CVE-2026-55558

MEDIUMPre-NVD 5.95.9—
EchelonGraph scoreMEDIUM confidence

This medium-severity CVE scores 5.9 under a secondary CVSS source (NVD's own analysis pending). EPSS exploit probability: 0.3%, top 82% of all CVEs by exploit prediction. GitHub Security Advisory data not yet ingested — confidence will rise once GHSA publishes (typical lag: hours to days for open-source ecosystem CVEs; never for infrastructure-only CVEs).

Triggered by: NVD CVSS baseline
Sources: epss, secondary
5.9EG
EchelonGraph verdictMonitorLow exploitation likelihood right now — keep watching.
  • Lower severity and no public exploit yet
CISA-KEV: Not listedEPSS PROB: 0%CVSS: 5.9Exploit: None knownExposed: 0

No vendor fix yet — apply a workaround or compensating control (WAF / firewall / segmentation) and watch for a patch.

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.

CVSS v3
5.9
EG Score
5.9(medium)
EG Risk
27(Track)
EG Risk 27/100SSVC: Track

EG Risk is EchelonGraph's 0–100 priority score: it fuses intrinsic severity with real-world exploitation and automatability so you can rank equal-severity CVEs and fix the most dangerous first. Higher = act sooner. Distinct from the 0–10 EG Score (severity).

How it’s computed
Severity59% × 45%
Exploitation0% × 40%
Automatability0% × 15%
Action: Routine — remediate on your standard cadence.
EPSS PROB
0%
EPSS %ILE
18%
KEV
Not listed

Published

August 20, 2026

Last Modified

August 20, 2026

Advisory Details (3)

Auto-updated Aug 20, 2026
Patch available. Sources: github, github_commit, github_release.
github_release Patch Available

v5.1.2

Patch available: cole/aiosmtplib v5.1.2

https://github.com/cole/aiosmtplib/releases/tag/v5.1.2
github_commit Patch Available

commit 9fab7ba1361d (cole/aiosmtplib)

Patch available: cole/aiosmtplib v5.1.2 (contains commit 9fab7ba1361d)

https://github.com/cole/aiosmtplib/commit/9fab7ba1361dbf7622ede1315a24be805cff09c9
github Patch Available

STARTTLS response injection · Advisory · cole/aiosmtplib · GitHub

https://github.com/cole/aiosmtplib/security/advisories/GHSA-vxj7-4xrp-5vr4

Vendor Advisories for CVE-2026-55558(1)

These vendors published their own advisory mentioning this CVE — often with vendor-specific remediation steps + affected product lists not in NVD.

Affected Packages

(1 across 1 ecosystem)
PyPI(1)
PackageVulnerable rangeFixed inDependents
aiosmtplib0.1 ... 5.1.1 (37 versions)5.1.2—

Weakness Classification(1)

MITRE Common Weakness Enumeration — the root-cause categories this CVE belongs to.

Data Freshness Timeline

(refreshed 9× in last 7d / 17× in last 30d)

Each row is a source pipeline that fetched or updated this CVE on that date, with what changed. For example, "NVD update" means NVD published or revised its analysis for this CVE; "MITRE cvelistV5" means we ingested or refreshed it from the CNA feed. Most recent first.

  1. 2026-08-30 01:22 UTCEPSS rescore
  2. 2026-08-29 17:44 UTCEG score recompute
  3. 2026-08-28 21:41 UTCEPSS rescore
  4. 2026-08-26 17:54 UTCEG score recompute
  5. 2026-08-26 14:46 UTCEPSS rescore
  6. 2026-08-25 17:14 UTCEG score recompute
  7. 2026-08-25 13:49 UTCEPSS rescore
  8. 2026-08-24 17:51 UTCEG score recompute
  9. 2026-08-23 18:28 UTCEG score recompute
  10. 2026-08-23 00:19 UTCEPSS rescore
  11. 2026-08-22 19:06 UTCEG score recompute
  12. 2026-08-21 23:49 UTCEPSS rescore
  13. 2026-08-20 20:20 UTCEG score recompute
  14. 2026-08-20 20:02 UTCEG score recompute
  15. 2026-08-20 15:27 UTCEG score recompute
  16. 2026-08-20 14:42 UTCEG score recompute
  17. 2026-08-20 14:41 UTCMITRE cvelistV5first tracked

Related CVEs(same CWE)

Frequently asked(5)

What is CVE-2026-55558?
CVE-2026-55558 is a medium vulnerability published on August 20, 2026. aiosmtplib is an asynchronous SMTP client for use with asyncio. Prior to 5.1.2, SMTPProtocol.starttls 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…
When was CVE-2026-55558 disclosed?
CVE-2026-55558 was first published in the National Vulnerability Database on August 20, 2026. EchelonGraph re-ingests CVE updates from NVD on a 2-hour cycle, so this page reflects the latest published state.
Is CVE-2026-55558 actively exploited?
CVE-2026-55558 is not currently on CISA's Known Exploited Vulnerabilities catalog. FIRST EPSS estimates a 0% probability of exploitation in the next 30 days, which ranks it in the top 82.5% of all scored CVEs.
What is the CVSS score of CVE-2026-55558?
CVE-2026-55558 has a CVSS v3 base score of 5.9 (NVD).
How do I remediate CVE-2026-55558?
Patch to the fixed version published by the affected vendor. Where vendor advisories exist for CVE-2026-55558, EchelonGraph cross-links them in the Vendor Advisories panel below — those typically contain the canonical remediation steps, fixed version numbers, and any vendor-specific mitigations.

Dependency Blast Radius

See which npm, PyPI, Go, and Maven packages are affected by CVE-2026-55558

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