Fetchmail before 6.4.22 fails to enforce STARTTLS session encryption in some circumstances, such as a certain situation with IMAP and PREAUTH.

report_vbuild in report.c in Fetchmail before 6.4.20 sometimes omits initialization of the vsnprintf va_list argument, which might allow mail servers to cause a denial of service or possibly have unspecified other impact via long error messages. NOTE: it is unclear whether use of Fetchmail on any realistic platform results in an impact beyond an inconvenience to the client user.

Fetchmail 5.0.8 through 6.3.21, when using NTLM authentication in debug mode, allows remote NTLM servers to (1) cause a denial of service (crash and delayed delivery of inbound mail) via a crafted NTLM response that triggers an out-of-bounds read in the base64 decoder, or (2) obtain sensitive information from memory via an NTLM Type 2 message with a crafted Target Name structure, which triggers an out-of-bounds read.

fetchmail 5.9.9 through 6.3.19 does not properly limit the wait time after issuing a (1) STARTTLS or (2) STLS request, which allows remote servers to cause a denial of service (application hang) by acknowledging the request but not sending additional packets.

fetchmail 4.6.3 through 6.3.16, when debug mode is enabled, does not properly handle invalid characters in a multi-character locale, which allows remote attackers to cause a denial of service (memory consumption and application crash) via a crafted (1) message header or (2) POP3 UIDL list.

socket.c in fetchmail before 6.3.11 does not properly handle a '\0' character in a domain name in the subject's Common Name (CN) field of an X.509 certificate, which allows man-in-the-middle attackers to spoof arbitrary SSL servers via a crafted certificate issued by a legitimate Certification Authority, a related issue to CVE-2009-2408.

sink.c in fetchmail before 6.3.9 allows context-dependent attackers to cause a denial of service (NULL dereference and application crash) by refusing certain warning messages that are sent over SMTP.