Older versions of libsoup are prone to integer overflow in memory requirement computation in the soup_base64_encode() in libsoup/soup-misc.c:

216 char *
217 soup_base64_encode (const char *text, int len)
218 {
219         unsigned char *out;
220         int state = 0, outlen,  save = 0;
221 
222         out = g_malloc (len * 4 / 3 + 5);

If the large enough untrusted input is passed to the function, insufficient amount of memory is allocated, followed by a heap-based buffer overflow with the Base64 encoded data.

Note: This issue does not affect current upstream versions of libsoup.  Newer version in 2.2.x branch implement base64 encoding / decoding functions as thin wrappers around functions provided by glib; 2.4.x branch does not provide base64 encoding / decoding functions any more.