CVE-2026-55558

MEDIUMPre-NVD 5.95.9
EchelonGraph scoreMEDIUM confidence

“—” beside the score: it is equal to the CVSS score beside it.

This medium-severity CVE scores 5.9 under a secondary CVSS source (NVD's own analysis pending). EPSS exploit probability: 0.4% (32nd percentile of EPSS-scored CVEs). No GitHub Security Advisory CVSS is on record for this CVE.

Triggered by: Headline CVSS from a secondary CVSS source
Sources: EPSS, a secondary CVSS source
5.9EG
EchelonGraph verdictMonitorRemediate within your standard update timelines (the SSVC decision on this page).
  • Lower severity and no public exploit yet
CISA-KEV: Not listedEPSS PROB: 0.4%CVSS: 5.9Exploit: None knownExposed services: Not assessed

A fix is available — apply it.

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
5.9MEDIUMmedium confidence
EG Risk
27
EG Risk 27/100

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 (value × EchelonGraph’s weight)
Severity59% × 45%
The EG Score, 5.9 of 10.
Exploitation0.4% × 40%
FIRST EPSS: the 30-day exploitation probability when EchelonGraph scored this CVE.
Automatability0% × 15%
CISA's SSVC Automatable value: no (Vulnrichment).
SSVC (CISA's decision table, applied by EchelonGraph): Track at every mission impact level.
Exploitation none (CISA Vulnrichment) · Automatable no (CISA Vulnrichment) · Technical impact partial (CISA Vulnrichment). All three are CISA's SSVC values (Vulnrichment). EchelonGraph applied CISA's decision table to them; CISA publishes SSVC inputs and the table, not a decision for each CVE.
Action: A fix is available. Apply it within your standard update timelines.
EPSS PROB
0.4%
EPSS %ILE
32nd
KEV
Not listed

SSVC (CISA's decision table, applied by EchelonGraph)Track at every mission impact level.

A fix is available. Apply it within your standard update timelines.

Exploitation none (CISA Vulnrichment) · Automatable no (CISA Vulnrichment) · Technical impact partial (CISA Vulnrichment). All three are CISA's SSVC values (Vulnrichment). EchelonGraph applied CISA's decision table to them; CISA publishes SSVC inputs and the table, not a decision for each CVE. Mission impact is CISA's Mission & Well-being decision point, and only you can judge it. CISA's table makes it high when Mission Prevalence is Essential — the vulnerable component "directly provides capabilities that constitute at least one MEF for at least one entity" (MEF: mission essential function) — or when Public Well-Being Impact is Irreversible: "multiple fatalities are likely", the cyber-physical system "is likely lost or destroyed", "extreme or serious externalities" are imposed on other parties, or social systems such as elections or the financial grid "are destabilized and potentially collapse". CISA's decision table

BOD 26-04 (CISA's remediation timeline, Table 1)60 days if the affected system is publicly exposed; Fix on system upgrade if it is not.

In CISA KEV no (CISA's KEV catalog) · Automatable no (CISA Vulnrichment) · Technical impact partial (CISA Vulnrichment). Whether your system is publicly exposed is yours to answer. BOD 26-04 binds US Federal Civilian Executive Branch agencies; for anyone else it is CISA's published timeline, a reference rather than an obligation. The directive · how EchelonGraph applies it

Published

August 20, 2026

Last Modified

September 18, 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 advisoryMentions a patch (unverified)

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

(4 across 4 ecosystems)
Debian:12(1)
PackageVulnerable rangeFix by version rangeDependents
aiosmtplib2.0.0-1 ... 5.1.3-1 (14 versions)
  • every version on: no fix on record
—
Debian:13(1)
PackageVulnerable rangeFix by version rangeDependents
aiosmtplib4.0.0-1 ... 5.1.3-1 (10 versions)
  • every version on: no fix on record
—
Debian:14(1)
PackageVulnerable rangeFix by version rangeDependents
aiosmtplib4.0.0-1 ... 5.1.0-3 (8 versions)
  • every version up to 5.1.2-1: fixed in 5.1.2-1
—
PyPI(1)
PackageVulnerable rangeFix by version rangeDependents
aiosmtplib0.1 ... 5.1.1 (37 versions)
  • every version up to 5.1.2: fixed in 5.1.2
—

Weakness Classification(1)

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

Data Freshness Timeline

(refreshed 12× in last 7d / 50× in last 30d)

Each row is a time one of our pipelines fetched, updated or re-scored this CVE. 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. An arrow marks a change the record states ("CVSS v3 → 9.1"); EPSS and GHSA rows show the values recorded at that time, not what changed (EPSS values are rounded; hover one for the exact figures). Most recent first.

  1. 2026-10-13 16:34 UTCEPSS rescoreEPSS 0.4% · 32nd percentile
  2. 2026-10-12 16:34 UTCEPSS rescoreEPSS 0.4% · 32nd percentile
  3. 2026-10-12 15:55 UTCEG score recompute
  4. 2026-10-11 19:39 UTCEG score recompute
  5. 2026-10-11 16:54 UTCEG score recompute
  6. 2026-10-11 14:17 UTCEG score recompute
  7. 2026-10-11 13:49 UTCEPSS rescoreEPSS 0.4% · 32nd percentile
  8. 2026-10-10 20:00 UTCEPSS rescoreEPSS 0.4% · 32nd percentile
  9. 2026-10-09 20:07 UTCEPSS rescoreEPSS 0.4% · 32nd percentile
  10. 2026-10-08 17:12 UTCEPSS rescoreEPSS 0.4% · 32nd percentile
  11. 2026-10-08 07:21 UTCEG score recompute
  12. 2026-10-07 23:48 UTCEPSS rescoreEPSS 0.4% · 32nd percentile
  13. 2026-10-07 07:21 UTCEG score recompute
  14. 2026-10-06 15:18 UTCEPSS rescoreEPSS 0.4% · 32nd percentile
  15. 2026-10-06 07:53 UTCEG score recompute
  16. 2026-10-05 17:43 UTCEPSS rescoreEPSS 0.4% · 32nd percentile
  17. 2026-10-05 05:24 UTCEG score recompute
  18. 2026-10-04 23:22 UTCEPSS rescoreEPSS 0.4% · 32nd percentile
  19. 2026-10-04 06:00 UTCEG score recompute
  20. 2026-10-03 14:26 UTCEPSS rescoreEPSS 0.4% · 32nd percentile
  21. 2026-10-03 06:38 UTCEG score recompute
  22. 2026-10-02 17:56 UTCEPSS rescoreEPSS 0.4% · 32nd percentile
  23. 2026-10-02 07:09 UTCEG score recompute
  24. 2026-10-01 19:50 UTCEPSS rescoreEPSS 0.4% · 32nd percentile
  25. 2026-10-01 07:43 UTCEG score recompute
Show 66 more
  1. 2026-09-30 15:03 UTCEPSS rescoreEPSS 0.4% · 32nd percentile
  2. 2026-09-30 08:21 UTCEG score recompute
  3. 2026-09-29 15:22 UTCEPSS rescoreEPSS 0.4% · 31st percentile
  4. 2026-09-29 02:07 UTCEG score recompute
  5. 2026-09-28 02:43 UTCEG score recompute
  6. 2026-09-27 13:48 UTCEPSS rescoreEPSS 0.4% · 31st percentile
  7. 2026-09-27 03:20 UTCEG score recompute
  8. 2026-09-26 15:59 UTCEPSS rescoreEPSS 0.4% · 31st percentile
  9. 2026-09-24 19:28 UTCEG score recompute
  10. 2026-09-24 14:04 UTCEPSS rescoreEPSS 0.4% · 31st percentile
  11. 2026-09-23 20:05 UTCEG score recompute
  12. 2026-09-23 17:54 UTCEPSS rescoreEPSS 0.3% · 18th percentile
  13. 2026-09-22 19:46 UTCEG score recompute
  14. 2026-09-21 18:33 UTCEG score recompute
  15. 2026-09-20 20:16 UTCEPSS rescoreEPSS 0.3% · 18th percentile
  16. 2026-09-19 19:43 UTCEG score recompute
  17. 2026-09-18 20:20 UTCEG score recompute
  18. 2026-09-18 19:28 UTCEPSS rescoreEPSS 0.3% · 18th percentile
  19. 2026-09-17 21:23 UTCEG score recompute
  20. 2026-09-17 19:31 UTCEPSS rescoreEPSS 0.3% · 18th percentile
  21. 2026-09-16 21:46 UTCEG score recompute
  22. 2026-09-16 14:08 UTCEPSS rescoreEPSS 0.3% · 17th percentile
  23. 2026-09-16 05:15 UTCEPSS rescoreEPSS 0.3% · 17th percentile
  24. 2026-09-15 21:49 UTCEG score recompute
  25. 2026-09-15 03:11 UTCEPSS rescoreEPSS 0.3% · 17th percentile
  26. 2026-09-13 23:00 UTCEG score recompute
  27. 2026-09-13 16:47 UTCEPSS rescoreEPSS 0.3% · 17th percentile
  28. 2026-09-12 23:37 UTCEG score recompute
  29. 2026-09-12 00:13 UTCEG score recompute
  30. 2026-09-11 14:53 UTCEPSS rescoreEPSS 0.3% · 17th percentile
  31. 2026-09-11 00:47 UTCEG score recompute
  32. 2026-09-10 09:35 UTCEPSS rescoreEPSS 0.3% · 17th percentile
  33. 2026-09-09 02:00 UTCEG score recompute
  34. 2026-09-08 22:01 UTCEPSS rescoreEPSS 0.3% · 17th percentile
  35. 2026-09-07 19:21 UTCEG score recompute
  36. 2026-09-07 16:01 UTCEPSS rescoreEPSS 0.3% · 17th percentile
  37. 2026-09-06 19:58 UTCEG score recompute
  38. 2026-09-06 13:47 UTCEPSS rescoreEPSS 0.3% · 17th percentile
  39. 2026-09-05 20:34 UTCEG score recompute
  40. 2026-09-04 21:12 UTCEG score recompute
  41. 2026-09-04 05:06 UTCEPSS rescoreEPSS 0.3% · 17th percentile
  42. 2026-09-02 21:11 UTCEG score recompute
  43. 2026-09-02 14:12 UTCEPSS rescoreEPSS 0.3% · 17th percentile
  44. 2026-09-01 19:01 UTCEG score recompute
  45. 2026-09-01 13:54 UTCEPSS rescoreEPSS 0.3% · 17th percentile
  46. 2026-09-01 04:40 UTCEPSS rescoreEPSS 0.3% · 17th percentile
  47. 2026-08-31 19:37 UTCEG score recompute
  48. 2026-08-30 19:17 UTCEPSS rescoreEPSS 0.3% · 17th percentile
  49. 2026-08-30 17:07 UTCEG score recompute
  50. 2026-08-30 01:22 UTCEPSS rescoreEPSS 0.3% · 17th percentile
  51. 2026-08-29 17:44 UTCEG score recompute
  52. 2026-08-28 21:41 UTCEPSS rescoreEPSS 0.3% · 17th percentile
  53. 2026-08-26 17:54 UTCEG score recompute
  54. 2026-08-26 14:46 UTCEPSS rescoreEPSS 0.3% · 17th percentile
  55. 2026-08-25 17:14 UTCEG score recompute
  56. 2026-08-25 13:49 UTCEPSS rescoreEPSS 0.3% · 17th percentile
  57. 2026-08-24 17:51 UTCEG score recompute
  58. 2026-08-23 18:28 UTCEG score recompute
  59. 2026-08-23 00:19 UTCEPSS rescoreEPSS 0.3% · 17th percentile
  60. 2026-08-22 19:06 UTCEG score recompute
  61. 2026-08-21 23:49 UTCEPSS rescoreEPSS 0.3% · 17th percentile
  62. 2026-08-20 20:20 UTCEG score recompute
  63. 2026-08-20 20:02 UTCEG score recompute
  64. 2026-08-20 15:27 UTCEG score recompute
  65. 2026-08-20 14:42 UTCEG score recomputeFirst assessed: 5.9 MEDIUM
  66. 2026-08-20 14:41 UTCMITRE cvelistV5first tracked

Related CVEs(same product + same CWE)

Same product

2 shown

PyPI:aiosmtplib · Debian:12:aiosmtplib

Frequently asked(5)

What is CVE-2026-55558?
CVE-2026-55558 is a medium-severity vulnerability; its CVE record was 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 the CVE record for CVE-2026-55558 published?
The CVE record for CVE-2026-55558 was published on August 20, 2026. That is the record's publication date; the vulnerability itself may have been made public earlier. EchelonGraph's copy of the record carries a last-modified date of September 18, 2026; the record at its source may have been updated since.
Is CVE-2026-55558 actively exploited?
CVE-2026-55558 is not currently on CISA's Known Exploited Vulnerabilities catalog. FIRST EPSS estimates a 0.4% probability of exploitation in the next 30 days (32nd percentile of EPSS-scored CVEs).
What is the CVSS score of CVE-2026-55558?
CVE-2026-55558 has a CVSS base score of 5.9 (a secondary CVSS source that NVD displays; NVD's own analysis pending).
How do I remediate CVE-2026-55558?
A fix for CVE-2026-55558 is available: update to the fixed version the vendor names in its advisory. The vendor advisories EchelonGraph has for CVE-2026-55558 are linked in the Vendor Advisories panel on this page.

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