CVE-2026-64537 in Linux
Summary
by MITRE • 07/28/2026
In the Linux kernel, the following vulnerability has been resolved:
bridge: cfm: reject invalid CCM interval at configuration time
ccm_tx_work_expired() re-arms itself via queue_delayed_work() using the configured exp_interval converted by interval_to_us(). When exp_interval is BR_CFM_CCM_INTERVAL_NONE or out of range, interval_to_us() returns 0, causing the worker to fire immediately in a tight loop that allocates skbs until OOM.
Fix this by validating exp_interval at configuration time:
- Constrain IFLA_BRIDGE_CFM_CC_CONFIG_EXP_INTERVAL to the valid range [BR_CFM_CCM_INTERVAL_3_3_MS, BR_CFM_CCM_INTERVAL_10_MIN] in the
netlink policy so userspace cannot set an invalid value.
- Reject starting CCM TX in br_cfm_cc_ccm_tx() when exp_interval has not yet been configured (defaults to 0 from kzalloc).
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Analysis
by VulDB Data Team • 07/28/2026
The vulnerability resides within the Linux kernel's bridge implementation specifically in the Connectivity Fault Management (CFM) subsystem which is responsible for monitoring connectivity and fault detection in network bridges. This flaw manifests as a denial of service condition that occurs when the CFM component attempts to process invalid configuration parameters. The issue stems from inadequate input validation during the configuration phase, allowing malformed values to propagate through the system and ultimately cause resource exhaustion. When userspace applications configure CFM parameters through netlink interfaces, they can inadvertently or maliciously specify invalid values that are not properly validated before being processed by kernel components.
The technical root cause involves the ccm_tx_work_expired() function which operates as a delayed work queue handler within the kernel's timer subsystem. This function re-arms itself using queue_delayed_work() API with timing parameters derived from interval_to_us() conversion function. When the configuration parameter exp_interval is set to BR_CFM_CCM_INTERVAL_NONE or falls outside acceptable ranges, the interval_to_us() function returns zero as its output value. This zero value causes the delayed work mechanism to schedule immediate execution of the worker function without proper delay intervals, creating an infinite loop scenario where the system continuously re-arms itself. The cascading effect of this tight loop results in rapid allocation of socket buffer structures (skbs) which consume available memory resources until the system reaches out-of-memory conditions and potentially crashes.
The operational impact of this vulnerability is severe as it can be exploited to cause system instability through resource exhaustion without requiring elevated privileges. Attackers can trigger this condition by configuring bridge interfaces with invalid CFM parameters, leading to denial of service for legitimate network operations. The vulnerability affects systems running Linux kernel versions where the bridge CFM functionality is enabled and actively used in network infrastructure deployments. This type of vulnerability aligns with CWE-129 Input Validation and CWE-400 Uncontrolled Resource Consumption, representing a classic case of improper input validation leading to resource exhaustion attacks.
The mitigation strategy implements two primary defensive measures that address the vulnerability at different layers of the system architecture. First, the netlink policy validation is strengthened by constraining the IFLA_BRIDGE_CFM_CC_CONFIG_EXP_INTERVAL parameter to the valid range between BR_CFM_CCM_INTERVAL_3_3_MS and BR_CFM_CCM_INTERVAL_10_MIN, preventing userspace applications from setting invalid values in the first place. This approach follows principle of least privilege and input sanitization best practices recommended by security frameworks. Second, runtime validation is implemented within the br_cfm_cc_ccm_tx() function to explicitly check for unconfigured exp_interval values that default to zero following kzalloc initialization. This dual-layer validation approach prevents the exploitation path at both configuration time and execution time, providing comprehensive protection against this specific denial of service vulnerability.
The fix addresses fundamental security principles outlined in MITRE ATT&CK framework under T1499-Resource Hijacking which covers techniques for consuming system resources to cause denial of service. This vulnerability also demonstrates the importance of proper parameter validation in kernel space components where invalid inputs can lead to critical system instability. The solution aligns with secure coding practices that emphasize early input validation and defensive programming techniques, ensuring that kernel subsystems properly validate all incoming parameters before processing them. This approach reduces attack surface area and prevents cascading failures that could compromise entire network infrastructure deployments relying on Linux bridge functionality for connectivity management.
The implementation of these fixes ensures that the bridge CFM subsystem operates within predictable resource constraints while maintaining proper functionality for legitimate use cases. The validation checks provide immediate protection against malformed configuration inputs while preserving the intended operational behavior of the CFM protocol. This vulnerability resolution demonstrates the critical importance of input validation in kernel space components where improper handling of user-provided data can lead to system-wide stability issues and potential denial of service conditions affecting network infrastructure reliability.