CVE-2026-63824 Overview
CVE-2026-63824 is a buffer overflow vulnerability in the Linux kernel's KEYS subsystem. The flaw resides in the keyctl_pkey_params_get_2() function, where the length calculation for the internal output buffer is incorrect. When a caller supplies a buffer smaller than required, the miscalculation results in a memory overflow condition. The fix allocates the internal output buffer using the maximum length of the cryptographic primitive rather than the caller-provided size.
Critical Impact
Local authenticated users can trigger memory corruption in the Linux kernel, potentially leading to privilege escalation, denial of service, or arbitrary code execution in kernel context.
Affected Products
- Linux kernel (multiple stable branches referenced in upstream fixes)
- Distributions shipping vulnerable kernel versions using the KEYS subsystem
- Systems relying on keyctl public key operations via the asymmetric key API
Discovery Timeline
- 2026-07-19 - CVE-2026-63824 published to NVD
- 2026-07-20 - Last updated in NVD database
Technical Details for CVE-2026-63824
Vulnerability Analysis
The vulnerability affects the Linux kernel KEYS subsystem, specifically the keyctl_pkey_params_get_2() helper. This function processes user-supplied parameters for public key operations such as sign, verify, encrypt, and decrypt performed through the keyctl syscall interface. The internal output buffer length is derived incorrectly from user input, and the kernel allocates a buffer smaller than what the cryptographic primitive can produce.
When the cryptographic operation writes its result into this undersized buffer, the write extends past the allocated region. This produces a kernel heap out-of-bounds write. Given the buffer is kernel-allocated and adjacent to other kernel objects, corruption of neighboring allocations becomes possible. Skilled attackers can convert such primitives into privilege escalation by targeting sensitive kernel structures.
Root Cause
The root cause is an incorrect length calculation for the internal output buffer inside keyctl_pkey_params_get_2(). The kernel used the caller-provided size when determining allocation size, rather than the maximum output length dictated by the cryptographic primitive in use. This constitutes a classic boundary condition error [CWE-131] combined with an out-of-bounds write [CWE-787].
Attack Vector
Exploitation requires local access with the ability to invoke the keyctl syscall and low privileges. An attacker crafts a keyctl_pkey_op request specifying an output buffer smaller than the cryptographic primitive's output size. When the kernel performs the operation, the mismatch triggers the overflow. No user interaction is required, and the attack executes entirely within a local context.
The vulnerability manifests during processing of asymmetric key operations. See the referenced upstream kernel commits for exact patch semantics and the corrected allocation logic.
Detection Methods for CVE-2026-63824
Indicators of Compromise
- Unexpected kernel oops or panic messages referencing keyctl_pkey_params_get_2 or the asymmetric key subsystem in dmesg output
- KASAN reports indicating slab-out-of-bounds writes originating from keyctl syscall paths
- Unprivileged processes making unusually frequent keyctl(KEYCTL_PKEY_*) invocations with malformed size parameters
Detection Strategies
- Enable KASAN (Kernel Address Sanitizer) on test kernels to surface out-of-bounds writes during fuzzing or QA
- Deploy audit rules to log keyctl syscall invocations, focusing on KEYCTL_PKEY_SIGN, KEYCTL_PKEY_VERIFY, KEYCTL_PKEY_ENCRYPT, and KEYCTL_PKEY_DECRYPT operations
- Correlate kernel crash telemetry with process ancestry to identify local users triggering repeated faults
Monitoring Recommendations
- Ingest /var/log/kern.log and dmesg output into a centralized logging platform for pattern analysis
- Alert on kernel panics or oops entries containing keyctl or asymmetric_key strings
- Track process behavior anomalies where non-privileged accounts interact with the keyring subsystem at high volume
How to Mitigate CVE-2026-63824
Immediate Actions Required
- Apply the upstream Linux kernel patches referenced in the stable tree commits and reboot affected systems
- Inventory all Linux hosts and identify kernel versions that predate the fix across the impacted stable branches
- Prioritize patching multi-tenant systems, container hosts, and shared build infrastructure where local users are untrusted
Patch Information
The fix modifies keyctl_pkey_params_get_2() to allocate the internal output buffer based on the maximum output length of the cryptographic primitive instead of the caller-supplied size. Patches are available in the following upstream commits: Kernel Commit 0f3058d7, Kernel Commit 5165f1cc, Kernel Commit 5966e4e2, Kernel Commit 622ec2dc, Kernel Commit 670fc6a3, Kernel Commit b11c1fa3, Kernel Commit b1e24733, and Kernel Commit cb481e59. Consult your Linux distribution vendor for backported packages.
Workarounds
- Restrict access to the keyctl syscall using seccomp filters on workloads that do not require asymmetric key operations
- Disable the CONFIG_ASYMMETRIC_KEY_TYPE and related kernel options in custom builds where public key keyring functionality is not needed
- Limit local shell access on shared systems until patched kernels are deployed
# Verify running kernel version and check for patch availability
uname -r
# Example seccomp restriction concept (pseudocode)
# Deny keyctl syscall for untrusted service accounts via systemd
# In the relevant service unit file:
# SystemCallFilter=~keyctl
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
