CVE-2025-66038: OpenSC Buffer Overflow Vulnerability

CVE-2025-66038 Overview

A buffer over-read vulnerability exists in OpenSC, an open source smart card tools and middleware package. The vulnerability resides in the sc_compacttlv_find_tag function, which searches a compact-TLV buffer for a given tag. When processing malformed compact-TLV data, the function fails to verify that the claimed value length fits within the remaining buffer, potentially returning out-of-bounds pointers that can lead to memory corruption when subsequently dereferenced.

Critical Impact

Attackers with physical access to a system can supply maliciously crafted smart card data or files to trigger out-of-bounds memory access, potentially leading to memory corruption and system compromise.

Affected Products

• OpenSC versions prior to 0.27.0
• Smart card middleware deployments using vulnerable OpenSC libraries
• Systems reading compact-TLV formatted data from untrusted sources

Discovery Timeline

• 2026-03-30 - CVE-2025-66038 published to NVD
• 2026-04-01 - Last updated in NVD database

Technical Details for CVE-2025-66038

Vulnerability Analysis

This vulnerability is classified as an Out-of-Bounds Read (CWE-126). The flaw exists in OpenSC's compact-TLV parsing logic where the sc_compacttlv_find_tag function processes encoded tag-length-value structures. In the compact-TLV format, a single byte encodes both the tag (high nibble) and value length (low nibble).

When the function encounters a malformed buffer such as {0x0A}, it interprets this as tag=0 and length=10. However, no actual value bytes follow this encoded byte. The function returns a pointer equal to buf+1 and sets outlen=10 without validating that the claimed value length of 10 bytes actually exists within the remaining buffer space.

This absence of bounds checking creates a dangerous condition where subsequent code attempting to dereference the returned pointer or read the indicated number of bytes will access memory beyond the allocated buffer boundaries.

Root Cause

The root cause is insufficient input validation in the sc_compacttlv_find_tag function. The function trusts the length value encoded in the compact-TLV byte without verifying that sufficient data exists in the buffer to satisfy the claimed length. This is a classic bounds-checking oversight where the encoded metadata is not validated against the actual buffer size before returning pointers to callers.

Attack Vector

The attack requires physical access to the target system. An attacker can exploit this vulnerability by:

1. Crafting a malicious smart card containing specially formatted compact-TLV data with inflated length values
2. Presenting the malicious card to a system running vulnerable OpenSC middleware
3. When the system reads and parses the card data, sc_compacttlv_find_tag returns an out-of-bounds pointer
4. Downstream code attempting to use this pointer triggers memory corruption

The vulnerability can also be triggered through maliciously crafted files that contain compact-TLV formatted data, provided the application uses OpenSC to parse such files from untrusted sources.

Detection Methods for CVE-2025-66038

Indicators of Compromise

• Unexpected application crashes or segmentation faults when processing smart card operations
• Memory access violations in processes utilizing OpenSC libraries
• Anomalous smart card read failures followed by system instability

Detection Strategies

• Monitor for crash dumps or core files from applications using OpenSC middleware
• Implement memory sanitizer tools (AddressSanitizer, Valgrind) during development and testing to detect out-of-bounds access
• Review system logs for unexpected termination of smart card-related services

Monitoring Recommendations

• Enable enhanced logging for smart card subsystem operations
• Monitor process health for applications that interact with smart cards or parse compact-TLV data
• Deploy endpoint detection solutions capable of identifying memory corruption exploitation attempts

How to Mitigate CVE-2025-66038

Immediate Actions Required

• Upgrade OpenSC to version 0.27.0 or later immediately
• Restrict physical access to systems processing smart card data from untrusted sources
• Review applications that parse compact-TLV formatted data from external sources and validate they use patched OpenSC versions

Patch Information

OpenSC has released version 0.27.0 which addresses this vulnerability. The fix ensures proper bounds checking in the sc_compacttlv_find_tag function before returning pointers to callers. The security patch is available via the GitHub Commit Update. Additional details are available in the GitHub Security Advisory GHSA-72x5-fwjx-2459 and the GitHub CVE-2025-66038 Information page.

Workarounds

• Implement application-level input validation before passing data to OpenSC compact-TLV parsing functions
• Restrict smart card operations to trusted, known-good cards until patches can be applied
• Consider disabling smart card functionality on systems where it is not strictly required until the update is deployed

# Verify OpenSC version to confirm patched status
opensc-tool --version
# Expected output should show version 0.27.0 or later

# Update OpenSC on Debian/Ubuntu systems
sudo apt update && sudo apt install opensc

# Update OpenSC on RHEL/CentOS systems
sudo yum update opensc