CVE-2020-6819
Under certain conditions, when running the nsDocShell destructor, a race condition can cause a use-after-free.
- Severity
- 8.1 / 10
- High · CVSS 3.1
- Exploit chance (EPSS)
- 3.04%
- Chance of exploitation in the next 30 days. Higher than 87.52% of scored vulnerabilities.
- Known exploited
- Yes
- In CISA's catalog since Nov 3, 2021
- Weakness
- CWE-362 Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition')
- CWE-416 Use After Free
- CWE-416 Use After Free
- Fix
- None yet
- No fixed version published
Description
Under certain conditions, when running the nsDocShell destructor, a race condition can cause a use-after-free. We are aware of targeted attacks in the wild abusing this flaw. This vulnerability affects Thunderbird < 68.7.0, Firefox < 74.0.1, and Firefox ESR < 68.6.1.
How to fix
No fixed version has been published yet. Until one is, remove or replace the package, or turn off or isolate the affected feature. This page updates when a fix ships.
Severity breakdown
CVSS 3.1 8.1
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
- Attack vector
- Network
- Attack complexity
- High
- Privileges required
- None
- User interaction
- None
- Scope
- Unchanged
- Confidentiality
- High
- Integrity
- High
- Availability
- High
Weakness
CWE-362: Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition')
The product contains a concurrent code sequence that requires temporary, exclusive access to a shared resource, but a timing window exists in which the shared resource can be modified by another code sequence operating concurrently.
How to prevent it
- Architecture and Design: In languages that support it, use synchronization primitives. Only wrap these around critical code to minimize the impact on performance.
- Architecture and Design: Use thread-safe capabilities such as the data access abstraction in Spring.
- Architecture and Design: Minimize the usage of shared resources in order to remove as much complexity as possible from the control flow and to reduce the likelihood of unexpected conditions occurring. Additionally, this will minimize the amount of synchronization necessary and may even help to reduce the likelihood of a denial of service where an attacker may be able to repeatedly trigger a critical section (CWE-400).
- Implementation: When using multithreading and operating on shared variables, only use thread-safe functions.
- Implementation: Use atomic operations on shared variables. Be wary of innocent-looking constructs such as "x++". This may appear atomic at the code layer, but it is actually non-atomic at the instruction layer, since it involves a read, followed by a computation, followed by a write.
- Implementation: Use a mutex if available, but be sure to avoid related weaknesses such as CWE-412.
- Implementation: Avoid double-checked locking (CWE-609) and other implementation errors that arise when trying to avoid the overhead of synchronization.
- Implementation: Disable interrupts or signals over critical parts of the code, but also make sure that the code does not go into a large or infinite loop.
- Implementation: Use the volatile type modifier for critical variables to avoid unexpected compiler optimization or reordering. This does not necessarily solve the synchronization problem, but it can help.
- Architecture and Design, Operation: Run your code using the lowest privileges that are required to accomplish the necessary tasks [REF-76]. If possible, create isolated accounts with limited privileges that are only used for a single task. That way, a successful attack will not immediately give the attacker access to the rest of the software or its environment. For example, database applications rarely need to run as the database administrator, especially in day-to-day operations.
CWE-416: Use After Free
The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory "belongs" to the code that operates on the new pointer.
How to prevent it
- Architecture and Design: Choose a language that provides automatic memory management.
- Implementation: When freeing pointers, be sure to set them to NULL once they are freed. However, the utilization of multiple or complex data structures may lower the usefulness of this strategy.
CWE-416: Use After Free
The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory "belongs" to the code that operates on the new pointer.
How to prevent it
- Architecture and Design: Choose a language that provides automatic memory management.
- Implementation: When freeing pointers, be sure to set them to NULL once they are freed. However, the utilization of multiple or complex data structures may lower the usefulness of this strategy.
References
Advisories
Exploits and evidence
Other links
Timeline
- Published
- Added to CISA's Known Exploited Vulnerabilities catalog
- Remediation due for U.S. federal agencies
- Last updated by the source
- Latest EPSS score
Related advisories
| Advisory | Severity | Why it's related |
|---|---|---|
| GHSA-5wjr-9356-4wc8 Tokio: Race condition in `tokio::time::Interval` | Medium | Same weakness, CWE-362 |
| GHSA-44jh-23m7-hpcf @better-auth/drizzle-adapter: Concurrent requests can exceed rate limits on PostgreSQL | Medium | Same weakness, CWE-362 |
| GHSA-494h-xx7x-5frh Microsoft Security Advisory CVE-2026-70183 - ASP.NET Core Security Feature Bypass Vulnerability | High | Same weakness, CWE-362 |
| GO-2026-6617 HTTP/2 server crash due to HPACK encoder race in net/http | High | Same weakness, CWE-362 |
| GHSA-xvgh-hmx8-9xxf Backstage: Improper task state validation in Scaffolder backend | High | Same weakness, CWE-362 |
| GHSA-pr7f-p5mw-fc87 vLLM: Incomplete CVE-2025-62164 remediation can be bypassed by concurrent prompt parts | Medium | Same weakness, CWE-362 |
| GHSA-2hw9-mc66-jc2q Wasmtime: Preemption and traps during bulk operations enable breaking internal VM state | Low | Same weakness, CWE-362 |
| GHSA-9h85-g7w3-rh49 ViewComponent: Reused Component Instances Retain Stale Render Context | Medium | Same weakness, CWE-362 |
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