fetchmail 6.3.5 and 6.3.6 before 6.3.6-rc4, when refusing a message delivered via the mda option, allows remote attackers to cause a denial of service (crash) via unknown vectors that trigger a NULL pointer dereference when calling the (1) ferror or (2) fflush functions.
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.