CVE-2026-5477

Description

An integer overflow existed in the wolfCrypt CMAC implementation, that could be exploited to forge CMAC tags. The function wc_CmacUpdate used the guard `if (cmac->totalSz != 0)` to skip XOR-chaining on the first block (where digest is all-zeros and the XOR is a no-op). However, totalSz is word32 and wraps to zero after 2^28 block flushes (4 GiB), causing the guard to erroneously discard the live CBC-MAC chain state. Any two messages sharing a common suffix beyond the 4 GiB mark then produce identical CMAC tags, enabling a zero-work prefix-substitution forgery. The fix removes the guard, making the XOR unconditional; the no-op property on the first block is preserved because digest is zero-initialized by wc_InitCmac_ex.

CVSS Details

CVSS Score

7.5

Severity

HIGH

CVSS Vector

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

Configurations (Affected Products)

cpe:2.3:a:wolfssl:wolfssl:*:*:*:*:*:*:*:* - VULNERABLE

wolfSSL (Specific versions prior to fix in PR #10102)

PoC / Exploit Code

⚠ For Security Research Only

The following code is for security research and authorized testing only.

python

#include <stdio.h>
#include <stdint.h>

// Simulation of the vulnerable logic in wc_CmacUpdate
void simulate_vulnerable_update(uint32_t totalSz, int isLastBlock) {
    // The vulnerable guard: checks if totalSz is not zero
    // In the real code, totalSz wraps around after 4GB of data
    if (totalSz != 0) {
        printf("[+] Performing XOR chaining with previous state.\n");
    } else {
        // This block is executed when totalSz wraps to 0 after 4GB
        printf("[!] VULNERABLE: Skipping XOR (treated as first block).\n");
        printf("[!] CBC-MAC chain state is discarded/reset.\n");
    }
}

int main() {
    // Scenario: Processing data that causes totalSz to wrap
    // 2^28 blocks * 16 bytes/block = 4GB (approx limit for word32)
    
    printf("--- Normal Operation (< 4GB) ---\n");
    simulate_vulnerable_update(16, 0); 

    printf("\n--- After 4GB Overflow (totalSz wraps to 0) ---\n");
    // Simulate the state where totalSz has wrapped around to 0
    // due to integer overflow after processing massive data.
    simulate_vulnerable_update(0, 0);

    printf("\n--- Impact ---\n");
    printf("An attacker can now send a message with a common suffix,\n");
    printf("and it will generate the same CMAC tag as the previous message.\n");

    return 0;
}

References

[1] CVE.org https://www.cve.org/CVERecord?id=CVE-2026-5477
[2] NVD NIST https://nvd.nist.gov/vuln/detail/CVE-2026-5477
[3] CVE Details https://www.cvedetails.com/cve/CVE-2026-5477/
[4] VulDB https://vuldb.com/cve/CVE-2026-5477
[5] https://github.com/wolfSSL/wolfssl/pull/10102

Raw JSON Data

JSON

{"cve": {"id": "CVE-2026-5477", "sourceIdentifier": "[email protected]", "published": "2026-04-10T06:16:05.243", "lastModified": "2026-04-27T17:51:47.143", "vulnStatus": "Analyzed", "cveTags": [], "descriptions": [{"lang": "en", "value": "An integer overflow existed in the wolfCrypt CMAC implementation, that could be exploited to forge CMAC tags. The function wc_CmacUpdate used the guard `if (cmac->totalSz != 0)` to skip XOR-chaining on the first block (where digest is all-zeros and the XOR is a no-op). However, totalSz is word32 and wraps to zero after 2^28 block flushes (4 GiB), causing the guard to erroneously discard the live CBC-MAC chain state. Any two messages sharing a common suffix beyond the 4 GiB mark then produce identical CMAC tags, enabling a zero-work prefix-substitution forgery. The fix removes the guard, making the XOR unconditional; the no-op property on the first block is preserved because digest is zero-initialized by wc_InitCmac_ex."}], "metrics": {"cvssMetricV40": [{"source": "[email protected]", "type": "Secondary", "cvssData": {"version": "4.0", "vectorString": "CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X", "baseScore": 8.2, "baseSeverity": "HIGH", "attackVector": "NETWORK", "attackComplexity": "LOW", "attackRequirements": "PRESENT", "privilegesRequired": "NONE", "userInteraction": "NONE", "vulnConfidentialityImpact": "NONE", "vulnIntegrityImpact": "HIGH", "vulnAvailabilityImpact": "NONE", "subConfidentialityImpact": "NONE", "subIntegrityImpact": "NONE", "subAvailabilityImpact": "NONE", "exploitMaturity": "NOT_DEFINED", "confidentialityRequirement": "NOT_DEFINED", "integrityRequirement": "NOT_DEFINED", "availabilityRequirement": "NOT_DEFINED", "modifiedAttackVector": "NOT_DEFINED", "modifiedAttackComplexity": "NOT_DEFINED", "modifiedAttackRequirements": "NOT_DEFINED", "modifiedPrivilegesRequired": "NOT_DEFINED", "modifiedUserInteraction": "NOT_DEFINED", "modifiedVulnConfidentialityImpact": "NOT_DEFINED", "modifiedVulnIntegrityImpact": "NOT_DEFINED", "modifiedVulnAvailabilityImpact": "NOT_DEFINED", "modifiedSubConfidentialityImpact": "NOT_DEFINED", "modifiedSubIntegrityImpact": "NOT_DEFINED", "modifiedSubAvailabilityImpact": "NOT_DEFINED", "Safety": "NOT_DEFINED", "Automatable": "NOT_DEFINED", "Recovery": "NOT_DEFINED", "valueDensity": "NOT_DEFINED", "vulnerabilityResponseEffort": "NOT_DEFINED", "providerUrgency": "NOT_DEFINED"}}], "cvssMetricV31": [{"source": "[email protected]", "type": "Primary", "cvssData": {"version": "3.1", "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N", "baseScore": 7.5, "baseSeverity": "HIGH", "attackVector": "NETWORK", "attackComplexity": "LOW", "privilegesRequired": "NONE", "userInteraction": "NONE", "scope": "UNCHANGED", "confidentialityImpact": "NONE", "integrityImpact": "HIGH", "availabilityImpact": "NONE"}, "exploitabilityScore": 3.9, "impactScore": 3.6}]}, "weaknesses": [{"source": "[email protected]", "type": "Secondary", "description": [{"lang": "en", "value": "CWE-190"}]}], "configurations": [{"nodes": [{"operator": "OR", "negate": false, "cpeMatch": [{"vulnerable": true, "criteria": "cpe:2.3:a:wolfssl:wolfssl:*:*:*:*:*:*:*:*", "versionEndIncluding": "5.9.0", "matchCriteriaId": "DA5C710C-46E8-470C-83AF-D33D1A40512D"}]}]}], "references": [{"url": "https://github.com/wolfSSL/wolfssl/pull/10102", "source": "[email protected]", "tags": ["Issue Tracking"]}]}}