Security Fields, Hotlists, and Issue Access / Visibility

Bug database labels are used very heavily for security bugs. We rely on the labels being correct for a variety of reasons, including driving fixing efforts, driving release management efforts (merges and release notes) and also historical queries and data mining.

Because of the extent to which we rely on labels, it is an important part of the Security Sheriff duty to ensure that all security bugs are correctly tagged and managed. But even if you are not the Security Shepherd, please fix any labeling errors you happen upon.

Any issue that relates to security should have one of the following:

  • Security Component Tag: Features that are related to security.
  • Type=Vulnerability: Designates a security vulnerability that impacts users. This label should not be used for new features that relate to security, or general remediation/refactoring ideas. (Use the Security Component Tag for that.)

Fields and Hotlists Relevant For Any Type=Vulnerability

  • Security_Severity-{Critical (S0), High (S1), Medium (S2), Low(S3), Unknown / Not Yet Assessed (S4)}: Designates the severity of a vulnerability according to our severity guidelines.

  • Priority: P#: Priority should generally match Severity (but should be higher if there is evidence of active exploitation):

    • Security_Severity-Critical: P0.
    • High and Medium: P1.
    • Low: P2.
  • Found In: MMM#: Designates which milestones of Chrome are impacted by the bug. Multiple milestones may be set in the Found In field, but the most important one is the earliest affected milestone. See ChromiumDash for current releases.

  • Security_Impact-{Head, Beta, Stable, Extended, None} hotlists: Derived from milestones set in the Found In field,, this hotlist specifies the earliest affected release channel. Should not normally be set by humans, except in the case of Security_Impact-None (hotlistID: 5433277) which means that the bug is in a disabled feature, or otherwise doesn't impact Chrome: see the section below for more details.

    • Note that Severity should still be set to the appropriate Severity (S0-S3) for Security_Impact-None issues, as if the feature were enabled or the code reachable.
  • Issue access level & collaborator groups{collaborators=,, Limited Visibility + Google, SecurityEmbargo hotlist}: settings that restrict access to the bug. Meaning and usage guidelines are as follows:

    • Issue Access level: Limited Visibility + collaborator group = Restricts access to members of This is the default that should be used for general security bugs that aren't sensitive otherwise.
    • Issue Access level: Limited Visibility + collaborator group = Restricts access to members of, which includes external parties who ship Chromium-based products and who need to know about available bug fixes. is a member of that group so the former is a superset of the latter. Collaborator group = is not suitable for sensitive bugs.
    • Issue Access level: Limited Visibility + collaborator group = As above, but additionally give access to, a community of downstream WebRTC embedders.
    • Issue Access level: Limited Visibility + Googlers: Restricts access to users that are Google employees (but also via their accounts). This should be used for bugs that aren't OK for external contributors to see (even if we trust them with security work), for example due to:
      • legal reasons (bug affects a partner Google is under NDA and the information is subject to that)
      • the bug affecting more Google products than Chrome and Chrome OS
    • SecurityEmbargo hotlist (hotlistID: 5432549): Keeps issues already set with Issue Access level: Limited Visibility + collaborator group = from being automatically de-restricted by Blintz and keeping the bug from being opened for public disclosure. Use this if the bug in question is subject to disclosure decisions made externally, such as:
      • We receive advance notice of security bugs from an upstream open source project or Google partner and they organize a coordinated disclosure process. We'd remove the restriction hotlist if/when the embargo gets lifted.
      • The reporter indicates a preference to remain anonymous and the bug history would give away the reporter‘s identity (if they file using an anonymous account, this doesn’t apply).
  • reward-{topanel, unpaid, na, inprocess, #} hotlists: Hotlists used for tracking bugs nominated for our Vulnerability Reward Program.

  • reward_to-external (hotlistID: 5432589): If a bug is filed by a Google or Chromium user on behalf of an external party, use reward_to-external to ensure the report is still properly credited to the external reporter in the release notes. IF, however, the reporter is an individual with an email address you should set the Reporter field to reflect the email address of the external reporter. If the reporter was an organization or entity with a specific email address, then do not alter the Reporter field and use the reward-to_external hotlist. Despite its name, you should add this label whether or not the reporter is in scope for the vulnerability rewards program, because external reports are credited in the release notes irrespective.

  • M-# field: Target milestone for the fix.

  • Chromium ‘Component Tags’: For bugs filed as Type=Vulnerability, we also want to track which Chromium component(s) the bug is in.

  • ReleaseBlock field = Stable: When we find a security bug regression that has not yet shipped to stable, we use this label to try and prevent the security regression from ever affecting users of the Stable channel.

  • OS-{Chrome, Linux, Windows, ...}: Denotes which operating systems are affected.

  • Merge: field{Request-?, Approved-?, Merged-?}: Security fixes are frequently merged to earlier release branches.

  • Security Release: #-M###: Denotes which exact patch a security fix made it into. This is more fine-grained than the M-# label. Release-0-M105 denotes the initial release of an M105 release to Stable channel.

  • CVE field: -####-####: For security bugs that get assigned a CVE, we update the CVE field for appropriate bug(s) with the CVE number for easy searching. Type=Vulnerability bugs should always have Severity of S0-S3, Found In - ### set, Security_Impact hotlist, OS, Priority, M, Component Tags, and an Assigner set.

When to use the Security_Impact-None hotlist

Security_Impact-None says that the bug can‘t affect any users running the default configuration of Chrome. It’s most commonly used for cases where code is entirely disabled or absent in the production build.

Other cases where it's OK to set Security_Impact-None:

  • The impacted code runs behind a feature flag which is disabled by default, and the field trial configuration has not been switched on.
  • The impacted code only runs behind a command-line flag or chrome://flags entry. (In particular, if a bug can only affect those who have set #enable-experimental-web-platform-features, it is Security_Impact-None.
  • It's a V8 feature behind flags such as --future, --es-staging or --wasm-staging or other experimental flags that are disabled by default.

Cases where it's not OK to set Security_Impact-None:

  • Features enabled via normal UI or settings which users might happen across in normal usage. For instance, accessibility features and the Chrome Labs experimental features accessible from the toolbar.
  • Origin trials. Origin trials are only active on some websites, but the affected code does run for Chrome users with the default Chrome configuration.
  • The impacted code runs behind a feature flag which is enabled by default, even if that field trial configuration has been switched off. That‘s because the code may be active for devices which can’t access the field trial configuration service.
  • The feature is turned on only for a small percent of users, e.g. 1%.
  • Feature or flag checks are done somewhere that the attacker could influence. For example a privilege escalation from a lower-privileged process (e.g. renderer) to a higher-privileged process (e.g. browser) assumes that the lower-privileged process is already compromised. The attacker could overwrite memory for any feature checks performed within that lower-privileged process; the bug only qualifies as impact None if checks are performed in the higher-privileged process.
  • If a bug involves a patch to a renderer or use of a flag to turn on MojoJS this may mean it's a simulation of a compromised renderer and the bug may still be a valid sandbox escape bug.

It‘s important to get this right, because this label influences how rapidly we merge and release the fix. Ask for help if you’re not sure.

Some Security_Impact-None bugs may still be subject to VRP rewards, if those bugs are found in code that we're likely to enable in the future.

OS Field

It can be hard to know which OS(s) a bug applies to. Here are some guidelines:

  • Blink is used on all platforms except iOS. A (say) UAF in Blink is probably not particular to whatever platform it was found on; it's probably applicable to all.
  • The same is true of Skia, and the net/ code.
  • If the bug is in a file named foo_{win,linux,mac,...}.cc, it's specific to the named platform.
  • Java code is particular to Android.
  • Objective-C++ ( is particular to macOS and iOS. (But note that most of our Objective-C++ is particular to macOS or iOS. You can usually tell by the pathname.)
  • Views code (e.g. ui/message_center/views) is used on Windows, Linux, Chrome OS, and perhaps Fuchsia (?). Views for macOS is increasingly a thing, but Cocoa code (e.g. ui/message_center/cocoa) is particular to macOS.

After the bug is fixed: Merge field labels

Once you've landed a complete fix for a security bug, please immediately mark the bug as Fixed. Do not request merges: Sheriffbot will request appropriate merges to beta or stable according to our guidelines. However, it is really helpful if you comment upon any unusual stability or compatibility risks of merging.

(Some Chromium teams traditionally deal with merges before marking bugs as Fixed. Please don't do that for security bugs.)

Please take the opportunity to consider whether there are any variants or related problems. It‘s very common for attackers to tweak working attack code to exploit a similar situation elsewhere. If you’ve even the remotest thought that there might be equivalent patterns or variants elsewhere, file a bug with type=Bug-Security. It can be nearly blank. The important thing is to record the fact that something may need doing.

Sheriffbot automation

Security labels guide the actions taken by Blintz. The source of truth for the actual rule set is go/chrome-blintz-source (sorry, Google employees only). The motivation behind these rules is to help automate the security bug life cycle so security shepherds and security engineers in general spend less time updating bugs and can do more useful work instead.

The following sections describe the current set of rules relevant to security bugs. The list below only describes rules that change the labels described above. There are additional rules for sending nag messages and janitorial tasks; check the Chrome blintz source for details.

Remove Invalid Release-# Field Entries

Only bugs that affect stable should carry a release designator, this rule removes release designators that are set on bugs not affecting stable.

Remove Invalid Security_Impact-X Hotlists

Each bug should be on exactly one Security_Impact-X and it should be one of the 5 valid Security_Impact hotlists (None, Extended, Stable, Beta, Head). This rule a bug from any invalid and excess Security_Impact hotlists.

Adjust Security_Impact-X Hotlists to match Found In

Based on Found In # milestone set in the field this rule assigns corresponding Security_Impact-X hotlists if they are incorrect or absent. Security_Impact-None is never changed.

Update M-# Field

Bugs that are set with milestones earlier than the current milestone will be updated to set the field to the current milestone and Security_Impact-Extended.

Bugs that carry a Security_Impact-X hotlist but are missing a milestone field being set will be updated so that the M-# field reflects the corresponding to the respective milestone.

Set ReleaseBlock field For Regressions

If there's a high or medium severity security regression in beta or ToT, update the ReleaseBlock field to Stable to prevent that regression from being shipped to users.

Similarly, critical security regressions are marked ReleaseBlock: Beta.

Adjust Priority P# To Match Severity

Adjust Priority P# according to the priority rules for severity labels described above. If there is evidence of active exploitation then a higher priority should be used.

Drop Visibility Group Restrictions From Old, Fixed Bugs for Disclosure

Remove **, and from Collaborator Groups and Update Issue Access level to Default Visibility for security bugs that have been closed (Fixed, Verified, Duplicate, WontFix,Invalid) more than 14 weeks ago, making them publicly accessible. The idea here is that by 14 weeks, important security fixes will have shipped in a Stable channel update and allowing users time to update.

Set as Collaborator On Fixed Bugs

While Issue Access level remains Limited Visibility removes as Collaborator field / Add Collaborator Groups and replaces updates with for fixed security bugs. Rationale is that while fixed bugs are generally not intended to become public immediately, we'd like to give access to external parties depending on Chromium via (Collaborator for WebRTC bugs is instead updated to

Update Merge Field with Request-X For Fixed Bugs

Fixed security bugs that affect stable or beta and are critical or high severity will automatically trigger a merge request for the current beta branch, and perhaps stable if also impacted.

Drop X from ReleaseBlock field For Security_Impact-None Bugs

No need to stop a release if the bug doesn't have any consequences.

An Example

Given the importance and volume of field, hotlists, and visiblity settings, an example might be useful.

  1. An external researcher files a security bug, with a repro that demonstrates memory corruption against the latest (e.g.) M123 dev channel. The will present as Type=Vulnerability and Visibility / Issue Access level will be set to Limited Visibility with set as Collaborator.
  2. The Security Shepherd triages the issue and uses ClusterFuzz to confirm that the bug is a novel and dangerous-looking buffer overflow in the renderer process. ClusterFuzz also confirms that all current releases are affected. Since M121 is the current Stable release, M120 is Extended Stable, and M122 is in Beta, we update the Found In field to 120 to reflect Extended Stable as the earliest / oldest affected release channel. The severity of a buffer overflow in a renderer implies High (S1) Severity and P1 Priority. Any external report for a confirmed vulnerability needs reward-topanel. Blintz will usually add it automatically once the bug is fixed. The stack trace provided by ClusterFuzz suggests that the bug is in the component Blink>DOM, and such bugs should be labeled as applying to all OSs except iOS (where Blink is not used): OS-{Linux, Windows, Android, Chrome, Fuchsia}. Blintz will check whether 120 is the current extended stable, stable, beta or head milestone; and will add the Security_Impact-Extended hotlist.
  3. Within a day or two, the Security Shepherd was able to get the bug assigned and — oh joy! — fixed very quickly. When the bug's status changes to Fixed, Blintz will update the Merge field with the appropriate request-MMM labels, and will change update Visibility by changing Collaborators from to
  4. Later that week, the Chrome Security VRP TL does a sweep of all reward-topanel bugs. This one gets rewarded, so that one reward label is replaced with two: reward-7000 and reward-unpaid. Later, reward-unpaid becomes reward-inprocess and is later still removed when all is done. Of course, reward-7000 remains forever. (We use it to track total payout!)
  5. A Chrome Security TPM with the responsibility of doing merge review regularly checks the security merge review queue based on Type=Vulnerability issues with Merge fields updated with *request-MMM or review-MMM entries. They review this issue and fix, having had enough bake time in Canary and being safe to merge, and approve it for merge, updating the Merge fields to Approved-MMM for each milestone the fix is being approved for backmerge to. There are weekly Stable channel updates, and this fix was approved for merge to M121, so it will be shipped in the following week's update of M121.
  6. Just before the release, a Chrome Security TPM runs a series of scripts and verifies the security fixes being shipped in that Stable channel update and applies the appropriate #-M121 tag in the Release field. Since the bug was externally reported, it will also be issued a CVE ID, and the CVE field is updated with the appropriate CVE for that issue: 2024-####.
  7. 14 weeks after the bug is marked Fixed, Blintz updates the Visibility from Issue access level -- Limited Visibility to Default Visibility and removes from Collaborators, making the issue publicly visible. There is one crucial exception -- Blintz will not update the Visibility or remove from Collaborators if the issue is on the SecurityEmbargo hotlist.