diff --git a/doc/03-cli.md b/doc/03-cli.md
index 8393d36ff..2a6be0911 100644
--- a/doc/03-cli.md
+++ b/doc/03-cli.md
@@ -768,6 +768,11 @@ some tools like git or curl will only use the lower-cased `http_proxy` version.
 Alternatively you can also define the git proxy using
 `git config --global http.proxy <proxy url>`.
 
+If you are using Composer in a non-CLI context (i.e. integration into a CMS or
+similar use case), and need to support proxies, please provide the `CGI_HTTP_PROXY`
+environment variable instead. See [httpoxy.org](https://httpoxy.org/) for further
+details.
+
 ### no_proxy
 
 If you are behind a proxy and would like to disable it for certain domains, you
diff --git a/src/Composer/Util/StreamContextFactory.php b/src/Composer/Util/StreamContextFactory.php
index 484145f45..2831c435d 100644
--- a/src/Composer/Util/StreamContextFactory.php
+++ b/src/Composer/Util/StreamContextFactory.php
@@ -39,12 +39,16 @@ final class StreamContextFactory
             'max_redirects' => 20,
         ));
 
-        // Handle system proxy
-        if (!empty($_SERVER['HTTP_PROXY']) || !empty($_SERVER['http_proxy'])) {
-            // Some systems seem to rely on a lowercased version instead...
+        // Handle HTTP_PROXY/http_proxy on CLI only for security reasons
+        if (PHP_SAPI === 'cli' && (!empty($_SERVER['HTTP_PROXY']) || !empty($_SERVER['http_proxy']))) {
             $proxy = parse_url(!empty($_SERVER['http_proxy']) ? $_SERVER['http_proxy'] : $_SERVER['HTTP_PROXY']);
         }
 
+        // Prefer CGI_HTTP_PROXY if available
+        if (!empty($_SERVER['CGI_HTTP_PROXY'])) {
+            $proxy = parse_url($_SERVER['CGI_HTTP_PROXY']);
+        }
+
         // Override with HTTPS proxy if present and URL is https
         if (preg_match('{^https://}i', $url) && (!empty($_SERVER['HTTPS_PROXY']) || !empty($_SERVER['https_proxy']))) {
             $proxy = parse_url(!empty($_SERVER['https_proxy']) ? $_SERVER['https_proxy'] : $_SERVER['HTTPS_PROXY']);