Severity Guidelines for Security Issues

Vendors shipping products based on Chromium might wish to rate the severity of security issues in the products they release. This document contains guidelines for how to rate these issues. Check out our security release management page for guidance on how to release fixes based on severity.

Any significant mitigating factors will generally reduce an issue's severity by one or more levels:

  • Not web accessible, reliant solely on direct UI interaction to trigger.
  • Unusual or unlikely user interaction will normally reduce severity by one level. This means interaction which may sometimes occur, but would not be typical of an average user engaging with Chrome or a particular feature in Chrome, nor could a user be easily convinced to perform by a persuasive web page.
  • Requiring profile destruction or browser shutdown will normally reduce severity by one level.

Bugs that require implausible interaction, interactions a user would not realistically be convinced to perform, will generally be downgraded to a functional bug and not considered a security bug.

Conversely, we do not consider it a mitigating factor if a vulnerability applies only to a particular group of users. For instance, a Critical vulnerability is still considered Critical even if it applies only to Linux or to those users running with accessibility features enabled.

Also note that most crashes do not indicate vulnerabilities. Chromium is designed to crash in a controlled manner (e.g., with a __debugBreak) when memory is exhausted or in other exceptional circumstances.

Critical severity

Critical severity issues allow an attacker to read or write arbitrary resources (including but not limited to the file system, registry, network, et c.) on the underlying platform, with the user's full privileges.

They are normally assigned priority Pri-0 and assigned to the current stable milestone (or earliest milestone affected). For critical severity bugs, SheriffBot will automatically assign the milestone.

For critical severity vulnerabilities, we aim to deploy the patch to all Chrome users in under 30 days.

Critical vulnerability details may be made public in 60 days, in accordance with Google's general vulnerability disclosure recommendations, or faster (7 days) if there is evidence of active exploitation.

Example bugs:

  • Memory corruption in the browser process (319125).
  • Exploit chains made up of multiple bugs that can lead to code execution outside of the sandbox (416449).
  • A bug that enables web content to read local files (962500).

Note that the individual bugs that make up the chain will have lower severity ratings.

High severity

High severity vulnerabilities allow an attacker to execute code in the context of, or otherwise impersonate other origins or read cross-origin data. Bugs which would normally be critical severity with unusual mitigating factors may be rated as high severity. For example, renderer sandbox escapes fall into this category as their impact is that of a critical severity bug, but they require the precondition of a compromised renderer. (Bugs which involve using MojoJS to trigger an exploitable browser process crash usually fall into this category). Another example are bugs that result in memory corruption in the browser process, which would normally be critical severity, but require browser shutdown or profile destruction, which would lower these issues to high severity. A bug with the precondition of browser shutdown or profile destruction should be considered to have a maximum severity of high and could potentially be reduced by other mitigating factors.

They are normally assigned priority Pri-1 and assigned to the current stable milestone (or earliest milestone affected). For high severity bugs, SheriffBot will automatically assign the milestone.

For high severity vulnerabilities, we aim to deploy the patch to all Chrome users in under 60 days.

Example bugs:

  • A bug that allows full circumvention of the same origin policy. Universal XSS bugs fall into this category, as they allow script execution in the context of an arbitrary origin (534923).
  • A bug that allows arbitrary code execution within the confines of the sandbox, such as renderer, network, or GPU process memory corruption (570427, 468936).
  • Complete control over the apparent origin in the omnibox (76666).
  • Memory corruption in the browser process that can only be triggered from a compromised renderer, leading to a sandbox escape (469152).
  • Kernel memory corruption that could be used as a sandbox escape from a compromised renderer (377392).
  • Memory corruption in the browser process that requires specific user interaction, such as granting a permission (455735).
  • Site Isolation bypasses:
    • Cross-site execution contexts unexpectedly sharing a renderer process (863069, 886976).
    • Cross-site data disclosure (917668, 927849).

Medium severity

Medium severity bugs allow attackers to read or modify limited amounts of information, or are not harmful on their own but potentially harmful when combined with other bugs. This includes information leaks that could be useful in potential memory corruption exploits, or exposure of sensitive user information that an attacker can exfiltrate. Bugs that would normally be rated at a higher severity level with unusual mitigating factors may be rated as medium severity.

They are normally assigned priority Pri-1 and assigned to the current stable milestone (or earliest milestone affected). If the fix seems too complicated to merge to the current stable milestone, they may be assigned to the next stable milestone.

Example bugs:

  • An out-of-bounds read in a renderer process (281480).
  • An uninitialized memory read in the browser process where the values are passed to a compromised renderer via IPC (469151).
  • Memory corruption that requires a specific extension to be installed (313743).
  • Memory corruption in the browser process, triggered by a browser shutdown that is not reliably triggered and/or is difficult to trigger (1230513).
  • Memory corruption in the browser process, requiring a non-standard flag and user interaction (1255332).
  • An HSTS bypass (461481).
  • A bypass of the same origin policy for pages that meet several preconditions (419383).
  • A bug that allows web content to tamper with trusted browser UI (550047).
  • A bug that reduces the effectiveness of the sandbox (338538).
  • A bug that allows arbitrary pages to bypass security interstitials (540949).
  • A bug that allows an attacker to reliably read or infer browsing history (381808).
  • An address bar spoof where only certain URLs can be displayed, or with other mitigating factors (265221).
  • Memory corruption in a renderer process that requires specific user interaction, such as dragging an object (303772).

Low severity

Low severity vulnerabilities are usually bugs that would normally be a higher severity, but which have extreme mitigating factors or highly limited scope.

They are normally assigned priority Pri-2. Milestones can be assigned to low severity bugs on a case-by-case basis, but they are not normally merged to stable or beta branches.

Example bugs:

  • Bypass requirement for a user gesture (256057).
  • Partial CSP bypass (534570).
  • A limited extension permission bypass (169632).
  • An uncontrolled single-byte out-of-bounds read (128163).

Can't impact Chrome users by default

If the bug can't impact Chrome users by default, this is denoted instead by the Security-Impact_None label. See the security labels document for more information. The bug should still have a severity set according to these guidelines.

Not a security bug

The security FAQ covers many of the cases that we do not consider to be security bugs, such as denial of service and, in particular, null pointer dereferences with consistent fixed offsets.

“MiraclePtr” protection against use-after-free

“MiraclePtr” is a technology designed to deterministically prevent exploitation of use-after-free bugs. Address sanitizer is aware of MiraclePtr and will report on whether a given use-after-free bug is protected or not:

MiraclePtr Status: NOT PROTECTED
No raw_ptr<T> access to this region was detected prior to the crash.

or

MiraclePtr Status: PROTECTED
The crash occurred while a raw_ptr<T> object containing a dangling pointer was being dereferenced.
MiraclePtr should make this crash non-exploitable in regular builds.

For now, ignore these messages while determining severity, because MiraclePtr is not yet active on all Chromium platforms. In the future, we'll use this protection to reduce the severity of these bugs or even (once we have a lot of practical experience) reclassify them as non-security bugs.