learn how a single argument, keep_blank_values=True, can make or break request validation when building serverless apps with AWS Lambda and AWS SAM.",coverImage:"https://d1fiydes8a4qgo.cloudfront.net/troubleshooting_hub/2025/cover-troubleshooting-hub-twilio-request-validation.png",image:"https://d1fiydes8a4qgo.cloudfront.net/troubleshooting_hub/2025/cover-troubleshooting-hub-twilio-request-validation.png",twitterImage:"https://d1fiydes8a4qgo.cloudfront.net/troubleshooting_hub/2025/cover-troubleshooting-hub-twilio-request-validation.png",ogImage:"https://d1fiydes8a4qgo.cloudfront.net/troubleshooting_hub/2025/cover-troubleshooting-hub-twilio-request-validation.png",sidebar_position:1,tags:["twilio","aws sam","aws lambda","serverless","signature validation","parse_qs keep_blank_values","AWS API gateway","twilio request validation","debugging"],keywords:["twilio signature validation","twilio request validation","validating twilio requests","aws lambda twilio","aws sam twilio","keep_blank_values parse_qs","twilio webhook security","debugging stir/shaken calls","empty form fields","AWS API gateway"]},o="How to Fix Twilio Signature Validation Failures in AWS Lambda",r={},d=[{value:"Why Twilio validates requests",id:"why-twilio-validates-requests",level:2},{value:"The mysterious signature failures",id:"the-mysterious-signature-failures",level:2},{value:"The Crucial Fix: <code>keep_blank_values=True</code>
1",id:"the-crucial-fix-keep_blank_valuestrue",level:2},{value:"Conclusion",id:"conclusion",level:3},{value:"The final working code",id:"the-final-working-code",level:2},{value:"Why this works",id:"why-this-works",level:2},{value:"Wrapping up",id:"wrapping-up",level:2}];function u(e){const i={admonition:"admonition",blockquote:"blockquote",code:"code",h1:"h1",h2:"h2",h3:"h3",header:"header",img:"img",li:"li",ol:"ol",p:"p",pre:"pre",strong:"strong",ul:"ul",...(0,l.R)(),...e.components};return(0,a.jsxs)(a.Fragment,{children:[(0,a.jsx)(i.header,{children:(0,a.jsx)(i.h1,{id:"how-to-fix-twilio-signature-validation-failures-in-aws-lambda",children:"How to Fix Twilio Signature Validation Failures in AWS Lambda"})}),"\n",(0,a.jsx)(i.p,{children:(0,a.jsx)(i.img,{src:"https://d1fiydes8a4qgo.cloudfront.net/troubleshooting_hub/2025/cover-troubleshooting-hub-twilio-request-validation.png",alt:"How to Fix Twilio Signature Validation Failures in AWS Lambda",title:"How to Fix Twilio Signature Validation Failures in AWS Lambda"})}),"\n",(0,a.jsxs)(i.p,{children:["Have you ever ",(0,a.jsx)(i.strong,{children:"tested Twilio webhook validation"})," in AWS Lambda, only to see ",(0,a.jsx)(i.code,{children:"validator.validate()"})," fail over and over again - even though your code looks perfect?\nYou parse the form data, flatten the parameters, and everything seems correct, yet ",(0,a.jsx)(i.strong,{children:"Twilio"})," insists that ",(0,a.jsx)(i.code,{children:"X-Twilio-Signature"})," doesn't match."]}),"\n",(0,a.jsxs)(i.p,{children:["If that sounds familiar, you might be missing ",(0,a.jsx)(i.strong,{children:"one tiny argument"})," in your parse logic:"]}),"\n",(0,a.jsx)(i.pre,{children:(0,a.jsx)(i.code,{className:"language-python",metastring:'title="lambda.py"',children:"params = urllib.parse.parse_qs(\n raw_body, \n # highlight-next-line\n keep_blank_values=True # \u26a0\ufe0f crucial \u26a0\ufe0f\n)\n"})}),"\n",(0,a.jsxs)(i.p,{children:["In this post, you'll learn why ",(0,a.jsx)(i.code,{children:"keep_blank_values=True"})," can make or break Twilio's HMAC signature validation when building serverless apps with ",(0,a.jsx)(i.strong,{children:"AWS SAM"})," and ",(0,a.jsx)(i.strong,{children:"AWS Lambda"}),"."]}),"\n",(0,a.jsx)(i.p,{children:"This single detail often goes unnoticed, but it can be the entire reason Twilio's signature fails. Let's dive in!"}),"\n",(0,a.jsx)(i.h2,{id:"why-twilio-validates-requests",children:"Why Twilio validates requests"}),"\n",(0,a.jsxs)(i.p,{children:["When Twilio sends webhooks to your application (e.g., for inbound calls, SMS messages, etc.), it includes an ",(0,a.jsx)(i.code,{children:"X-Twilio-Signature"})," header to ",(0,a.jsx)(i.strong,{children:"prove"})," the request is truly from Twilio. You can verify the authenticity by:"]}),"\n",(0,a.jsxs)(i.ol,{children:["\n",(0,a.jsx)(i.li,{children:"Constructing the exact URL Twilio called (including path and optional trailing slash)"}),"\n",(0,a.jsx)(i.li,{children:"Parsing all POST fields"}),"\n",(0,a.jsx)(i.li,{children:"Sorting those fields by key"}),"\n",(0,a.jsx)(i.li,{children:"Combining them into a string and hashing with your Twilio Auth Token via HMAC-SHA1"}),"\n"]}),"\n",(0,a.jsxs)(i.p,{children:["The ",(0,a.jsx)(i.code,{children:"twilio.request_validator.RequestValidator"})," does this automatically under the hood."]}),"\n",(0,a.jsxs)(i.p,{children:["If anything is off - like missing parameters, double-encoded strings, or ",(0,a.jsx)(i.strong,{children:"empty fields"})," - the signature won't match, and you'll see ",(0,a.jsx)(i.code,{children:"False"})," instead of ",(0,a.jsx)(i.code,{children:"True"}),"."]}),"\n",(0,a.jsx)(i.h2,{id:"the-mysterious-signature-failures",children:"The mysterious signature failures"}),"\n",(0,a.jsxs)(i.p,{children:[(0,a.jsx)(i.strong,{children:"Picture this:"})," You've set your Twilio webhook to ",(0,a.jsx)(i.code,{children:"https://YOUR-API/Prod/endpoint/"}),"."]}),"\n",(0,a.jsx)(i.p,{children:"In your AWS Lambda code, you parse the form data via:"}),"\n",(0,a.jsx)(i.pre,{children:(0,a.jsx)(i.code,{className:"language-python",metastring:'title="lambda.py"',children:"params = urllib.parse.parse_qs(raw_body)\n"})}
1),"\n",(0,a.jsx)(i.p,{children:"Then you flatten it:"}),"\n",(0,a.jsx)(i.pre,{children:(0,a.jsx)(i.code,{className:"language-python",metastring:'title="lambda.py"',children:"flattened_params = {k: v[0] for k, v in params.items()}\n"})}),"\n",(0,a.jsx)(i.p,{children:"Finally, you do:"}),"\n",(0,a.jsx)(i.pre,{children:(0,a.jsx)(i.code,{className:"language-python",metastring:'title="lambda.py"',children:"is_valid = validator.validate(full_url, flattened_params, twilio_signature)\n"})}),"\n",(0,a.jsxs)(i.p,{children:[(0,a.jsx)(i.strong,{children:"But"})," Twilio responds with ",(0,a.jsx)(i.code,{children:"False"}),"."]}),"\n",(0,a.jsx)(i.p,{children:"You confirm you have \u2705 correct trailing slash, the correct \u2705 Auth Token, and you're sure the \u2705 phone numbers match."}),"\n",(0,a.jsx)(i.p,{children:"What gives?"}),"\n",(0,a.jsxs)(i.h2,{id:"the-crucial-fix-keep_blank_valuestrue",children:["The Crucial Fix: ",(0,a.jsx)(i.code,{children:"keep_blank_values=True"})]}),"\n",(0,a.jsxs)(i.p,{children:[(0,a.jsx)(i.strong,{children:"One small tweak"})," often solves it:"]}),"\n",(0,a.jsx)(i.pre,{children:(0,a.jsx)(i.code,{className:"language-python",metastring:'title="lambda.py"',children:"params = urllib.parse.parse_qs(\n raw_body, \n # highlight-next-line\n keep_blank_values=True\n)\n"})}),"\n",(0,a.jsxs)(i.p,{children:["By default, Python's ",(0,a.jsx)(i.code,{children:"parse_qs()"})," will ignore keys that have empty values - meaning if Twilio sends ",(0,a.jsx)(i.code,{children:"Param=&AnotherParam=Hello"}),", the first param (which is empty) might get lost or stripped."]}),"\n",(0,a.jsx)(i.admonition,{title:"Twilio includes that empty parameter",type:"danger",children:(0,a.jsxs)(i.p,{children:["Twilio, however, ",(0,a.jsx)(i.strong,{children:"still"})," includes that empty parameter in the HMAC signature. The mismatch kills your validation."]})}),"\n",(0,a.jsxs)(i.p,{children:["Looking at the ",(0,a.jsx)(i.code,{children:'raw_body = event.get("body", "")'})," I noticed that Twilio sent several keys with empty values:"]}),"\n",(0,a.jsx)(i.pre,{children:(0,a.jsx)(i.code,{className:"language-bash",children:"...&CalledZip=&ApiVersion=2010-04-01&CalledCity=&CallStatus=ringing...\n"})}),"\n",(0,a.jsxs)(i.p,{children:["If we parse it, we'll notice 4 empty keys ",(0,a.jsx)(i.code,{children:"ToZip"}),",",(0,a.jsx)(i.code,{children:"CalledZip"}),",",(0,a.jsx)(i.code,{children:"CalledCity"})," and ",(0,a.jsx)(i.code,{children:"ToCity"}),":"]}),"\n",(0,a.jsx)(i.pre,{children:(0,a.jsx)(i.code,{className:"language-python",children:"parsed_params = {\n 'CallerCountry': ['US'],\n 'Direction': ['inbound'],\n 'CallerState': ['CA'],\n # highlight-start\n 'ToZip': [''], # empty key\n 'CalledZip': [''], # empty key\n 'CalledCity': [''], # empty key\n 'ToCity': [''], # empty key\n # highlight-end\n ...\n}\n"})}),"\n",(0,a.jsxs)(i.p,{children:["So when I went ahead and parsed the ",(0,a.jsx)(i.code,{children:"raw_body"})," without ",(0,a.jsx)(i.code,{children:"keep_blank_values=True"}),", those empty keys were ignored, resulting in ",(0,a.jsx)(i.code,{children:"parsed_params"})," missing those values:"]}),"\n",(0,a.jsx)(i.pre,{children:(0,a.jsx)(i.code,{className:"language-python",children:"parsed_params = {\n 'CallerCountry': ['US'],\n 'Direction': ['inbound'],\n 'CallerState': ['CA'],\n ...\n}\n"})}),"\n",(0,a.jsx)(i.h3,{id:"conclusion",children:"Conclusion"}),"\n",(0,a.jsxs)(i.ul,{children:["\n",(0,a.jsxs)(i.li,{children:[(0,a.jsx)(i.strong,{children:"Raw body:"})," ",(0,a.jsx)(i.code,{children:"CalledZip=&ApiVersion=2010-04-01..."})]}),"\n",(0,a.jsxs)(i.li,{children:[(0,a.jsx)(i.strong,{children:"Without"})," ",(0,a.jsx)(i.code,{children:"keep_blank_values=True"}),", the empty ",(0,a.jsx)(i.code,{children:"CalledZip"})," might vanish from your final dictionary"]}),"\n",(0,a.jsxs)(i.li,{children:["Twilio's signature logic includes ",(0,a.jsx)(i.code,{children:'CalledZip=""'})]}),"\n",(0,a.jsxs)(i.li,{children:["Your logic omits ",(0,a.jsx)(i.code,{children:"CalledZip"})," entirely, generating a different string for HMAC"]}),"\n",(0,a.jsx)(i.li,{children:"Result: validation fails"}),"\n"]}),"\n",(0,a.jsx)(i.h2,{id:"the-final-working-code",children:"The final working code"}),"\n",(0,a.jsxs)(i.p,{children:["Below is a minimal ",(0,a.jsx)(i.strong,{children:"AWS Lambda"})," function using AWS SAM that fixes the signature by including empty parameters:"]}
1),"\n",(0,a.jsx)(i.pre,{children:(0,a.jsx)(i.code,{className:"language-python",metastring:'title="lambda.py"',children:'import os\nimport json\nimport urllib.parse\nfrom twilio.request_validator import RequestValidator\n\ndef lambda_handler(event, context):\n # 1. Load Twilio Auth Token\n auth_token = os.environ["TWILIO_AUTH_TOKEN"]\n validator = RequestValidator(auth_token)\n\n # 2. Construct the exact URL Twilio called\n host = event["headers"].get("Host", "")\n path = event["requestContext"]["path"] # Your endpoint path\n full_url = f"https://{host}{path}" # If Twilio has a trailing slash, keep it\n\n # 3. Extract Twilio signature\n twilio_signature = event["headers"].get("X-Twilio-Signature", "")\n\n # 4. Parse body with keep_blank_values=True\n raw_body = event.get("body", "")\n params = urllib.parse.parse_qs(\n raw_body, \n # highlight-next-line\n keep_blank_values=True # \u26a0\ufe0f crucial \u26a0\ufe0f\n )\n\n # 5. Flatten multi-value fields\n flattened_params = {k: v[0] if len(v) == 1 else v for k, v in params.items()}\n\n # 6. Validate\n is_valid = validator.validate(full_url, flattened_params, twilio_signature)\n\n print("URL:", full_url)\n print("Twilio Signature:", twilio_signature)\n print("Flattened Params:", json.dumps(flattened_params, indent=2))\n print("Is Valid:", is_valid)\n\n if not is_valid:\n return {\n "statusCode": 403,\n "body": "Signature validation failed"\n }\n\n return {\n "statusCode": 200,\n "headers": {"Content-Type": "application/xml"},\n "body": "<Response><Say>All Good!</Say></Response>"\n }\n\n'})}),"\n",(0,a.jsx)(i.p,{children:"That's it."}),"\n",(0,a.jsxs)(i.p,{children:["Just ",(0,a.jsx)(i.strong,{children:"one small parameter"}),": ",(0,a.jsx)(i.code,{children:"keep_blank_values=True"}),"."]}),"\n",(0,a.jsx)(i.p,{children:"Now, your dictionary includes placeholders for those empty fields, matching exactly what Twilio expects."}),"\n",(0,a.jsx)(i.h2,{id:"why-this-works",children:"Why this works"}),"\n",(0,a.jsx)(i.p,{children:(0,a.jsx)(i.strong,{children:"1. Twilio doesn't ignore blank fields"})}),"\n",(0,a.jsxs)(i.ul,{children:["\n",(0,a.jsxs)(i.li,{children:["In Twilio's HMAC calculation, a param like ",(0,a.jsx)(i.code,{children:"CalledZip="})," still contributes ",(0,a.jsx)(i.code,{children:"CalledZip"})," with an empty string to the final hash string."]}),"\n"]}),"\n",(0,a.jsx)(i.p,{children:(0,a.jsxs)(i.strong,{children:["2. Python's ",(0,a.jsx)(i.code,{children:"parse_qs()"})," defaults"]})}),"\n",(0,a.jsxs)(i.ul,{children:["\n",(0,a.jsxs)(i.li,{children:["Without ",(0,a.jsx)(i.code,{children:"keep_blank_values=True"}),", ",(0,a.jsx)(i.code,{children:"parse_qs()"})," discards such parameters or sets them to ",(0,a.jsx)(i.code,{children:"[]"}),", messing up your final param dictionary."]}),"\n"]}),"\n",(0,a.jsx)(i.p,{children:(0,a.jsx)(i.strong,{children:"3. Signature mismatch"})}),"\n",(0,a.jsxs)(i.ul,{children:["\n",(0,a.jsxs)(i.li,{children:["Once your dictionary is missing an empty param that Twilio included, the concatenated HMAC string is different. Twilio's signature doesn't match, so ",(0,a.jsx)(i.code,{children:"False"})," is returned."]}),"\n"]}),"\n",(0,a.jsxs)(i.p,{children:["By explicitly telling Python to ",(0,a.jsx)(i.strong,{children:"keep blank values"}),", you preserve the parameter as ",(0,a.jsx)(i.code,{children:'{"CalledZip": [""]}'})," instead of dropping or ignoring it."]}),"\n",(0,a.jsxs)(i.p,{children:["When flattened and sorted, it lines up perfectly with Twilio's internal logic, giving you a ",(0,a.jsx)(i.code,{children:"True"})," result."]}),"\n",(0,a.jsx)(i.h2,{id:"wrapping-up",children:"Wrapping up"}),"\n",(0,a.jsx)(i.p,{children:"When everything looks correct but Twilio validation still fails, remember:"}),"\n",(0,a.jsxs)(i.blockquote,{children:["\n",(0,a.jsxs)(i.p,{children:[(0,a.jsx)(i.code,{children:"keep_blank_values=True"})," can make or break your signature check"]}),"\n"]}),"\n",(0,a.jsxs)(i.p,{children:["It's a tiny detail that's easy to miss, especially if you're new to Twilio or Python's ",(0,a.jsx)(i.co
1de,{children:"parse_qs()"}),"."]}),"\n",(0,a.jsx)(i.p,{children:"With this fix, you'll ensure you're capturing all parameters - blank or otherwise - and matching Twilio's exact HMAC calculation."}),"\n",(0,a.jsx)(i.p,{children:"Now your AWS SAM-deployed Lambda should confidently validate Twilio requests."}),"\n",(0,a.jsxs)(i.p,{children:["That's it! If you've spent hours seeing ",(0,a.jsx)(i.code,{children:"False"})," from Twilio's RequestValidator (like I did), hopefully this quick fix spares you any more headaches."]}),"\n",(0,a.jsx)(i.p,{children:(0,a.jsx)(i.strong,{children:"Happy coding!"})})]})}function h(e={}){const{wrapper:i}={...(0,l.R)(),...e.components};return i?(0,a.jsx)(i,{...e,children:(0,a.jsx)(u,{...e})}):u(e)}},28453:(e,i,n)=>{n.d(i,{R:()=>s,x:()=>o});var t=n(96540);const a={},l=t.createContext(a);function s(e){const i=t.useContext(l);return t.useMemo((function(){return"function"==typeof e?e(i):{...i,...e}}),[i,e])}function o(e){let i;return i=e.disableParentContext?"function"==typeof e.components?e.components(a):e.components||a:s(e.components),t.createElement(l.Provider,{value:i},e.children)}}}]);
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