Introducing the ASan Nightly Project

Every day, countless Mozillians spend numerous hours testing Firefox to ensure that Firefox users get a stable and secure product. However, no product is bug free and, despite all of our testing efforts, browsers still crash sometimes. When we investigate our crash reports, some of them even look like lingering security issues (e.g. use-after-free or other memory corruptions) but the data we have in these reports is often not sufficient for them to be actionable on their own (i.e. they do not provide enough information for a developer to be able to find and fix the problem). This is particularly true for use-after-free problems and some other types of memory corruptions where the actual crash happens a lot later than the memory violation itself.

In our automated integration and fuzz testing, we have been using AddressSanitizer (ASan), a compile-time instrumentation, very successfully for over 5 years. The information it provides about use-after-free is much more actionable than a simple crash stack: It not only tells you immediately when the violation happens, but also includes the location where the memory was free’d previously.

In order to leverage the combined power of Nightly testing and ASan we have joined them together to form the ASan Nightly Project. For this purpose we made a custom ASan Nightly build that is equipped with a special ASan reporter addon. This addon is capable of collecting and reporting ASan errors back to Mozilla, once they are detected. We launched this project to find errors in the wild and then leverage the ASan error report to identify and fix the problem, even though it might not be reproducible. So far, we made these builds for Linux only, but we are actively working on Windows and Mac builds.

Of course this approach comes with a drawback: While ASan’s performance can almost compete with the performance of a regular build, its already higher memory usage grows the longer you run the browser as ASan needs to retain freed memory for a while in order to detect use-after-free on it. Hence, running such a build requires you to have enough RAM (at least 16 GB is recommended) and to restart the browser once or twice a day to free memory.

However, if you are willing to browse the web using this new Firefox environment, you might be eligible to earn a bug bounty: We will treat the automated reporter submissions as if they were filed in Bugzilla (with no test case) which means that if the issue is 1) an eligible security problem and 2) can be fixed by our developers, you will receive a bug bounty for it. All rules of the Mozilla Bug Bounty Program apply. If you would like to participate, ensure that you read the Bug Bounty section carefully and set the right preference, so your report can be attributed to you.

This project can only succeed if enough people are using it. So if you meet the current requirements, we would be very happy if you joined the project.

Root Store Policy Updated

After several months of discussion on the mailing list, our Root Store Policy governing Certification Authorities (CAs) that are trusted in Mozilla products has been updated. Version 2.6 has an effective date of July 1st, 2018.

More than one dozen issues were addressed in this update, including the following changes:

  • Section 2.2 “Validation Practices” now requires CAs with the email trust bit to clearly disclose their email address validation methods in their CP/CPS.
  • The use of IP Address validation methods defined by the CA has been banned in certain circumstances.
  • Methods used for IP Address validation must now be clearly specified in the CA’s CP/CPS.
  • Section 3.1 “Audits” increases the WebTrust EV minimum version to 1.6.0 and removes ETSI TS 102 042 and 101 456 from the list of acceptable audit schemes in favor of EN 319 411.
  • Section 3.1.4 “Public Audit Information” formalizes the requirement for an English language version of the audit statement supplied by the Auditor.
  • Section 5.2 “Forbidden and Required Practices” moves the existing ban on CA key pair generation for SSL certificates into our policy.
  • After January 1, 2019, CAs will be required to create separate intermediate certificates for issuing SSL and S/MIME certificates. Newly issued Intermediate certificates will need to be restricted with an EKU extension that doesn’t contain anyPolicy, or both serverAuth and emailProtection. Intermediate certificates issued prior to 2019 that do not comply with this requirement may continue to be used to issue new end-entity certificates.
  • Section 5.3.2 “Publicly Disclosed and Audited” clarifies that Mozilla expects newly issued intermediate certificates to be included on the CA’s next periodic audit report. As long as the CA has current audits, no special audit is required when issuing a new intermediate. This matches the requirements in the CA/Browser Forum’s Baseline Requirements (BR) section 8.1.
  • Section 7.1 “Inclusions” adds a requirement that roots being added to Mozilla’s program must have complied with Mozilla’s Root Store Policy from the time that they were created. This effectively means that roots in existence prior to 2014 that did not receive BR audits after 2013 are not eligible for inclusion in Mozilla’s program. Roots with documented BR violations may also be excluded from Mozilla’s root store under this policy.
  • Section 8 “CA Operational Changes” now requires notification when an intermediate CA certificate is transferred to a third party.

A comparison of all the policy changes is available here.

Scanning for breached accounts with k-Anonymity

The new Firefox Monitor service will use anonymized range query API endpoints from Have I Been Pwned (HIBP). This new Firefox feature allows users to check for compromised online accounts while preserving their privacy.

An API request reveals sensitive data about the requesting party.

An API request can reveal subject identifiers like cookies, IP address, etc.

Anonymizing Account Identifiers

Operations like ‘search’ often need plaintext, or simply-hashed data. But, as Cloudflare has described in their own HIBP integration, searching with plain account data introduces privacy & security risks that allow an adversary, or even the service itself, to use the data to breach the searched account.

As an alternative, a user search client could download an entire set of data. Unfortunately this practice discloses all the service data to the client, which could abuse the data of all other users.

Anonymized Data Sharing

To mitigate these risks, Mozilla is working with Troy Hunt – creator and maintainer of HIBP – to use new hash range query API endpoints for breached account data in the Firefox Monitor project.

Hash range queries add k-Anonymity to the data that Mozilla exchanges with HIBP. Data with k-Anonymity protects individuals who are the subjects of the data from re-identification while preserving the utility of the data.

When a user submits their email address to Firefox Monitor, it hashes the plaintext value and sends the first 6 characters to the HIBP API. For example, the value “” hashes to 567159d622ffbb50b11b0efd307be358624a26ee. We send this hash prefix to the API endpoint:


The API responds with many suffixes and the list of breaches that include the full value:

    "HashSuffix": "D622FFBB50B11B0EFD307BE358624A26EE",
    "Websites": [
    "HashSuffix": "0000000000000000000000000000000000",
    "Websites": [
    "HashSuffix": "1111111111111111111111111111111111",
    "Websites": [

When Firefox Monitor receives this response, it loops thru the objects to find which (if any) prefix and breached account HashSuffix equals the the user-submitted hash value. The following pseudo code describes the algorithm in more detail:

if (fullUserHash === userHashPrefix + breachedAccount.HashSuffix)

Using the running example from above, for the first HashSuffix, the expression evaluates to:

if (‘567159D622FFBB50B11B0EFD307BE358624A26EE’ ===

Firefox Monitor discovers that “” appears in the LinkedIn breach, but does not disclose plaintext or even hashes of sensitive user data. Further, HIBP does not disclose its entire set of hashes, which allows Firefox users to maintain their privacy, and protects breached users from further exposure.

Brute Force Attacks

Hashed data is still vulnerable to brute-force attacks. An adversary could still loop thru a dictionary of email addresses to find the plaintext of all the range query results. To reduce this attack surface, Firefox Monitor does not store the range queries nor any results in its database. Instead, it caches a user’s results in an encrypted client session. We also monitor our scan endpoint to prevent abuse by an adversary attempting a brute force breached-account enumeration attack against our service.

Helping Subjects of Data Breaches

HIBP contains billions of records of email addresses. Troy has done an outstanding job to raise awareness and educate users about breaches globally. Breached sites embrace HIBP, even self-submitting their breached data. HIBP is there to help victims of data breaches after things go wrong, and Firefox Monitor is extending that help to more people.

Blocking FTP subresource loads within non-FTP documents in Firefox 61

Firefox 61 will block subresource loads that rely on the insecure FTP protocol unless the document itself is an FTP document. For example, Firefox will block FTP subresource loads within HTTP(S) pages.

The File Transfer Protocol (FTP) enables file exchange between computers on a network. While this standard protocol is supported by all major browsers and allows convenient file sharing within a network, it’s one of the oldest protocols in use today and has a number of security issues.

The fundamental underlying problem with FTP is that any data transferred will be unencrypted and hence sent across networks in plain text, allowing attackers to steal, spoof and even modify the data transmitted. To date, many malware distribution campaigns rely on compromising FTP servers, downloading malware on an end users device using the FTP protocol. Further, FTP makes HSTS protection somewhat useless, because the automated upgrading from an unencrypted to an encrypted connection that HSTS promises does not apply to FTP.

Following through to our intent to deprecate non-secure HTTP and aligning with Mozilla’s effort to improve adoption of HTTPS Firefox will block subresource loads, like images, scripts and iframes, relying on the insecure FTP protocol. Starting with Firefox 61, loading an FTP subresource within a non-FTP document will be blocked and the following message will be logged to the console:

For the Mozilla Security Team:
Tom Schuster and Christoph Kerschbaumer


Supporting Same-Site Cookies in Firefox 60

Firefox 60 will introduce support for the same-site cookie attribute, which allows developers to gain more control over cookies. Since browsers will include cookies with every request to a website, most sites rely on this mechanism to determine whether users are logged in.

Attackers can abuse the fact that cookies are automatically sent with every request to force a user to perform unwanted actions on the site where they are currently logged in. Such attacks, known as cross-site request forgeries (CSRF), allow attackers who control third-party code to perform fraudulent actions on the user’s behalf. Unfortunately current web architecture does not allow web applications to reliably distinguish between actions initiated by the user and those that are initiated by any of the third-party gadgets or scripts that they rely on.

To compensate, the same-site cookie attribute allows a web application to advise the browser that cookies should only be sent if the request originates from the website the cookie came from. Requests triggered from a URL different than the one that appears in the URL bar will not include any of the cookies tagged with this new attribute.

The same-site attribute can take one of two values: ‘strict’ or ‘lax’. In strict mode, same-site cookies will be withheld for any kind of cross-site usage. This includes all inbound links from external sites to the application. Visitors clicking on such a link will initially be treated as ‘not being logged in’ whether or not they have an active session with the site.

The lax mode caters to applications which are incompatible with these restrictions. In this mode, same-site cookies will be withheld on cross-domain subrequests (e.g. images or frames), but will be sent whenever a user navigates safely from an external site, for example by following a link.

For the Mozilla Security Team:
Christoph Kerschbaumer, Mark Goodwin, Francois Marier

Distrust of Symantec TLS Certificates

A Certification Authority (CA) is an organization that browser vendors (like Mozilla) trust to issue certificates to websites. Last year, Mozilla published and discussed a set of issues with one of the oldest and largest CAs run by Symantec. The discussion resulted in the adoption of a consensus proposal to gradually remove trust in all Symantec TLS/SSL certificates from Firefox. The proposal includes a number of phases designed to minimize the impact of the change to Firefox users:

  • January 2018 (Firefox 58): Notices in the Browser Console warn about Symantec certificates issued before 2016-06-01, to encourage site owners to replace their TLS certificates.
  • May 2018 (Firefox 60): Websites will show an untrusted connection error if they use a TLS certificate issued before 2016-06-01 that chains up to a Symantec root certificate.
  • October 2018 (Firefox 63): Distrust of Symantec root certificates for website server TLS authentication.

After the consensus proposal was adopted, the Symantec CA was acquired by DigiCert; however, that fact has not changed Mozilla’s commitment to implement the proposal.

Firefox 60 is expected to enter Beta on March 13th carrying with it the removal of trust for Symantec certificates issued prior to June 1st, 2016, with the exception of certificates issued by a few subordinate CAs that are controlled by Apple and Google. This change affects all Symantec brands including GeoTrust, RapidSSL, Thawte, and VeriSign. The change is already in effect in Firefox Nightly.

Mozilla telemetry currently shows that a significant number of sites – roughly 1% of the top one million – are still using TLS certificates that are no longer trusted in Firefox 60. While the number of affected sites has been declining steadily, we do not expect every website to be updated prior to the Beta release of Firefox 60. We strongly encourage operators of affected sites to take immediate action to replace these certificates.

If you attempt to visit a site that is using a TLS certificate that is no longer trusted in Firefox 60, you will encounter the following error:

Clicking on the “Advanced” button will allow you to bypass the error and reach the site:

These changes are expected to be included in the final version of Firefox 60 that is planned to be release on May 9th, 2018.

In Firefox 63, trust will be removed for all Symantec TLS certificates regardless of the date issued (with the exception of certificates issued by Apple and Google subordinate CAs as described above).

Wayne Thayer
Kathleen Wilson

Analysis of the Alexa Top 1M Sites

Prior to the release of the Mozilla Observatory in June of 2016, I ran a scan of the Alexa Top 1M websites. Despite being available for years, the usage rates of modern defensive security technologies was frustratingly low. A lack of tooling combined with poor and scattered documentation had led to minimal awareness around countermeasures such as Content Security Policy (CSP), HTTP Strict Transport Security (HSTS), and Subresource Integrity (SRI).

Since then, a number of additional assessments have done, including in October 2016 and June 2017. Both of those surveys demonstrated clear and continual improvement in the state of internet security. But now that tools like the Mozilla Observatory, and Hardenize have become more commonplace, has the excitement for improvement been tempered?

February 2018 Scan

Technology June 2017 February 2018 % Change
(June 2017)
% Change
Content Security Policy (CSP) .018%2
Cookies (Secure/HttpOnly)4 6.50% 8.97% +38% +139%
Cross-origin Resource Sharing (CORS)5 96.55% 96.89% +.35% +3.3%
HTTPS 45.80% 54.31% +19% +83%
HTTP → HTTPS Redirection 14.38%6
Public Key Pinning (HPKP) 0.71% 1.07% +51% +148%
   — HPKP Preloaded8 0.43% 0.70% +63% +71%
Strict Transport Security (HSTS)9 4.37% 6.03% +38% +245%
   — HSTS Preloaded8 .337% .631% +87% +299%
Subresource Integrity (SRI) 0.113%10 0.182%11 +61% +1113%
X-Content-Type-Options (XCTO) 9.41% 11.72% +21% +89%
X-Frame-Options (XFO)12 10.98% 12.55% +14% +84%
X-XSS-Protection (XXSSP)13 8.12% 10.36% +28% +106%

Improvement across the web appears to be continuing at a steady rate. Although a 19% increase in the number of sites that support HTTPS might seem small, the absolute numbers are quite large — it represents over 83,000 websites, a slight slowdown from the previous survey’s 119,000 jump, but still a great sign of progress in encrypting the web’s long tail.

Not only that, but an additional 97,000 of the top websites have chosen to be HTTPS by default, with another 16,000 of them forbidding any HTTP access at all through the use of HTTP Strict Transport Security (HSTS). Also notable is the jump in websites that have chosen to opt into being preloaded in major web browsers, via a process known as HSTS preloading. Until browsers switch to HTTPS by default, HSTS preloading is the best method for solving the trust-on-first-use problem in HSTS.

Content Security Policy (CSP) — one of the most important recent advances due to its ability to prevent cross-site scripting (XSS) attacks — continues to see strong growth. Growth is faster in policies that ignore inline stylesheets (CSS), perhaps reflecting the difficulties that many sites have with separating their presentation from their content. Nevertheless, improvements brought about by specification additions such as 'strict-dynamic' and policy generators such as the Mozilla Laboratory continue to push forward CSP adoption.

Mozilla Observatory Grading

Despite this progress, the vast majority of top websites around the web continue not to use Content Security Policy, Strict Transport Security, or Subresource Integrity. As these technologies — when properly used — can nearly eliminate huge classes of attacks against sites and their users, they are given a significant amount of weight in Observatory scans.

As a result of their low usage rates amongst top websites, they typically receive failing grades from the Observatory:

Grade April 2016 October 2016 June 2017 February 2018 % Change
  A+ .003% .008% .013% .018% +38%
A .006% .012% .029% .011% -62%
B .202% .347% .622% 1.08% +74%
C .321% .727% 1.38% 2.04% +48%
D 1.87% 2.82% 4.51% 6.12% +36%
F 97.60% 96.09% 93.45% 90.73% -2.9%

We do see some significant improvements. As 976,930 scans were successfully completed in the last survey, a decrease in failing grades by 2.9% implies that over 27,000 of the top sites in the world have improved from a failing grade in the last eight months alone. Note that the drop in A grades is due to a recent change where extra credit points can no longer be used to move up to an A grade..

Thus far, over 140,000 websites around the web have directly used the Mozilla Observatory to improve their grades, indicated by making an improvement to their website after an initial scan. Of these 140,000 websites, over 2,800 have improved all the way from a failing grade to an A or A+ grade.

When I first built the Observatory at Mozilla, I had never imagined that it would see such widespread use. 6.6M scans across 2.3M unique domains later, it seems to have made a significant difference across the internet. I couldn’t have done it without the support of Mozilla and the security researchers who have helped to improve it.

Please share the Mozilla Observatory so that the web can continue to see improvements over the years to come!


  1. Since April 2016
  2. Allows 'unsafe-inline' in neither script-src nor style-src
  3. Allows 'unsafe-inline' in style-src only
  4. Amongst sites that set cookies
  5. Disallows foreign origins from reading the domain’s contents within user’s context
  6. Redirects from HTTP to HTTPS on the same domain, which allows HSTS to be set
  7. Redirects from HTTP to HTTPS, regardless of the final domain
  8. As listed in the Chromium preload list
  9. max-age set to at least six months
  10. Percentage is of sites that load scripts from a foreign origin
  11. Percentage is of sites that load scripts
  12. CSP frame-ancestors directive is allowed in lieu of an XFO header
  13. Strong CSP policy forbidding 'unsafe-inline' is allowed in lieu of an XXSSP header

Restricting AppCache to Secure Contexts

The Application Cache (AppCache) interface provides a caching mechanism that allows websites to run offline. Using this API, developers can specify resources that the browser should cache and make available to users offline. Unfortunately, AppCache has limitations in revalidating its cache, which allows attackers to trick the browser into never revalidate the cache by setting a manifest to a malformed cache file. Removing AppCache over HTTP connections removes the risk that users could see stale cached content that came from a malicious connection indefinitely.

Consider the following attack scenario: A user logs onto a coffee shop WiFi where an attacker can manipulate the WiFi that is served over HTTP. Even if the user only visits one HTTP page over the WiFi, the attacker can plant many insecure iframes using AppCache which allows the attacker to rig the cache with malicious content manipulating all of those sites indefinitely. Even a cautious user who decides only to login to their websites at home is at risk due to this stale cache.

In line with our previous stated intents of deprecating HTTP and requiring HTTPS for all new APIs, we are continuing to remove features from sites served over insecure connections. This means that websites wishing to preserve all their functionality should transition their sites to using TLS encryption as soon as possible.

In Firefox 60+ Beta and Nightly, Application Cache access from HTTP pages is denied. Starting with Firefox 62 Release, Application Cache over HTTP will be fully removed for all release channels. All other browsers have also stated their intent to remove: Chrome, Edge, WebKit. This change will also be reflected in the HTML standard.

Going forward, Firefox will deprecate more APIs over insecure connections in an attempt to increase adoption of HTTPS and improve the safety of the internet as a whole.

Preventing data leaks by stripping path information in HTTP Referrers

To help prevent third party data leakage while browsing privately, Firefox Private Browsing Mode will remove path information from referrers sent to third parties starting in Firefox 59.

Referrers can leak sensitive data

Screenshot of requests. Source: EFF

An example of personal health data being sent to third parties from Source: EFF

When you click a link in your browser to navigate to a new site, the new site you visit receives the exact address of the site you came from through the so-called “Referrer value”. For example, if you came to this Mozilla Security Blog from, the browser would send this:


This leaks user data to websites, telling websites the exact page you were looking at when you clicked the link. To make things worse, browsers also send a referrer value when requesting sub-resources, like ads, or other social media snippets integrated in a modern web site. In other words, embedded content also knows exactly what page you are visiting

Most sites log this data for operational and statistical purposes. Many sites also log this data to collect as much information about their users as possible.  They can then use that data for a variety of purposes, or even sell that data – e.g., for re-targeting.

While the data above may not be a problem, consider this example:

Referer: https://www.

EFF researchers discovered this leak of personal health data from to DoubleClick. As indicated, the referrer in this case leaks information about your age, your zip code, whether you are a smoker or not, and potentially even your income. Other companies (link1, link2) have disclosed similar vulnerabilities and leaks.

Private Browsing will strip paths in HTTP referrers

Screenshot: Firefox Private Browsing window

To prevent this type of data leakage when Firefox users are browsing privately, we are changing the way Firefox sends referrers in Private Browsing Mode.

Starting with Firefox 59, Private Browsing will remove path information from referrer values sent to third parties (i.e. technically, setting a Referrer Policy of strict-origin-when-cross-origin).

In the previous examples, this setting would remove the path and query string data from the referrer values so that they are stripped down to:




This change prevents site authors from accidentally leaking user data to third parties when their users choose Private Browsing Mode.  We made this change only after first ensuring that this would have minimal to no effect on web usability.

Other ways of controlling referrers

Vendors and authors continue to propose changes to Referrers to improve web privacy, security, and functionality.

In 2014, the W3C Web Application Security Working Group started its Referrer Policy Recommendation. This Policy lets vendors and authors control referrer values. For example, it defines a secure-by-default no-referrer-when-downgrade policy for user agents, which does not send referrers to HTTP resources from an HTTPS page. In Firefox Regular and Private Browsing Mode, if a site specifically sets a more restrictive or more liberal Referrer Policy than the browser default, the browser will honor the websites request since the site author is intentionally changing the value.

Users can also change their default referrer options in Firefox.  These will override the browser’s default Referrer Policy and override the site author’s Referrer Policy, putting the users choice first.


January 2018 CA Communication

Mozilla has sent a CA Communication to inform Certificate Authorities (CAs) who have root certificates included in Mozilla’s program about current events related to domain validation for SSL certificates and to remind them of a number of upcoming deadlines. This CA Communication has been emailed to the Primary Point of Contact (POC) and an email alias for each CA in Mozilla’s program, and they have been asked to respond to the following 6 action items:

  1. Disclose Use of Baseline Requirements Methods or for Domain Validation – Recently discovered vulnerabilities in these methods of domain validation have led Mozilla to require CAs to disclose their use of these methods and to describe how they have mitigated these vulnerabilities.
  2. Disclose Use of Methods or for Domain Validation – Significant concerns were recently raised about the reliability of these methods that are defined in the Baseline Requirements for the Issuance and Management of Publicly-Trusted Certificates.
  3. Disclose All Non-Technically-Constrained Subordinate CA Certificates – CAs have until April 15, 2018 to disclose all non-technically constrained subordinate CA certificates – including subordinate CA certificates that are constrained via EKU to S/MIME but do not have Name Constraints – as required by version 2.5 of Mozilla’s Root Store Policy.
  4. Complete BR Self Assessment – Mozilla has asked all CAs to complete a Baseline Requirements Self-Assessment by January 31, 2018, or by April 15, 2018 if an extension was requested.
  5. Update CP/CPS to Comply with version 2.5 of Mozilla’s Root Store Policy – In the November 2017 CA Communication, a number of CAs indicated that their CP/CPS does not yet comply with version 2.5 of the Mozilla Root Store Policy. The deadline for compliance has been extended to April 15, 2018.
  6. Reduce SSL Certificate Validity Periods to 825 Days or Less by March 1, 2018 – On March 17, 2017, in ballot 193, the CA/Browser Forum set a deadline of March 1, 2018 after which newly-issued SSL certificates must not have a validity period greater than 825 days, and the re-use of validation information must be limited to 825 days.

The full action items can be read here. Responses to the survey will be automatically and immediately published by the CCADB.

With this CA Communication, we reiterate that participation in Mozilla’s CA Certificate Program is at our sole discretion, and we will take whatever steps are necessary to keep our users safe. Nevertheless, we believe that the best approach to safeguard that security is to work with CAs as partners, to foster open and frank communication, and to be diligent in looking for ways to improve.

Mozilla Security Team