CVE-2026-3548: wolfSSL Buffer Overflow Vulnerability

CVE-2026-3548 Overview

Two buffer overflow vulnerabilities have been identified in the wolfSSL CRL (Certificate Revocation List) parser when parsing CRL numbers. The vulnerabilities consist of a heap-based buffer overflow that can occur when improperly storing the CRL number as a hexadecimal string, and a stack-based overflow that can be triggered with sufficiently sized CRL numbers. With appropriately crafted CRLs, either of these out-of-bounds write conditions could be exploited. This vulnerability specifically affects builds that have CRL support enabled, and requires the user to load a CRL from an untrusted source.

Critical Impact

Remote attackers can potentially achieve code execution or cause denial of service by exploiting heap and stack buffer overflows in the wolfSSL CRL parser through maliciously crafted Certificate Revocation Lists.

Affected Products

• wolfSSL library builds with CRL support enabled
• Applications using wolfSSL for TLS/SSL certificate validation with CRL checking
• Embedded systems and IoT devices implementing wolfSSL with CRL functionality

Discovery Timeline

• 2026-03-19 - CVE CVE-2026-3548 published to NVD
• 2026-03-19 - Last updated in NVD database

Technical Details for CVE-2026-3548

Vulnerability Analysis

This vulnerability comprises two distinct buffer overflow conditions within the wolfSSL CRL parser component. The first is a heap-based buffer overflow (CWE-122) that manifests during the conversion and storage of CRL numbers to hexadecimal string format. The improper handling of CRL number conversion results in memory being written beyond allocated heap buffer boundaries.

The second vulnerability is a stack-based buffer overflow that occurs when processing CRL numbers of sufficient size. Both vulnerabilities are triggered during the parsing of specially crafted CRL files, potentially allowing attackers to corrupt memory, hijack program execution flow, or cause application crashes.

The attack is network-accessible and requires no authentication or user interaction beyond the victim system loading the malicious CRL. However, the attack requires specific preconditions: the target must have CRL support explicitly enabled in their wolfSSL build, and must be configured to load CRLs from sources under attacker control.

Root Cause

The root cause of these vulnerabilities lies in insufficient bounds checking within the CRL number parsing routines. When storing CRL numbers as hexadecimal strings, the parser fails to properly validate that the destination buffer is large enough to accommodate the converted data. Similarly, the stack-based vulnerability results from inadequate size validation when handling CRL numbers that exceed expected lengths, causing stack buffer overwrites.

Attack Vector

The attack vector is network-based, where an adversary would need to supply a maliciously crafted CRL file to a vulnerable application. This could be achieved through several scenarios: compromising a CRL distribution point, performing man-in-the-middle attacks on CRL fetches, or convincing users to import untrusted CRLs. The crafted CRL would contain specially constructed CRL number fields designed to trigger either the heap or stack overflow conditions during parsing.

The exploitation requires the target system to have wolfSSL compiled with CRL support enabled and to process the attacker-controlled CRL content. Once the malicious CRL is parsed, the overflow can corrupt adjacent memory, potentially enabling arbitrary code execution or denial of service.

Detection Methods for CVE-2026-3548

Indicators of Compromise

• Unexpected crashes or segmentation faults in applications using wolfSSL with CRL functionality
• Memory corruption indicators or heap/stack anomalies in processes handling certificate revocation
• Unusually large or malformed CRL files being processed by the system
• Evidence of CRL fetches from suspicious or unexpected sources

Detection Strategies

• Monitor wolfSSL-enabled applications for abnormal termination or memory access violations during CRL processing
• Implement network monitoring to detect potentially malicious CRL downloads from untrusted sources
• Deploy memory protection mechanisms (ASLR, stack canaries, heap guards) to detect exploitation attempts
• Use application logging to track CRL parsing operations and identify anomalous CRL number sizes

Monitoring Recommendations

• Enable crash reporting and core dump analysis for applications utilizing wolfSSL CRL validation
• Implement intrusion detection signatures for network traffic containing oversized or malformed CRL structures
• Monitor system logs for repeated CRL parsing failures that may indicate exploitation attempts
• Track CRL distribution point connections and alert on unexpected or unauthorized endpoints

How to Mitigate CVE-2026-3548

Immediate Actions Required

• Update wolfSSL to a patched version that addresses these buffer overflow vulnerabilities
• If updating is not immediately possible, disable CRL support in wolfSSL builds until patches can be applied
• Restrict CRL loading to trusted, authenticated sources only
• Implement network controls to prevent loading CRLs from untrusted or external sources

Patch Information

wolfSSL has addressed these vulnerabilities through security patches. Organizations should update to the latest patched version of wolfSSL. For technical details on the fixes, refer to GitHub Pull Request #9628 and GitHub Pull Request #9873.

Workarounds

• Disable CRL support in wolfSSL builds if CRL checking is not required for your use case
• Implement strict input validation on CRL sources, only accepting CRLs from pre-approved, authenticated distribution points
• Deploy network-level controls to filter and validate CRL content before processing
• Use OCSP (Online Certificate Status Protocol) as an alternative to CRL-based revocation checking where feasible

# Configuration example - Rebuild wolfSSL without CRL support as temporary mitigation
./configure --disable-crl
make clean
make
make install