Executive Summary

[{'type': 'paragraph', 'content': "CVE-2026-3548 involves two buffer overflow vulnerabilities in the wolfSSL library's Certificate Revocation List (CRL) parser when handling CRL numbers."}, {'type': 'list_item', 'content': 'A heap-based buffer overflow occurs due to improper sizing of the buffer used to store the CRL number as a hexadecimal string. The buffer was sized based on the number of bytes rather than the length of the hexadecimal string, which requires twice the space plus a null terminator.'}, {'type': 'list_item', 'content': 'A stack-based buffer overflow happens when parsing CRL numbers larger than 56 octets. The CRL number data is written into a fixed 64-byte stack buffer, which is insufficient for larger CRL numbers, causing overflow into adjacent stack frames.'}, {'type': 'paragraph', 'content': 'Both vulnerabilities can be triggered by loading a specially crafted CRL from an untrusted source in builds that enable CRL support.'}] [1, 2]

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Impact Analysis

Exploitation of these buffer overflow vulnerabilities can lead to memory corruption within the wolfSSL library.

• Heap-based overflow may allow attackers to overwrite memory beyond the allocated buffer, potentially leading to crashes or arbitrary code execution.
• Stack-based overflow can overwrite adjacent stack frames, which may result in control flow hijacking or denial of service.

Since the vulnerabilities require loading a CRL from an untrusted source with crafted CRL numbers, the impact depends on whether such CRLs are processed by the application using wolfSSL with CRL support enabled.

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Compliance Impact

I don't know

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Detection Guidance

This vulnerability can be detected by monitoring for the presence of wolfSSL builds that enable CRL support and by analyzing Certificate Revocation Lists (CRLs) loaded from untrusted sources for unusually large or malformed CRL numbers.

Specifically, detection involves checking if CRL numbers exceed expected sizes (e.g., larger than 20 octets or 56 octets) which could trigger the buffer overflow.

While no explicit commands are provided in the resources, a practical approach includes using network capture tools like tcpdump or Wireshark to capture CRL traffic and then inspecting the CRL number fields for abnormal sizes.

• Use tcpdump to capture CRL traffic: tcpdump -i <interface> port 80 or port 443 -w crl_capture.pcap
• Analyze captured CRLs with OpenSSL to inspect CRL numbers: openssl crl -in <crl_file> -noout -text
• Check wolfSSL version and build configuration to confirm if CRL support is enabled, as only those builds are vulnerable.

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Mitigation Strategies

Immediate mitigation steps include updating wolfSSL to a version that contains the patches fixing this vulnerability, specifically versions including the fixes merged around January 2026 and targeting release 5.9.0.

The patches fix the buffer overflow by correctly sizing the buffer for hexadecimal CRL numbers and by adding length checks to reject CRL numbers exceeding the maximum allowed size.

Additionally, avoid loading CRLs from untrusted sources, especially those with unusually large CRL numbers.

• Apply the official wolfSSL patch that corrects buffer sizing for CRL numbers.
• Ensure the wolfSSL build enables CRL support only if necessary and consider disabling it if not required.
• Implement input validation or filtering to reject CRLs with CRL numbers larger than the defined maximum size (20 octets).

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