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.
The product receives input or data, but it does not validate or incorrectly validates that the input has the properties that are required to process the data safely and correctly.