fix: force gzip when no accept encoding header is sent and use identity if gzip is not sent (#28026)

Co-authored-by: Chris Breiding <[email protected]>

Author: ryanthemanuel

cli/CHANGELOG.md

@@ -3,6 +3,10 @@
 
 _Released 10/25/2023 (PENDING)_
 
+**Bugfixes:**
+
+- Fixed an issue with Accept Encoding headers by forcing gzip when no accept encoding header is sent and using identity if gzip is not sent. Fixes [#28025](https://github.com/cypress-io/cypress/issues/28025).
+
 **Dependency Updates:**
 
 - Upgraded [`@babel/core`](https://www.npmjs.com/package/@babel/core) from `7.22.9` to `7.23.2` to address the [SNYK-JS-SEMVER-3247795](https://snyk.io/vuln/SNYK-JS-SEMVER-3247795) security vulnerability. Upgraded [`@babel/traverse`](https://www.npmjs.com/package/@babel/traverse) from `7.22.8` to `7.23.2` to address the [SNYK-JS-BABELTRAVERSE-5962462](https://snyk.io/vuln/SNYK-JS-BABELTRAVERSE-5962462) security vulnerability. Upgraded [`react-docgen`](https://www.npmjs.com/package/react-docgen) from `6.0.0-alpha.3` to `6.0.4` to address the [SNYK-JS-BABELTRAVERSE-5962462](https://snyk.io/vuln/SNYK-JS-BABELTRAVERSE-5962462) security vulnerability. Addressed in [#28063](https://github.com/cypress-io/cypress/pull/28063).

packages/proxy/lib/http/request-middleware.ts

@@ -348,7 +348,7 @@ const EndRequestsToBlockedHosts: RequestMiddleware = function () {
 const StripUnsupportedAcceptEncoding: RequestMiddleware = function () {
   const span = telemetry.startSpan({ name: 'strip:unsupported:accept:encoding', parentSpan: this.reqMiddlewareSpan, isVerbose })
 
-  // Cypress can only support plaintext or gzip, so make sure we don't request anything else
+  // Cypress can only support plaintext or gzip, so make sure we don't request anything else, by either filtering down to `gzip` or explicitly specifying `identity`
   const acceptEncoding = this.req.headers['accept-encoding']
 
   span?.setAttributes({
@@ -365,8 +365,12 @@ const StripUnsupportedAcceptEncoding: RequestMiddleware = function () {
     if (doesAcceptHeadingIncludeGzip) {
       this.req.headers['accept-encoding'] = 'gzip'
     } else {
-      delete this.req.headers['accept-encoding']
+      this.req.headers['accept-encoding'] = 'identity'
     }
+  } else {
+    // If there is no accept-encoding header, it means to accept everything (https://www.rfc-editor.org/rfc/rfc9110#name-accept-encoding).
+    // In that case, we want to explicitly filter that down to `gzip` and identity
+    this.req.headers['accept-encoding'] = 'gzip,identity'
   }
 
   span?.end()