CVE-2026-56211 Overview
CVE-2026-56211 is a remote code execution vulnerability in libaom, the reference AV1 video codec implementation maintained by the Alliance for Open Media. The flaw stems from insufficient bounds validation in the AV1 encoder's Scalable Video Coding (SVC) layer ID control. Attackers can supply crafted video frame pixels that overlap internal encoder layer context structures. In fork-based video processing services, an attacker can hijack the cyclic refresh map pointer, brute-force the process base address through a crash oracle, and redirect control flow to execute arbitrary commands. The vulnerability is tracked as an out-of-bounds write [CWE-787].
Critical Impact
Successful exploitation grants arbitrary command execution on services that use libaom with SVC encoding enabled and accept attacker-supplied video frames.
Affected Products
- libaom — reference AV1 codec implementation (AOMedia)
- Applications and services that link libaom with SVC encoding enabled
- Downstream distributions tracking the AOMedia source tree (see Red Hat advisory)
Discovery Timeline
- 2026-06-19 - CVE-2026-56211 published to the National Vulnerability Database
- 2026-06-25 - Last updated in NVD database
Technical Details for CVE-2026-56211
Vulnerability Analysis
The vulnerability resides in the AV1 encoder's SVC layer ID control path within libaom. SVC enables a single encoded bitstream to carry multiple spatial and temporal layers. The encoder maintains internal layer context structures to track per-layer state, including pointers to auxiliary maps such as the cyclic refresh map.
The control logic that accepts SVC layer identifiers does not adequately validate bounds before writing data derived from frame pixels. As a result, attacker-controlled pixel data can overlap and corrupt these adjacent layer context structures. The corruption primitive is powerful enough to overwrite live pointers used by subsequent encoder operations.
Root Cause
The root cause is missing bounds validation on inputs to the SVC layer ID control flow, classified as an out-of-bounds write [CWE-787]. The encoder treats the supplied layer index and associated frame data as trusted, allowing writes that extend past the intended layer context buffer into neighboring structures, including the cyclic refresh map pointer.
Attack Vector
Exploitation requires the target service to use libaom with SVC encoding enabled and to accept attacker-supplied video frames. The attacker submits crafted frames whose pixel data is positioned to overlap internal encoder structures.
In fork-based video processing services, the attacker uses repeated submissions to brute-force the process base address by observing crash behavior as a side channel. Once the base address is recovered, the attacker overwrites the cyclic refresh map pointer to redirect control flow and achieve arbitrary command execution. The high attack complexity and required user interaction reflect the need for a permissive ingestion path and repeated probing.
No verified public proof-of-concept code is available. Refer to the AOM Media Code Update and the Chromium Issue Tracker Entry for technical details.
Detection Methods for CVE-2026-56211
Indicators of Compromise
- Repeated worker process crashes or segmentation faults in services that perform AV1 encoding with SVC layers enabled.
- Anomalous AV1 input streams containing rapidly cycling or out-of-range SVC layer IDs across consecutive frames.
- Unexpected child processes or shell invocations spawned by media encoding workers.
Detection Strategies
- Inspect AV1 encoding pipelines for crash loops correlated with a single client or upload session, a signature of base-address brute forcing.
- Validate SVC layer ID ranges at the application boundary and alert when submitted values fall outside the negotiated encoder configuration.
- Monitor libaom worker memory regions for writes outside expected layer context buffer ranges using runtime instrumentation such as AddressSanitizer in canary environments.
Monitoring Recommendations
- Log the source identity, frame count, and crash signal for every encoder worker termination and aggregate by client.
- Forward media-service crash telemetry and child-process creation events to a centralized analytics platform for correlation.
- Alert on encoder workers initiating outbound network connections or executing interpreters such as /bin/sh, bash, or python.
How to Mitigate CVE-2026-56211
Immediate Actions Required
- Update libaom to the fixed revision referenced in the AOM Media Code Update and rebuild any statically linked applications.
- Apply distribution updates as they become available, including those tracked by the Red Hat CVE Advisory.
- Audit production services to identify any encoder paths that expose SVC controls to untrusted input.
Patch Information
The upstream fix is committed in the AOMedia source tree at revision a93ba0ffaa. Vendors integrating libaom should rebase to a release that includes this commit. Track the Red Hat Bug Report for distribution-specific package versions and backports.
Workarounds
- Disable SVC encoding in libaom configurations where the multilayer feature is not required by the application.
- Sandbox encoder workers with seccomp, namespaces, or equivalent isolation to limit the impact of a hijacked process.
- Reject or normalize uploaded video frames before passing them to the encoder, including strict validation of SVC layer parameters.
- Disable process forking models that share address space layouts across workers, or enable per-worker address space layout randomization re-randomization to defeat crash-oracle brute forcing.
# Configuration example: verify the installed libaom version includes the fix
pkg-config --modversion aom
# Rebuild applications against a patched libaom and restart media workers
systemctl restart media-encoder.service
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
