aiosmtplib: STARTTLS response injection

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

5.9 · CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N

EPSS

• CVE-2026-55558: 0.40% estimated exploitation activity in the next 30 days; percentile 34.58 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Impact When a connection is upgraded with STARTTLS, aiosmtplib reads the server's 220 go-ahead reply and immediately performs the TLS handshake without discarding any data still sitting in the receive buffer. Bytes the protocol read off the plaintext socket before the handshake survive across the plaintext→TLS boundary (the asyncio transport is swapped in place, so the protocol object and its buffer are reused), and are then parsed as though they had arrived inside the TLS session. Who is affected: Any caller that uses STARTTLS by passing starttls=True or starttls=None when the server advertises STARTTLS, and whose traffic can be intercepted by an active network attacker on the plaintext leg of the connection. A man in the middle can send, in a single segment immediately after the client's STARTTLS command, the 220 reply followed by attacker-chosen response lines (e.g. 220 Go ahead\r\n250-mx.evil\r\n250 AUTH LOGIN\r\n). aiosmtplib consumes only the 220, leaves the injected lines…

Published by the source

27 Aug 2026, 23:32 UTC

Last modified by the source

27 Aug 2026, 23:32 UTC

Source last checked

26 Oct 2026, 13:50 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-vxj7-4xrp-5vr4

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# Patch Next advisory brief: CVE-2026-55558

Advisory: aiosmtplib: STARTTLS response injection
IDs: CVE-2026-55558, GHSA-vxj7-4xrp-5vr4
Record: https://patchnext.com/advisories/cve-2026-55558 (revision 1, source checked 26 Oct 2026, 13:50 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 5.9)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: aiosmtplib 5.1.2. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI aiosmtplib: affected <= 5.1.1; fixed in 5.1.2

## Suggested inspection (read-only)
- `python -m pip show aiosmtplib`: prints the installed version. Run with the environment that serves production active.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on aiosmtplib.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-vxj7-4xrp-5vr4 (GitHub-reviewed advisory GHSA-vxj7-4xrp-5vr4 lists affected ranges: PyPI aiosmtplib >=0 <=5.1.1)

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