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CVE-2026-55558— aiosmtplib: STARTTLS response injection

Quick assessment

Affected
cole aiosmtplib
Exploitation
No confirmed in-the-wild exploitation; assess based on exposure
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

Cole Maclean aiosmtplib是Cole Maclean个人开发者的一个异步SMTP客户端库。 Cole Maclean aiosmtplib 5.1.2之前版本存在输入验证错误漏洞，该漏洞源于SMTPProtocol.start_tls在启动TLS握手时未清除SMTPProtocol._buffer，攻击者可在明文220响应后插入响应行，导致后续SMTP命令和响应对失步。

CVSS 5.9 · Medium EPSS 0.40% · P33

Affected Version Matrix 1

VendorProduct Version RangeStatus
cole aiosmtplib < 5.1.2 affected
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I. Basic Information for CVE-2026-55558

Vulnerability Information
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Vulnerability Title
aiosmtplib: STARTTLS response injection
Source: CVE Program / CVE List V5
Vulnerability Description
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.
Source: CVE Program / CVE List V5
CVSS Information
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N
Source: CVE Program / CVE List V5
Vulnerability Type
输出中的特殊元素转义处理不恰当(注入)
Source: CVE Program / CVE List V5
Vulnerability Title
Cole Maclean aiosmtplib 输入验证错误漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
Cole Maclean aiosmtplib是Cole Maclean个人开发者的一个异步SMTP客户端库。 Cole Maclean aiosmtplib 5.1.2之前版本存在输入验证错误漏洞，该漏洞源于SMTPProtocol.start_tls在启动TLS握手时未清除SMTPProtocol._buffer，攻击者可在明文220响应后插入响应行，导致后续SMTP命令和响应对失步。
Source: CNNVD (China National Vulnerability Database)
CVSS Information
N/A
Source: CNNVD (China National Vulnerability Database)
Vulnerability Type
N/A
Source: CNNVD (China National Vulnerability Database)

Affected Products

Vendor Product Affected Versions CPE Subscribe
cole aiosmtplib < 5.1.2 -

II. Public POCs for CVE-2026-55558

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III. Intelligence Information for CVE-2026-55558

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Patches & Fixes for CVE-2026-55558 (1)

Vendor Advisories for CVE-2026-55558 (1)

Vendor Pages for CVE-2026-55558 (1)

IV. Related Vulnerabilities

Same product: aiosmtplib
Same vendor: cole
Same weakness: CWE-74