CVE-2026-115559

Description

IMAP4.starttls() wraps the socket in TLS but leaves self._readbuf, imaplib's own read-ahead buffer, untouched. Any plaintext bytes that arrived in the same recv() as the tagged OK to STARTTLS are returned by readline() after the handshake, so the client treats them as the first responses on the protected channel. A network attacker who cannot break TLS can append forged lines after the server's OK, let the handshake complete with the real server and a valid certificate, and have the client accept the forged CAPABILITY and the forged tagged completion of the next command. This is the STARTTLS command/response injection class of CVE-2011-0411.

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EPSS — Exploit Prediction

0.0029

Probability of exploitation

0.22%

Percentile rank

EPSS estimates the probability that this vulnerability will be exploited in the wild within the next 30 days. A higher score means more likely to be exploited.

References

Other References

https://github.com/python/cpython/issues/160369 https://github.com/python/cpython/pull/160370 https://mail.python.org/archives/list/[email protected]/thread/LVAGFOFG26QVXEM6VXXPRMMOZBHM4CVB/

Frequently Asked Questions

What is CVE-2026-115559? +

IMAP4.starttls() wraps the socket in TLS but leaves self._readbuf, imaplib's own read-ahead buffer, untouched. Any plaintext bytes that arrived in the same recv() as the tagged OK to STARTTLS are returned by readline() after the handshake, so the client treats them as the first responses on the protected channel. A network attacker who cannot break TLS can append forged lines after the server's OK, let the handshake complete with the real server and a valid certificate, and have the client accept the forged CAPABILITY and the forged tagged completion of the next command. This is the STARTTLS command/response injection class of CVE-2011-0411.

How do I check if I'm vulnerable to CVE-2026-115559? +

You can use Secably's free Website Scanner to check your website for known vulnerabilities. For infrastructure scanning, use the Port Scanner to identify exposed services that may be affected. Check the vendor advisories linked above for specific patch and version information.