CVE-2026-13757: p11-kit Denial of Service Vulnerability

CVE-2026-13757 Overview

CVE-2026-13757 is a denial-of-service vulnerability in p11-kit, a library that provides a way to load and enumerate PKCS#11 modules on Linux systems. The flaw resides in the RPC message attribute parsing functions p11_rpc_message_get_attribute() and p11_rpc_message_get_attribute_array_value(), which form a mutually-recursive call chain without any recursion depth limit. An unauthenticated attacker with local access to the p11-kit RPC Unix domain socket can send a crafted request containing deeply nested template attributes. This triggers stack exhaustion, crashing the p11-kit server process and any dependent services relying on it for cryptographic operations.

Critical Impact
Local unauthenticated attackers can crash the p11-kit RPC server through unbounded recursion, disrupting PKCS#11-dependent cryptographic services.

Affected Products

p11-kit (upstream library)
Red Hat Enterprise Linux distributions bundling vulnerable p11-kit versions
Linux systems exposing the p11-kit RPC Unix domain socket to local users

Discovery Timeline

2026-06-29 - CVE-2026-13757 published to NVD
2026-07-01 - Last updated in NVD database

Technical Details for CVE-2026-13757

Vulnerability Analysis

The flaw is classified as uncontrolled recursion [CWE-674]. The p11-kit RPC protocol serializes PKCS#11 attributes over a Unix domain socket. Certain attribute types — CKA_WRAP_TEMPLATE, CKA_UNWRAP_TEMPLATE, and CKA_DERIVE_TEMPLATE — encode nested attribute arrays. Parsing these attributes requires the server to descend into each nested template and re-invoke attribute parsing on its children.

The functions p11_rpc_message_get_attribute() and p11_rpc_message_get_attribute_array_value() call each other recursively to walk this structure. Neither function enforces a maximum nesting depth. An attacker who supplies attributes nested thousands of levels deep drives the server's call stack past its allocated size. The resulting stack overflow terminates the p11-kit process and any service depending on it for key management or cryptographic operations.

Root Cause

The root cause is missing input validation on the nesting depth of PKCS#11 template attributes during RPC message deserialization. The mutually-recursive parser trusts client-supplied structure depth, converting a bounded protocol feature into an unbounded stack consumer.

Attack Vector

Exploitation requires local access to the p11-kit RPC Unix domain socket. No authentication is required beyond the socket's filesystem permissions. The attacker constructs a serialized RPC request in which a CKA_WRAP_TEMPLATE, CKA_UNWRAP_TEMPLATE, or CKA_DERIVE_TEMPLATE attribute contains another such template, repeated to a depth sufficient to exhaust the server thread's stack. Sending this message crashes the daemon.

No verified public proof-of-concept code has been released. Refer to the Red Hat CVE-2026-13757 Advisory and Red Hat Bug Report #2494556 for technical details.

Detection Methods for CVE-2026-13757

Indicators of Compromise

Unexpected termination or repeated restarts of the p11-kit-server or p11-kit process on affected hosts.
Segmentation fault entries in system logs (journalctl, dmesg) referencing p11-kit with stack-related fault addresses.
Application errors from PKCS#11 consumers reporting lost connections to the p11-kit RPC socket.

Detection Strategies

Monitor process exit codes and core dumps for p11-kit services and correlate with SIGSEGV or stack overflow signatures.
Audit access to the p11-kit RPC Unix domain socket path and flag connections from unexpected local UIDs.
Track anomalous volumes of RPC messages or oversized serialized attribute payloads against baseline usage.

Monitoring Recommendations

Enable core dump collection for p11-kit processes to preserve forensic evidence of exploitation attempts.
Alert on repeated crashes of PKCS#11-dependent services such as GnuTLS or NSS-consuming applications.
Review auditd rules covering the p11-kit socket path for connection tracking on multi-user systems.

How to Mitigate CVE-2026-13757

Immediate Actions Required

Apply vendor-supplied updates for p11-kit as they become available through your Linux distribution's package manager.
Restrict filesystem permissions on the p11-kit RPC Unix domain socket to trusted local users only.
Identify services that depend on p11-kit and prepare restart procedures in case of daemon crashes.

Patch Information

Consult the Red Hat CVE-2026-13757 Advisory for current package status and errata. Upstream fixes introduce a recursion depth limit in p11_rpc_message_get_attribute() and p11_rpc_message_get_attribute_array_value() to reject excessively nested template attributes before stack exhaustion occurs.

Workarounds

Disable the p11-kit RPC server on hosts that do not require remote PKCS#11 module sharing.
Tighten socket permissions so only intended service accounts can connect to the p11-kit RPC endpoint.
Run p11-kit under a resource-constrained systemd unit that automatically restarts on failure to reduce service disruption.

bash

# Restrict p11-kit RPC socket access to a dedicated group
chown root:p11kit-users /run/user/$(id -u)/p11-kit/pkcs11
chmod 0660 /run/user/$(id -u)/p11-kit/pkcs11

# Enable automatic restart on crash via systemd override
systemctl edit p11-kit-server.service
# [Service]
# Restart=on-failure
# RestartSec=2s