--- a/caps/nsIAddonPolicyService.idl
+++ b/caps/nsIAddonPolicyService.idl
@@ -25,8 +25,23 @@ interface nsIAddonPolicyService : nsISup
    */
   boolean extensionURILoadableByAnyone(in nsIURI aURI);
 
   /**
    * Maps an extension URI to the ID of the addon it belongs to.
    */
   AString extensionURIToAddonId(in nsIURI aURI);
 };
+
+/**
+ * This interface exposes functionality related to add-on content policy
+ * enforcement.
+ */
+[scriptable,uuid(7a4fe60b-9131-45f5-83f3-dc63b5d71a5d)]
+interface nsIAddonContentPolicy : nsISupports
+{
+  /**
+   * Checks a custom content security policy string, to ensure that it meets
+   * minimum security requirements. Returns null for valid policies, or a
+   * string describing the error for invalid policies.
+   */
+  AString validateAddonCSP(in AString aPolicyString);
+};

new file mode 100644
--- /dev/null
+++ b/toolkit/components/extensions/test/xpcshell/test_csp_validator.js
@@ -0,0 +1,65 @@
+/* -*- Mode: indent-tabs-mode: nil; js-indent-level: 2 -*- */
+/* vim: set sts=2 sw=2 et tw=80: */
+"use strict";
+
+const cps = Cc["@mozilla.org/addons/content-policy;1"].getService(Ci.nsIAddonContentPolicy);
+
+add_task(function* test_csp_validator() {
+  let checkPolicy = (policy, expectedResult, message = null) => {
+    do_print(`Checking policy: ${policy}`);
+
+    let result = cps.validateAddonCSP(policy);
+    equal(result, expectedResult);
+  };
+
+  checkPolicy("script-src 'self'; object-src 'self';",
+              null);
+
+  let hash = "'sha256-NjZhMDQ1YjQ1MjEwMmM1OWQ4NDBlYzA5N2Q1OWQ5NDY3ZTEzYTNmMzRmNjQ5NGU1MzlmZmQzMmMxYmIzNWYxOCAgLQo='";
+
+  checkPolicy(`script-src 'self' https://com https://*.example.com moz-extension://09abcdef blob: filesystem: ${hash} 'unsafe-eval'; ` +
+              `object-src 'self' https://com https://*.example.com moz-extension://09abcdef blob: filesystem: ${hash}`,
+              null);
+
+  checkPolicy("",
+              "Policy is missing a required 'script-src' directive");
+
+  checkPolicy("object-src 'none';",
+              "Policy is missing a required 'script-src' directive");
+
+  checkPolicy("script-src 'self';",
+              "Policy is missing a required 'object-src' directive");
+
+  checkPolicy("script-src 'none'; object-src 'none'",
+              "'script-src' must include the source 'self'");
+
+  checkPolicy("script-src 'self'; object-src 'none';",
+              null);
+
+  checkPolicy("script-src 'self' 'unsafe-inline'; object-src 'self';",
+              "'script-src' directive contains a forbidden 'unsafe-inline' keyword");
+
+
+  let directives = ["script-src", "object-src"];
+
+  for (let [directive, other] of [directives, directives.slice().reverse()]) {
+    for (let src of ["https://*", "https://*.blogspot.com", "https://*"]) {
+      checkPolicy(`${directive} 'self' ${src}; ${other} 'self';`,
+                  `https: wildcard sources in '${directive}' directives must include at least one non-generic sub-domain (e.g., *.example.com rather than *.com)`);
+    }
+
+    checkPolicy(`${directive} 'self' https:; ${other} 'self';`,
+                `https: protocol requires a host in '${directive}' directives`);
+
+    checkPolicy(`${directive} 'self' http://example.com; ${other} 'self';`,
+                `'${directive}' directive contains a forbidden http: protocol source`);
+
+    for (let protocol of ["http", "ftp", "meh"]) {
+      checkPolicy(`${directive} 'self' ${protocol}:; ${other} 'self';`,
+                  `'${directive}' directive contains a forbidden ${protocol}: protocol source`);
+    }
+
+    checkPolicy(`${directive} 'self' 'nonce-01234'; ${other} 'self';`,
+                `'${directive}' directive contains a forbidden 'nonce-*' keyword`);
+  }
+});

--- a/toolkit/components/extensions/test/xpcshell/xpcshell.ini
+++ b/toolkit/components/extensions/test/xpcshell/xpcshell.ini
@@ -1,9 +1,10 @@
 [DEFAULT]
 head = head.js
 tail =
 firefox-appdir = browser
 skip-if = toolkit == 'android' || toolkit == 'gonk'
 
+[test_csp_validator.js]
 [test_locale_data.js]
 [test_locale_converter.js]
 [test_ext_schemas.js]

new file mode 100644
--- /dev/null
+++ b/toolkit/locales/en-US/chrome/global/extensions.properties
@@ -0,0 +1,12 @@
+
+csp.error.missing-directive = Policy is missing a required '%S' directive
+
+csp.error.illegal-keyword = '%1$S' directive contains a forbidden %2$S keyword
+
+csp.error.illegal-protocol = '%1$S' directive contains a forbidden %2$S: protocol source
+
+csp.error.missing-host = %2$S: protocol requires a host in '%1$S' directives
+
+csp.error.missing-source = '%1$S' must include the source %2$S
+
+csp.error.illegal-host-wildcard = %2$S: wildcard sources in '%1$S' directives must include at least one non-generic sub-domain (e.g., *.example.com rather than *.com)

--- a/toolkit/locales/jar.mn
+++ b/toolkit/locales/jar.mn
@@ -40,16 +40,17 @@
   locale/@AB_CD@/global/customizeToolbar.properties     (%chrome/global/customizeToolbar.properties)
 #endif
   locale/@AB_CD@/global/datetimepicker.dtd              (%chrome/global/datetimepicker.dtd)
   locale/@AB_CD@/global/dateFormat.properties           (%chrome/global/dateFormat.properties)
   locale/@AB_CD@/global/dialogOverlay.dtd               (%chrome/global/dialogOverlay.dtd)
 #ifndef MOZ_FENNEC
   locale/@AB_CD@/global/editMenuOverlay.dtd             (%chrome/global/editMenuOverlay.dtd)
 #endif
+  locale/@AB_CD@/global/extensions.properties           (%chrome/global/extensions.properties)
   locale/@AB_CD@/global/fallbackMenubar.properties      (%chrome/global/fallbackMenubar.properties)
   locale/@AB_CD@/global/filefield.properties            (%chrome/global/filefield.properties)
   locale/@AB_CD@/global/filepicker.dtd                  (%chrome/global/filepicker.dtd)
   locale/@AB_CD@/global/filepicker.properties           (%chrome/global/filepicker.properties)
 #ifndef MOZ_FENNEC
   locale/@AB_CD@/global/findbar.dtd                     (%chrome/global/findbar.dtd)
   locale/@AB_CD@/global/findbar.properties              (%chrome/global/findbar.properties)
   locale/@AB_CD@/global/finddialog.dtd                  (%chrome/global/finddialog.dtd)

--- a/toolkit/mozapps/extensions/AddonContentPolicy.cpp
+++ b/toolkit/mozapps/extensions/AddonContentPolicy.cpp
@@ -1,45 +1,56 @@
 /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
 /* vim: set ts=8 sts=2 et sw=2 tw=80: */
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 #include "AddonContentPolicy.h"
 
+#include "mozilla/dom/nsCSPUtils.h"
 #include "nsCOMPtr.h"
 #include "nsContentPolicyUtils.h"
 #include "nsContentTypeParser.h"
 #include "nsContentUtils.h"
 #include "nsIConsoleService.h"
+#include "nsIContentSecurityPolicy.h"
 #include "nsIContent.h"
 #include "nsIDocument.h"
+#include "nsIEffectiveTLDService.h"
 #include "nsIScriptError.h"
+#include "nsIStringBundle.h"
+#include "nsIUUIDGenerator.h"
 #include "nsIURI.h"
+#include "nsNetCID.h"
+#include "nsNetUtil.h"
+
+using namespace mozilla;
 
 /* Enforces content policies for WebExtension scopes. Currently:
  *
  *  - Prevents loading scripts with a non-default JavaScript version.
+ *  - Checks custom content security policies for sufficiently stringent
+ *    script-src and object-src directives.
  */
 
 #define VERSIONED_JS_BLOCKED_MESSAGE \
   MOZ_UTF16("Versioned JavaScript is a non-standard, deprecated extension, and is ") \
   MOZ_UTF16("not supported in WebExtension code. For alternatives, please see: ") \
   MOZ_UTF16("https://developer.mozilla.org/Add-ons/WebExtensions/Tips")
 
 AddonContentPolicy::AddonContentPolicy()
 {
 }
 
 AddonContentPolicy::~AddonContentPolicy()
 {
 }
 
-NS_IMPL_ISUPPORTS(AddonContentPolicy, nsIContentPolicy)
+NS_IMPL_ISUPPORTS(AddonContentPolicy, nsIContentPolicy, nsIAddonContentPolicy)
 
 static nsresult
 GetWindowIDFromContext(nsISupports* aContext, uint64_t *aResult)
 {
   NS_ENSURE_TRUE(aContext, NS_ERROR_FAILURE);
 
   nsCOMPtr<nsIContent> content = do_QueryInterface(aContext);
   NS_ENSURE_TRUE(content, NS_ERROR_FAILURE);
@@ -75,16 +86,19 @@ LogMessage(const nsAString &aMessage, ns
 
   nsCOMPtr<nsIConsoleService> console = do_GetService(NS_CONSOLESERVICE_CONTRACTID);
   NS_ENSURE_TRUE(console, NS_ERROR_OUT_OF_MEMORY);
 
   console->LogMessage(error);
   return NS_OK;
 }
 
+
+// Content policy enforcement:
+
 NS_IMETHODIMP
 AddonContentPolicy::ShouldLoad(uint32_t aContentType,
                                nsIURI* aContentLocation,
                                nsIURI* aRequestOrigin,
                                nsISupports* aContext,
                                const nsACString& aMimeTypeGuess,
                                nsISupports* aExtra,
                                nsIPrincipal* aRequestPrincipal,
@@ -138,8 +152,317 @@ AddonContentPolicy::ShouldProcess(uint32
                                   int16_t* aShouldProcess)
 {
   MOZ_ASSERT(aContentType == nsContentUtils::InternalContentPolicyTypeToExternal(aContentType),
              "We should only see external content policy types here.");
 
   *aShouldProcess = nsIContentPolicy::ACCEPT;
   return NS_OK;
 }
+
+
+// CSP Validation:
+
+static const char* allowedSchemes[] = {
+  "blob",
+  "filesystem",
+};
+
+static const char* allowedHostSchemes[] = {
+  "https",
+  "moz-extension",
+};
+
+/**
+ * Validates a CSP directive to ensure that it is sufficiently stringent.
+ * In particular, ensures that:
+ *
+ *  - No remote sources are allowed other than from https: schemes
+ *
+ *  - No remote sources specify host wildcards for generic domains
+ *    (*.blogspot.com, *.com, *)
+ *
+ *  - All remote sources and local extension sources specify a host
+ *
+ *  - No scheme sources are allowed other than blob:, filesystem:,
+ *    moz-extension:, and https:
+ *
+ *  - No keyword sources are allowed other than 'none', 'self', 'unsafe-eval',
+ *    and hash sources.
+ */
+class CSPValidator final : public nsCSPSrcVisitor {
+  public:
+    CSPValidator(nsAString& aURL, CSPDirective aDirective) :
+      mURL(aURL),
+      mDirective(CSP_CSPDirectiveToString(aDirective)),
+      mFoundSelf(false)
+    {
+      // Start with the default error message for a missing directive, since no
+      // visitors will be called if the directive isn't present.
+      formatError("csp.error.missing-directive");
+    }
+
+
+    // Visitors
+
+    bool visitSchemeSrc(const nsCSPSchemeSrc& src) override
+    {
+      nsAutoString scheme;
+      src.getScheme(scheme);
+
+      if (schemeIsAllowed(scheme, allowedHostSchemes)) {
+        formatError("csp.error.missing-host", scheme);
+        return false;
+      }
+      if (!schemeIsAllowed(scheme, allowedSchemes)) {
+        formatError("csp.error.illegal-protocol", scheme);
+        return false;
+      }
+      return true;
+    };
+
+    bool visitHostSrc(const nsCSPHostSrc& src) override
+    {
+      nsAutoString scheme, host;
+
+      src.getScheme(scheme);
+      src.getHost(host);
+
+      if (scheme.LowerCaseEqualsLiteral("https")) {
+        if (!hostIsAllowed(host)) {
+          formatError("csp.error.illegal-host-wildcard", scheme);
+          return false;
+        }
+      } else if (scheme.LowerCaseEqualsLiteral("moz-extension")) {
+        // The CSP parser silently converts 'self' keywords to the origin
+        // URL, so we need to reconstruct the URL to see if it was present.
+        if (!mFoundSelf) {
+          nsAutoString url(MOZ_UTF16("moz-extension://"));
+          url.Append(host);
+
+          mFoundSelf = url.Equals(mURL);
+        }
+
+        if (host.IsEmpty() || host.EqualsLiteral("*")) {
+          formatError("csp.error.missing-host", scheme);
+          return false;
+        }
+      } else if (!schemeIsAllowed(scheme, allowedSchemes)) {
+        formatError("csp.error.illegal-protocol", scheme);
+        return false;
+      }
+
+      return true;
+    };
+
+    bool visitKeywordSrc(const nsCSPKeywordSrc& src) override
+    {
+      switch (src.getKeyword()) {
+      case CSP_NONE:
+      case CSP_SELF:
+      case CSP_UNSAFE_EVAL:
+        return true;
+
+      default:
+        NS_ConvertASCIItoUTF16 keyword(CSP_EnumToKeyword(src.getKeyword()));
+
+        formatError("csp.error.illegal-keyword", keyword);
+        return false;
+      }
+    };
+
+    bool visitNonceSrc(const nsCSPNonceSrc& src) override
+    {
+      formatError("csp.error.illegal-keyword", NS_LITERAL_STRING("'nonce-*'"));
+      return false;
+    };
+
+    bool visitHashSrc(const nsCSPHashSrc& src) override
+    {
+      return true;
+    };
+
+
+    // Accessors
+
+    inline nsAString& getError()
+    {
+      return mError;
+    };
+
+    inline bool foundSelf()
+    {
+      return mFoundSelf;
+    };
+
+
+    // Formatters
+
+    template <typename... T>
+    inline void formatError(const char* aName, const T ...aParams)
+    {
+      const char16_t* params[] = { mDirective.get(), aParams.get()... };
+      formatErrorParams(aName, params, PR_ARRAY_SIZE(params));
+    };
+
+  private:
+    // Validators
+
+    bool hostIsAllowed(nsAString& host)
+    {
+      if (host.First() == '*') {
+        if (host.EqualsLiteral("*") || host[1] != '.') {
+          return false;
+        }
+
+        host.Cut(0, 2);
+
+        nsCOMPtr<nsIEffectiveTLDService> tldService =
+          do_GetService(NS_EFFECTIVETLDSERVICE_CONTRACTID);
+
+        if (!tldService) {
+          return false;
+        }
+
+        NS_ConvertUTF16toUTF8 cHost(host);
+        nsAutoCString publicSuffix;
+
+        nsresult rv = tldService->GetPublicSuffixFromHost(cHost, publicSuffix);
+
+        return NS_SUCCEEDED(rv) && !cHost.Equals(publicSuffix);
+      }
+
+      return true;
+    };
+
+    template <typename T>
+    bool schemeIsAllowed(nsAString& scheme, T& allowedSchemes)
+    {
+      for (uint32_t i = 0; i < PR_ARRAY_SIZE(allowedSchemes); i++) {
+        if (scheme.LowerCaseEqualsASCII(allowedSchemes[i])) {
+          return true;
+        }
+      }
+      return false;
+    };
+
+
+    // Formatters
+
+    already_AddRefed<nsIStringBundle>
+    getStringBundle()
+    {
+      nsCOMPtr<nsIStringBundleService> sbs =
+        mozilla::services::GetStringBundleService();
+      NS_ENSURE_TRUE(sbs, nullptr);
+
+      nsCOMPtr<nsIStringBundle> stringBundle;
+      sbs->CreateBundle("chrome://global/locale/extensions.properties",
+                        getter_AddRefs(stringBundle));
+
+      return stringBundle.forget();
+    };
+
+    void formatErrorParams(const char* aName, const char16_t** aParams, int32_t aLength)
+    {
+      nsresult rv = NS_ERROR_FAILURE;
+
+      nsCOMPtr<nsIStringBundle> stringBundle = getStringBundle();
+
+      if (stringBundle) {
+        NS_ConvertASCIItoUTF16 name(aName);
+
+        rv = stringBundle->FormatStringFromName(name.get(), aParams, aLength,
+                                                getter_Copies(mError));
+      }
+
+      if (NS_WARN_IF(NS_FAILED(rv))) {
+        mError.AssignLiteral("An unexpected error occurred");
+      }
+    };
+
+
+    // Data members
+
+    nsAutoString mURL;
+    NS_ConvertASCIItoUTF16 mDirective;
+    nsXPIDLString mError;
+
+    bool mFoundSelf;
+};
+
+/**
+ * Validates a custom content security policy string for use by an add-on.
+ * In particular, ensures that:
+ *
+ *  - Both object-src and script-src directives are present, and meet
+ *    the policies required by the CSPValidator class
+ *
+ *  - The script-src directive includes the source 'self'
+ */
+NS_IMETHODIMP
+AddonContentPolicy::ValidateAddonCSP(const nsAString& aPolicyString,
+                                     nsAString& aResult)
+{
+  nsresult rv;
+
+  // Validate against a randomly-generated extension origin.
+  // There is no add-on-specific behavior in the CSP code, beyond the ability
+  // for add-ons to specify a custom policy, but the parser requires a valid
+  // origin in order to operate correctly.
+  nsAutoString url(MOZ_UTF16("moz-extension://"));
+  {
+    nsCOMPtr<nsIUUIDGenerator> uuidgen = services::GetUUIDGenerator();
+    NS_ENSURE_TRUE(uuidgen, NS_ERROR_FAILURE);
+
+    nsID id;
+    rv = uuidgen->GenerateUUIDInPlace(&id);
+    NS_ENSURE_SUCCESS(rv, rv);
+
+    char idString[NSID_LENGTH];
+    id.ToProvidedString(idString);
+
+    url.AppendASCII(idString + 1, NSID_LENGTH - 2);
+  }
+
+
+  RefPtr<BasePrincipal> principal =
+    BasePrincipal::CreateCodebasePrincipal(NS_ConvertUTF16toUTF8(url));
+
+  nsCOMPtr<nsIContentSecurityPolicy> csp;
+  rv = principal->EnsureCSP(nullptr, getter_AddRefs(csp));
+  NS_ENSURE_SUCCESS(rv, rv);
+
+
+  csp->AppendPolicy(aPolicyString, false, false);
+
+  const nsCSPPolicy* policy = csp->GetPolicy(0);
+  if (!policy) {
+    CSPValidator validator(url, nsIContentSecurityPolicy::SCRIPT_SRC_DIRECTIVE);
+    aResult.Assign(validator.getError());
+    return NS_OK;
+  }
+
+
+  aResult.SetIsVoid(true);
+  {
+    CSPDirective directive = nsIContentSecurityPolicy::SCRIPT_SRC_DIRECTIVE;
+    CSPValidator validator(url, directive);
+
+    if (!policy->visitDirectiveSrcs(directive, &validator)) {
+      aResult.Assign(validator.getError());
+    } else if (!validator.foundSelf()) {
+      validator.formatError("csp.error.missing-source", NS_LITERAL_STRING("'self'"));
+      aResult.Assign(validator.getError());
+    }
+  }
+
+  if (aResult.IsVoid()) {
+    CSPDirective directive = nsIContentSecurityPolicy::OBJECT_SRC_DIRECTIVE;
+    CSPValidator validator(url, directive);
+
+    if (!policy->visitDirectiveSrcs(directive, &validator)) {
+      aResult.Assign(validator.getError());
+    }
+  }
+
+  return NS_OK;
+}

--- a/toolkit/mozapps/extensions/AddonContentPolicy.h
+++ b/toolkit/mozapps/extensions/AddonContentPolicy.h
@@ -1,19 +1,22 @@
 /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
 /* vim: set ts=8 sts=2 et sw=2 tw=80: */
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 #include "nsIContentPolicy.h"
+#include "nsIAddonPolicyService.h"
 
-class AddonContentPolicy : public nsIContentPolicy
+class AddonContentPolicy : public nsIContentPolicy,
+                           public nsIAddonContentPolicy
 {
 protected:
   virtual ~AddonContentPolicy();
 
 public:
   AddonContentPolicy();
 
   NS_DECL_ISUPPORTS
   NS_DECL_NSICONTENTPOLICY
+  NS_DECL_NSIADDONCONTENTPOLICY
 };