CVE-2026-55558 is a medium-severity (CVSS 5.9) CWE-74 vulnerability in aiosmtplib. A fix is available for aiosmtplib — see the affected versions and patch details below.
aiosmtplib: STARTTLS response injection
Exploitation Status
No confirmed exploitation observed yet
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-55558.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
CVE-2026-55558 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.
Where this sits among everything scored
Of 385,987 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.
Real-World Exposure
aiosmtplibReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.
Description
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 start_tls=True or start_tls=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 buffered, completes the handshake, and then parses the attacker's pre-staged plaintext as the first post-TLS server response. This also desynchronizes every subsequent command/response pair inside the "encrypted" session.
Not affected: Connections using implicit/direct TLS (use_tls=True) have no plaintext phase and are not vulnerable. The attack requires an active man in the middle via network compromise; a passive eavesdropper cannot exploit it.
Patches
A fix is available in aiosmtplib 5.1.2. All earlier versions that support STARTTLS are affected; upgrade to 5.1.2 or later.
The fix treats any data buffered after the 220 STARTTLS reply and before the handshake as a protocol violation, per RFC 3207 §4.2 ("the client MUST discard any knowledge obtained from the server … which was not obtained from the TLS negotiation itself").
Workarounds
If you cannot upgrade immediately:
- Use implicit TLS instead of STARTTLS. Connect with
use_tls=True. This removes the plaintext phase entirely. - If STARTTLS is unavoidable, restrict connections to servers reached over a trusted network path.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐍PyPI | aiosmtplib | all versions | 5.1.2pip install --upgrade 'aiosmtplib==5.1.2' |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for aiosmtplib, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update aiosmtplib to 5.1.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-55558 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
Frequently Asked Questions
Is CVE-2026-55558 in your dependencies?
Find it across PyPI, including transitive dependencies.