CVE-2026-43861

Description

mutt before 2.3.2 does not check for '\0' in url_pct_decode.

CVSS Details

CVSS Score

3.7

Severity

LOW

CVSS Vector

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

Configurations (Affected Products)

No configuration data available.

Mutt < 2.3.2

PoC / Exploit Code

⚠ For Security Research Only

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

python

/*
 * Conceptual PoC for CVE-2026-43861
 * Demonstrating the lack of null byte checks in URL decoding.
 */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

// Simulates the vulnerable url_pct_decode function
void vulnerable_decode(char *dst, const char *src) {
    while (*src) {
        if (*src == '%' && *(src + 1) && *(src + 2)) {
            char hex[3] = {0};
            hex[0] = *(src + 1);
            hex[1] = *(src + 2);
            // Vulnerability: No check if the decoded char is '\0'
            unsigned char val = (unsigned char)strtol(hex, NULL, 16);
            *dst++ = (char)val;
            src += 3;
        } else {
            *dst++ = *src++;
        }
    }
    *dst = '\0';
}

int main() {
    char decoded[256];
    // Payload: 'http://safe.com' + null byte + 'http://evil.com'
    // Encoded as: http://safe.com%00http://evil.com
    const char *payload = "http://safe.com%00http://evil.com";
    
    printf("Payload: %s\n", payload);
    vulnerable_decode(decoded, payload);
    
    printf("Decoded Result: %s\n", decoded);
    
    // Verify if the string was terminated early
    if (strstr(decoded, "evil.com") == NULL) {
        printf("[+] Vulnerability confirmed: String terminated at null byte.\n");
        printf("    Intended 'evil.com' part was dropped.\n");
    } else {
        printf("[-] Vulnerability not reproduced or patched.\n");
    }
    
    return 0;
}

References

[1] CVE.org https://www.cve.org/CVERecord?id=CVE-2026-43861
[2] NVD NIST https://nvd.nist.gov/vuln/detail/CVE-2026-43861
[3] CVE Details https://www.cvedetails.com/cve/CVE-2026-43861/
[4] VulDB https://vuldb.com/cve/CVE-2026-43861
[5] https://github.com/muttmua/mutt/commit/12f54fe3b61f761c096fe95e95d5e3072af00ed2

Raw JSON Data

JSON

{"cve": {"id": "CVE-2026-43861", "sourceIdentifier": "[email protected]", "published": "2026-05-04T07:16:00.730", "lastModified": "2026-05-05T19:44:42.893", "vulnStatus": "Deferred", "cveTags": [], "descriptions": [{"lang": "en", "value": "mutt before 2.3.2 does not check for '\\0' in url_pct_decode."}], "metrics": {"cvssMetricV31": [{"source": "[email protected]", "type": "Secondary", "cvssData": {"version": "3.1", "vectorString": "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N", "baseScore": 3.7, "baseSeverity": "LOW", "attackVector": "NETWORK", "attackComplexity": "HIGH", "privilegesRequired": "NONE", "userInteraction": "NONE", "scope": "UNCHANGED", "confidentialityImpact": "NONE", "integrityImpact": "LOW", "availabilityImpact": "NONE"}, "exploitabilityScore": 2.2, "impactScore": 1.4}]}, "weaknesses": [{"source": "[email protected]", "type": "Primary", "description": [{"lang": "en", "value": "CWE-158"}]}], "references": [{"url": "https://github.com/muttmua/mutt/commit/12f54fe3b61f761c096fe95e95d5e3072af00ed2", "source": "[email protected]"}]}}