{"dataType": "CVE_RECORD", "dataVersion": "5.2", "cveMetadata": {"cveId": "CVE-2026-55558", "assignerOrgId": "a0819718-46f1-4df5-94e2-005712e83aaa", "state": "PUBLISHED", "assignerShortName": "GitHub_M", "dateReserved": "2026-06-16T23:11:20.213Z", "datePublished": "2026-08-20T14:29:58.580Z", "dateUpdated": "2026-08-20T14:29:58.580Z"}, "containers": {"cna": {"title": "aiosmtplib: STARTTLS response injection", "problemTypes": [{"descriptions": [{"cweId": "CWE-74", "lang": "en", "description": "CWE-74: Improper Neutralization of Special Elements in Output Used by a Downstream Component ('Injection')", "type": "CWE"}]}], "metrics": [{"cvssV3_1": {"attackComplexity": "HIGH", "attackVector": "NETWORK", "availabilityImpact": "NONE", "baseScore": 5.9, "baseSeverity": "MEDIUM", "confidentialityImpact": "NONE", "integrityImpact": "HIGH", "privilegesRequired": "NONE", "scope": "UNCHANGED", "userInteraction": "NONE", "vectorString": "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N", "version": "3.1"}}], "references": [{"name": "https://github.com/cole/aiosmtplib/security/advisories/GHSA-vxj7-4xrp-5vr4", "tags": ["x_refsource_CONFIRM"], "url": "https://github.com/cole/aiosmtplib/security/advisories/GHSA-vxj7-4xrp-5vr4"}, {"name": "https://github.com/cole/aiosmtplib/commit/9fab7ba1361dbf7622ede1315a24be805cff09c9", "tags": ["x_refsource_MISC"], "url": "https://github.com/cole/aiosmtplib/commit/9fab7ba1361dbf7622ede1315a24be805cff09c9"}, {"name": "https://github.com/cole/aiosmtplib/releases/tag/v5.1.2", "tags": ["x_refsource_MISC"], "url": "https://github.com/cole/aiosmtplib/releases/tag/v5.1.2"}], "affected": [{"vendor": "cole", "product": "aiosmtplib", "versions": [{"version": "< 5.1.2", "status": "affected"}]}], "providerMetadata": {"orgId": "a0819718-46f1-4df5-94e2-005712e83aaa", "shortName": "GitHub_M", "dateUpdated": "2026-08-20T14:29:58.580Z"}, "descriptions": [{"lang": "en", "value": "aiosmtplib is an asynchronous SMTP client for use with asyncio. Prior to 5.1.2, SMTPProtocol.start_tls in src/aiosmtplib/protocol.py consumes the server's 220 response and starts the TLS handshake without clearing SMTPProtocol._buffer. An active network attacker can place attacker-chosen SMTP response lines after the plaintext 220 response in the same network segment. The method then calls loop.start_tls; those bytes survive the transport upgrade and are parsed as the first response from inside the TLS session, desynchronizing subsequent SMTP command and response pairs. Connections using start_tls=True or opportunistic STARTTLS are affected, while connections using use_tls=True are not. This issue is fixed in version 5.1.2."}], "source": {"advisory": "GHSA-vxj7-4xrp-5vr4", "discovery": "UNKNOWN"}}}}

{}

{"affected": [{"affectedData": [{"product": "aiosmtplib", "vendor": "cole", "versions": [{"status": "affected", "version": "< 5.1.2"}]}], "source": "security-advisories@github.com"}], "cveTags": [], "descriptions": [{"lang": "en", "value": "aiosmtplib is an asynchronous SMTP client for use with asyncio. Prior to 5.1.2, SMTPProtocol.start_tls in src/aiosmtplib/protocol.py consumes the server's 220 response and starts the TLS handshake without clearing SMTPProtocol._buffer. An active network attacker can place attacker-chosen SMTP response lines after the plaintext 220 response in the same network segment. The method then calls loop.start_tls; those bytes survive the transport upgrade and are parsed as the first response from inside the TLS session, desynchronizing subsequent SMTP command and response pairs. Connections using start_tls=True or opportunistic STARTTLS are affected, while connections using use_tls=True are not. This issue is fixed in version 5.1.2."}], "id": "CVE-2026-55558", "lastModified": "2026-08-20T15:17:31.980", "metrics": {"cvssMetricV31": [{"cvssData": {"attackComplexity": "HIGH", "attackVector": "NETWORK", "availabilityImpact": "NONE", "baseScore": 5.9, "baseSeverity": "MEDIUM", "confidentialityImpact": "NONE", "integrityImpact": "HIGH", "privilegesRequired": "NONE", "scope": "UNCHANGED", "userInteraction": "NONE", "vectorString": "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N", "version": "3.1"}, "exploitabilityScore": 2.2, "impactScore": 3.6, "source": "security-advisories@github.com", "type": "Secondary"}]}, "published": "2026-08-20T15:17:31.980", "references": [{"source": "security-advisories@github.com", "url": "https://github.com/cole/aiosmtplib/commit/9fab7ba1361dbf7622ede1315a24be805cff09c9"}, {"source": "security-advisories@github.com", "url": "https://github.com/cole/aiosmtplib/releases/tag/v5.1.2"}, {"source": "security-advisories@github.com", "url": "https://github.com/cole/aiosmtplib/security/advisories/GHSA-vxj7-4xrp-5vr4"}], "sourceIdentifier": "security-advisories@github.com", "vulnStatus": "Received", "weaknesses": [{"description": [{"lang": "en", "value": "CWE-74"}], "source": "security-advisories@github.com", "type": "Primary"}]}

{}

{}