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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.

aiosmtplib 是一个用于 asyncio 的异步 SMTP 客户端库。在版本 5.1.2 之前， 中的 方法会消耗服务器返回的 220 响应，并在未清空 的情况下启动 TLS 握手。活跃的网络攻击者可以在同一网络段中，在明文 220 响应之后插入由攻击者控制的 SMTP 响应行。随后，该方法调用 ；这些残留字节会在传输层升级后继续存在，并被解析为 TLS 会话内的第一个 SMTP 响应，从而导致后续的 SMTP 命令与响应对发生不同步。使用 或 opportunistic STARTTLS 的连接会受到此漏

CVSS 5.9 · Medium EPSS 0.26% · P17

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

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

V. Comments for CVE-2026-55558

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