CVE-2026-40528: OpenSC Buffer Overflow Vulnerability

CVE-2026-40528 Overview

CVE-2026-40528 is a stack and heap buffer overrun vulnerability in OpenSC versions before 0.27.0. The flaw resides in the do_key_value() function within src/pkcs15init/profile.c. When pkcs15-init parses a crafted profile configuration file, a key value entry beginning with = followed by more than sizeof(keybuf) characters is copied into keybuf through memcpy without a length check. This results in corruption of both stack and heap memory regions. The issue is tracked under CWE-121: Stack-based Buffer Overflow and was fixed in commit 0358817.

Critical Impact

An attacker who supplies a malicious profile configuration file can corrupt memory during pkcs15-init invocation, potentially affecting confidentiality, integrity, and availability of the smartcard initialization process.

Affected Products

• OpenSC versions before 0.27.0
• Systems invoking pkcs15-init with attacker-controlled profile files
• Smartcard tooling distributions bundling vulnerable OpenSC builds

Discovery Timeline

• 2026-05-29 - CVE-2026-40528 published to NVD
• 2026-06-03 - Last updated in NVD database

Technical Details for CVE-2026-40528

Vulnerability Analysis

The vulnerability resides in the profile parser used by OpenSC's pkcs15-init tool. OpenSC reads profile configuration files to determine how to initialize PKCS#15 structures on smartcards. The do_key_value() function in src/pkcs15init/profile.c handles key value entries within these profile files.

When a key value entry begins with the = character, the parser treats the following characters as raw key material. The function then copies the supplied bytes into a fixed-size stack buffer named keybuf using memcpy. The copy operation does not validate that the source length is less than or equal to sizeof(keybuf). Supplying more than sizeof(keybuf) bytes overruns both the stack buffer and adjacent heap allocations associated with the parsing context.

The physical attack vector and required user interaction limit remote exploitation, but local users or operators who load untrusted profiles are exposed. See the VulnCheck Advisory on Buffer Overrun for additional analysis.

Root Cause

The root cause is a missing length validation before invoking memcpy. The parser trusts attacker-controlled input length from the profile file and copies bytes into a fixed-size destination without bounds checking, violating safe memory copy practices outlined in [CWE-121].

Attack Vector

An attacker crafts a malicious profile configuration file containing a key value entry starting with = followed by an oversized byte sequence. When an administrator or automated tooling invokes pkcs15-init and references the crafted profile, the overrun executes during parsing. Exploitation requires physical or local access and user interaction to load the malicious profile.

No verified proof-of-concept code is publicly available. Refer to the upstream GitHub Commit Update for the precise input handling pattern that triggers the overrun.

Detection Methods for CVE-2026-40528

Indicators of Compromise

• Unexpected crashes or aborts of the pkcs15-init process during smartcard provisioning
• Profile configuration files containing key value entries that begin with = followed by abnormally long byte sequences
• Core dumps referencing do_key_value or profile.c in OpenSC binaries

Detection Strategies

• Inventory installed OpenSC versions across endpoints and compare against 0.27.0 to identify vulnerable hosts
• Scan profile directories used by pkcs15-init for files containing oversized =-prefixed key value entries
• Monitor process telemetry for pkcs15-init invocations referencing profile paths outside trusted locations

Monitoring Recommendations

• Log all pkcs15-init executions, including the profile path argument and invoking user
• Alert on crashes of OpenSC binaries that produce segmentation faults or stack corruption signatures
• Track file integrity of profile configuration files in /etc/opensc or equivalent paths

How to Mitigate CVE-2026-40528

Immediate Actions Required

• Upgrade OpenSC to version 0.27.0 or later, which contains commit 0358817
• Restrict write access to profile configuration directories to trusted administrators only
• Avoid invoking pkcs15-init with profile files sourced from untrusted users or locations

Patch Information

The fix is available in OpenSC commit 0358817ec74aeca654f83e7709c7720b14c5db59. Rebuild OpenSC from the patched source or install a distribution package containing version 0.27.0 or later. Refer to the GitHub Commit Update for the exact source changes.

Workarounds

• Set restrictive filesystem permissions on profile directories so only root or designated administrators can modify them
• Validate profile configuration files before use by inspecting key value entries for excessive length
• Disable or remove pkcs15-init on systems that do not require PKCS#15 initialization workflows

# Configuration example: restrict OpenSC profile directory permissions
chown -R root:root /usr/share/opensc/
chmod -R 755 /usr/share/opensc/
find /usr/share/opensc/ -name '*.profile' -exec chmod 644 {} \;