GHSA-vxj7-4xrp-5vr4: aiosmtplib (CVE-2026-55558)

Package	Ecosystem	Affected	Fixed in
aiosmtplib	PyPI	<5.1.2	5.1.2

Machine-written summary

Summary written by Claude Haiku from the source advisory. Not reviewed by a person.

Affected: aiosmtplib versions before 5.1.2. When a connection is upgraded with STARTTLS (start_tls=True or None), the client keeps data buffered after the server's 220 reply and parses it as if it came over the encrypted session. An attacker who can actively intercept the plaintext connection can inject fake responses that the client accepts after the TLS handshake. Upgrade to aiosmtplib 5.1.2 or later. Until then, use implicit TLS (use_tls=True), which has no plaintext phase.

Written 1 Nov 2026.

Not analysed yet

We haven’t published our analysis of this Advisory yet. Once a person has reviewed it, this page will say what’s vulnerable, what has to be true for your app to be affected, and how to check.

Severity and exploitation

Score	Rating	CVSS	Vector	Source
5.9	Medium	3.1	CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N	CNA (GitHub_M)

CISA KEV Not listed.
EPSS 0.0040, the 35th percentile, on 12 Nov 2026: FIRST’s estimate of the chance of exploitation in the next 30 days.

Timeline

31 Oct 2026, 14:10 UTC First seen by agentprime
31 Oct 2026, 14:10 UTC Confirmed by OSV

Sources

Facts on this page come from the GitHub Advisory Database (CC-BY 4.0), OSV and the other sources listed, as published. We don’t reproduce full advisory descriptions, which can contain exploit code.