https://vulnerability.circl.lu/sightings/feed 2026-08-29T21:52:03.429580+00:00 Vulnerability-Lookup info@circl.lu python-feedgen Contains only the most 10 recent sightings. https://vulnerability.circl.lu/sighting/b44f3356-2296-42a4-991d-bea8f228f8b8/export 2026-08-29T21:52:03.455526+00:00 Automation user https://vulnerability.circl.lu/user/automation {"uuid": "b44f3356-2296-42a4-991d-bea8f228f8b8", "vulnerability_lookup_origin": "1a89b78e-f703-45f3-bb86-59eb712668bd", "author": "9f56dd64-161d-43a6-b9c3-555944290a09", "vulnerability": "CVE-2026-55558", "type": "seen", "source": "https://gist.github.com/alon710/a5f4630118da96558b479a90bb02c09c", "content": "# CVE-2026-55558: CVE-2026-55558: STARTTLS Response Injection in aiosmtplib\n\n> **CVSS Score:** 5.9\n> **Published:** 2026-08-27\n> **Full Report:** https://cvereports.com/reports/CVE-2026-55558\n\n## Summary\nAn input buffering vulnerability exists in the aiosmtplib asynchronous SMTP client library before version 5.1.2. When upgrading a plaintext connection to TLS via STARTTLS, the library processes buffered plaintext responses after transport negotiation has completed. This behavior allows a network-positioned attacker to inject spoofed server responses prior to negotiation, leading to command/response desynchronization, arbitrary capability injection, and potential credential theft.\n\n## TL;DR\naiosmtplib does not clear its internal read buffer prior to executing a TLS handshake during STARTTLS negotiation. An attacker positioned on the network path can pipeline malicious plaintext responses that survive the transport upgrade, hijacking subsequent encrypted commands.\n\n## Exploit Status: POC\n\n## Technical Details\n\n- **CWE ID**: CWE-74\n- **Attack Vector**: Network\n- **CVSS v3.1 Score**: 5.9 (Medium)\n- **EPSS Score**: 0.00261\n- **Impact**: Integrity Loss / Session Desynchronization\n- **Exploit Status**: Proof of Concept (PoC) documented in patch unit tests\n- **KEV Status**: Not listed in CISA KEV\n\n## Affected Systems\n\n- Python asynchronous applications utilizing aiosmtplib for SMTP transport\n- **aiosmtplib**: < 5.1.2 (Fixed in: `5.1.2`)\n\n## Mitigation\n\n- Upgrade aiosmtplib to version 5.1.2 or later.\n- Reconfigure SMTP clients to use Implicit TLS (use_tls=True) on port 465 to bypass plaintext negotiation entirely.\n- Implement DNSSEC to prevent DNS-based Adversary-in-the-Middle hijacking of SMTP servers.\n\n**Remediation Steps:**\n1. Identify all deployment instances using the aiosmtplib library.\n2. Update requirements.txt, Pipfile, or pyproject.toml to define aiosmtplib >= 5.1.2.\n3. Execute dependency installation pipeline (e.g., pip install --upgrade aiosmtplib).\n4. Validate connection configurations and verify fallback behaviors are disabled.\n\n## References\n\n- [GitHub Security Advisory GHSA-vxj7-4xrp-5vr4](https://github.com/cole/aiosmtplib/security/advisories/GHSA-vxj7-4xrp-5vr4)\n- [Official Security Patch Commit](https://github.com/cole/aiosmtplib/commit/9fab7ba1361dbf7622ede1315a24be805cff09c9)\n- [aiosmtplib v5.1.2 Release Notes](https://github.com/cole/aiosmtplib/releases/tag/v5.1.2)\n- [National Vulnerability Database (NVD) Entry](https://nvd.nist.gov/vuln/detail/CVE-2026-55558)\n- [CVE.org Authority Record](https://www.cve.org/CVERecord?id=CVE-2026-55558)\n- [Wiz Vulnerability Database Details](https://www.wiz.io/vulnerability-database/cve/cve-2026-55558)\n\n\n---\n*Generated by [CVEReports](https://cvereports.com/reports/CVE-2026-55558) - Automated Vulnerability Intelligence*", "creation_timestamp": "2026-08-28T00:31:38.557700Z"} 2026-08-28T00:31:38.557700+00:00