CVE-2025-9640: Samba Information Disclosure Vulnerability

CVE-2025-9640 Overview

CVE-2025-9640 is an information disclosure vulnerability in Samba's vfs_streams_xattr module. The flaw allows uninitialized heap memory to be written into alternate data streams (ADS) exposed through the module. An authenticated user with access to an affected share can read residual heap content that may contain sensitive data from the smbd process. The issue is tracked under CWE-908: Use of Uninitialized Resource.

Critical Impact
Authenticated remote users can retrieve fragments of smbd heap memory through alternate data streams, potentially exposing credentials, file contents, or other in-process secrets.

Affected Products

Samba distributions shipping the vfs_streams_xattr VFS module
Red Hat Enterprise Linux Samba packages (see Red Hat CVE-2025-9640 Advisory)
Debian LTS Samba packages (see Debian LTS Security Announcement)

Discovery Timeline

2025-10-15 - CVE-2025-9640 published to the National Vulnerability Database
2025-10-15 - Public disclosure on the OpenWall OSS Security list
2025-10-16 - Follow-up discussion on the OpenWall OSS Security list
2025-11 - Debian LTS advisory released for Samba packages
2026-09-01 - Last updated in NVD database

Technical Details for CVE-2025-9640

Vulnerability Analysis

Samba's vfs_streams_xattr module implements SMB alternate data streams by storing stream content inside extended attributes (xattr) on the underlying filesystem. When the module handles write operations that extend a stream beyond its previously written length, it fails to zero-initialize the newly allocated heap buffer that backs the stream. The uninitialized bytes are then persisted into the xattr and returned to any client that later reads the stream.

An authenticated SMB client can trigger the condition by writing at a non-zero offset or by truncating a stream to a larger size, then reading the stream back to recover the residual heap contents. The disclosed data originates from prior allocations inside smbd, which may include portions of file data, authentication material, or protocol state.

Root Cause

The root cause is missing initialization of heap-allocated memory before that memory is written to an alternate data stream via extended attributes. The defect maps to [CWE-908: Use of Uninitialized Resource]. Instead of clearing the sparse region between the old and new stream length, the module copies uninitialized allocator memory into the xattr backing store.

Attack Vector

Exploitation requires network access to an SMB share exported with the streams_xattr VFS object enabled and valid credentials for that share. The attacker connects to the share, creates or opens a file with an alternate data stream using the filename:streamname syntax, and issues a write at an offset larger than the current stream size. Reading the stream returns the uninitialized bytes. No user interaction is required and the attack does not modify integrity or availability of the target.

No verified public exploit code is available at the time of writing. For technical background, refer to the Red Hat Bug Report #2391698 and the Samba Security History page.

Detection Methods for CVE-2025-9640

Indicators of Compromise

SMB sessions issuing repeated SMB2 WRITE operations at large offsets against zero-length or short alternate data streams.
Unusual getxattr/setxattr activity on user.DosStream.* attributes on files stored in shares configured with vfs objects = streams_xattr.
Authenticated users reading many alternate data streams from files they did not create or modify.

Detection Strategies

Enable Samba full audit logging with vfs objects = full_audit streams_xattr and alert on pwrite/pread operations targeting stream names.
Correlate SMB2 create requests containing the : stream separator with subsequent read operations returning non-zero content from previously empty streams.
Hunt across SMB telemetry for a single authenticated principal touching alternate data streams across many unrelated files in a short window.

Monitoring Recommendations

Ingest smbd audit logs and Linux auditd xattr syscall events into a centralized analytics platform for retention and correlation.
Baseline normal alternate data stream usage per share; most Linux-hosted shares see little to no ADS activity, making anomalies high-signal.
Track Samba package versions across the fleet and alert when hosts run versions predating the fix referenced in vendor advisories.

How to Mitigate CVE-2025-9640

Immediate Actions Required

Inventory all Samba servers and identify shares that load streams_xattr in the vfs objects parameter of smb.conf.
Apply the vendor-supplied Samba security update for your distribution as soon as it is available.
Restrict share access to trusted authenticated users and remove guest or anonymous access from shares using streams_xattr.

Patch Information

Refer to the Samba Security History page for the fixed upstream Samba release and to distribution advisories for backported packages. Red Hat customers should follow the Red Hat CVE-2025-9640 Advisory for errata mapping to affected RHEL streams. Debian LTS users should apply the update described in the Debian LTS Security Announcement.

Workarounds

Remove streams_xattr from the vfs objects line in smb.conf on affected shares if alternate data streams are not required.
Tighten share ACLs so only trusted, authenticated principals can write to files on shares that must retain streams_xattr.
Restart smbd after any configuration change and validate with testparm that streams_xattr is no longer loaded on shares where it was removed.

bash

# Example: disable streams_xattr on a Samba share in /etc/samba/smb.conf
[data]
    path = /srv/samba/data
    read only = no
    # Remove or comment out streams_xattr
    # vfs objects = streams_xattr
    vfs objects =

# Validate configuration and reload smbd
testparm -s
systemctl reload smb