CVE-2026-59691: GStreamer Buffer Overflow Vulnerability

CVE-2026-59691 Overview

CVE-2026-59691 is a heap buffer overflow vulnerability in the GStreamer rfbsrc plugin, which implements the Remote Framebuffer (RFB/VNC) client source. When a GStreamer client connects to a malicious RFB/VNC server that advertises a 16 bits-per-pixel (bpp) framebuffer and sends Hextile-encoded updates, the Hextile background fill path writes 32-bit pixel values into a buffer allocated for 16-bit pixels. This type mismatch produces an out-of-bounds heap write. The flaw is classified as [CWE-787] Out-of-Bounds Write and can lead to denial of service or memory corruption in any application that consumes untrusted RFB streams through GStreamer.

Critical Impact
A malicious RFB/VNC server can crash the GStreamer client process and potentially corrupt heap memory when a user initiates a connection.

Affected Products

GStreamer rfbsrc plugin (RFB/VNC source element)
GStreamer distributions shipping the vulnerable rfbsrc element (see Red Hat CVE-2026-59691 Advisory)
Downstream applications and media pipelines that consume RFB/VNC streams via GStreamer

Discovery Timeline

2026-07-09 - CVE-2026-59691 published to NVD
2026-07-09 - Last updated in NVD database

Technical Details for CVE-2026-59691

Vulnerability Analysis

The rfbsrc plugin negotiates framebuffer parameters with the RFB/VNC server during session setup, including the pixel format and bits-per-pixel value. When the server advertises a 16bpp framebuffer, rfbsrc allocates its internal pixel buffer sized for 2-byte pixels. The bug occurs later in the Hextile decoder, where the background fill routine treats each pixel as a 32-bit (4-byte) value and writes past the end of the 16bpp allocation. The result is a linear heap write with attacker-influenced pixel data.

Exploitation requires user interaction because a user must initiate a connection to the attacker-controlled server. However, no authentication is required on the server side, and the attack traverses the network. Impact is primarily on availability, with a secondary risk of integrity loss through heap corruption.

Root Cause

The root cause is a pixel-size type mismatch between allocation and write paths in the Hextile decoder. The allocator honors the negotiated 16bpp format, while the Hextile background fill unconditionally writes guint32 values. This is a classic [CWE-787] Out-of-Bounds Write caused by inconsistent width assumptions across code paths that share the same pixel buffer.

Attack Vector

An attacker operates a malicious RFB/VNC server. When a victim application using GStreamer's rfbsrc connects to that server, the server advertises a 16bpp framebuffer during ServerInit and then sends a framebuffer update encoded with Hextile that triggers the background fill path. The oversized write occurs during Hextile decoding, corrupting adjacent heap memory. Refer to the GStreamer Security Merge Request and GStreamer Work Item #5173 for the upstream code changes that document the vulnerable path.

No verified public exploit code is available. The vulnerability is described in prose in the referenced advisories rather than through a released proof of concept.

Detection Methods for CVE-2026-59691

Indicators of Compromise

Unexpected crashes or SIGSEGV signals in processes linking libgstrfbsrc or loading the rfbsrc GStreamer element.
Heap corruption diagnostics from glibc such as malloc_consolidate(): invalid chunk size or double free or corruption in GStreamer-based media applications.
Outbound TCP connections from media or conferencing applications to untrusted hosts on RFB/VNC ports (typically 5900-5906).

Detection Strategies

Inventory hosts that install GStreamer plugin packages containing rfbsrc and compare versions against fixed releases listed in the Red Hat CVE-2026-59691 Advisory.
Run vulnerable applications under AddressSanitizer or Valgrind in test environments to confirm the out-of-bounds write when connecting to a controlled 16bpp RFB endpoint.
Monitor process crash telemetry and core dumps for GStreamer pipelines that include RFB/VNC sources.

Monitoring Recommendations

Alert on repeated crashes of the same GStreamer-based process within a short interval, which may indicate exploitation attempts.
Log all outbound connections to RFB/VNC ports from workstations and correlate with process command lines invoking gst-launch or embedded pipelines using rfbsrc.
Track package versions of GStreamer plugin sets across the fleet through a software bill of materials (SBOM) pipeline.

How to Mitigate CVE-2026-59691

Immediate Actions Required

Update GStreamer packages that ship the rfbsrc element to the fixed versions published by your distribution vendor.
Restrict outbound connectivity to RFB/VNC ports from workstations that run GStreamer-based media pipelines.
Instruct users not to open RFB/VNC URLs from untrusted sources in applications built on GStreamer.

Patch Information

Upstream fixes are tracked in the GStreamer Security Merge Request and GStreamer Work Item #5173. Distribution-specific package updates are enumerated in the Red Hat CVE-2026-59691 Advisory and the corresponding Red Hat Bug 2497343 Report. Apply vendor updates as soon as they are released for your platform.

Workarounds

Remove or disable the rfbsrc GStreamer element on systems that do not require RFB/VNC ingestion until patches are deployed.
Block egress traffic to TCP ports 5900-5906 at the host or perimeter firewall for endpoints running vulnerable GStreamer builds.
Run affected media pipelines under least-privilege service accounts and with sandboxing (for example, seccomp or systemd NoNewPrivileges) to reduce the impact of memory corruption.

bash

# Remove the rfbsrc plugin file to disable the vulnerable element
# Path may vary by distribution and architecture
sudo rm -f /usr/lib64/gstreamer-1.0/libgstrfbsrc.so

# Verify the element is no longer registered
gst-inspect-1.0 rfbsrc || echo "rfbsrc disabled"

# Block outbound VNC ports at the host firewall (example: firewalld)
sudo firewall-cmd --permanent --direct --add-rule ipv4 filter OUTPUT 0 \
    -p tcp --dport 5900:5906 -j REJECT
sudo firewall-cmd --reload