CVE-2021-20316
Description
A flaw was found in the way Samba handled file/directory metadata. This flaw allows an authenticated attacker with permissions to read or modify share metadata, to perform this operation outside of the share.
Statement
As per upstream, "Prior to Samba 4.15.0, patches for this are not possible due to the prior design of the Samba VFS layer, which used pathname-based calls for most meta-data operations. A two and a half year effort was undertaken to completely re-write the Samba VFS layer to stop the use of pathname-based calls in all cases involving reading and writing of metadata returned to the client. This work has finally been completed in Samba 4.15.0."
Mitigation
Do not enable SMB1 (please note SMB1 is disabled by default in Samba from version 4.11.0 and onwards). This prevents the creation of symbolic links via SMB1. If SMB1 must be enabled for backward compatibility then add the parameter:
unix extensions = no
to the [global] section of your smb.conf and restart smbd. This prevents SMB1 clients from creating symlinks on the exported file system.
However, if the same region of the file system is also exported using NFS, NFS clients can create symlinks that potentially can also hit the race condition. For non-patched versions of Samba, we recommend only exporting areas of the file system by either SMB2 or NFS, not both.
Common Vulnerability Scoring System (CVSS) Score Details
Info alert:Important note
CVSS scores for open source components depend on vendor-specific factors (e.g. version or build chain). Therefore, Red Hat's score and impact rating can be different from NVD and other vendors. Red Hat remains the authoritative CVE Naming Authority (CNA) source for its products and services (see Red Hat classifications).
CVSS v3 Score Breakdown
| Red Hat | NVD | cve.org | |
|---|---|---|---|
| Base Score | 5.9 | 6.8 | N/A |
| Attack Vector | Network | Network | N/A |
| Attack Complexity | High | High | N/A |
| Privileges Required | Low | Low | N/A |
| User Interaction | None | None | N/A |
| Scope | Unchanged | Unchanged | N/A |
| Confidentiality | Low | High | N/A |
| Integrity Impact | High | High | N/A |
| Availability Impact | None | None | N/A |
Vector
Red Hat: CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:L/I:H/A:N
NVD: CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:N
Understanding the Weakness (CWE)
Availability
Technical Impact: DoS: Resource Consumption (CPU); DoS: Resource Consumption (Memory); DoS: Resource Consumption (Other)
When a race condition makes it possible to bypass a resource cleanup routine or trigger multiple initialization routines, it may lead to resource exhaustion.
Availability
Technical Impact: DoS: Crash, Exit, or Restart; DoS: Instability
When a race condition allows multiple control flows to access a resource simultaneously, it might lead the product(s) into unexpected states, possibly resulting in a crash.
Confidentiality,Integrity
Technical Impact: Read Files or Directories; Read Application Data
When a race condition is combined with predictable resource names and loose permissions, it may be possible for an attacker to overwrite or access confidential data (CWE-59).
Access Control
Technical Impact: Execute Unauthorized Code or Commands; Gain Privileges or Assume Identity; Bypass Protection Mechanism
This can have security implications when the expected synchronization is in security-critical code, such as recording whether a user is authenticated or modifying important state information that should not be influenced by an outsider.
Acknowledgements
Red Hat would like to thank Michael Hanselmann (Google) for reporting this issue. Upstream acknowledges the Samba project as the original reporter.
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