Code: static int burl_normalize_2F_to_slash_fix (buffer *b, int qs, int i)
{
    char * const s = b->ptr;
    const int blen = (int)buffer_string_length(b);
    const int used = qs < 0 ? blen : qs;
    int j = i;
    for (; i < used; ++i, ++j) {
        s[j] = s[i];
        if (s[i] == '%' && s[i+1] == '2' && s[i+2] == 'F') {
            s[j] = '/';
            i+=2;
         }
     }
     if (qs >= 0) {
        memmove(s+j, s+qs, blen - qs);
        j += blen - qs;
     }
     buffer_string_set_length(b, j);
     return qs;
}

Vulnerability Type: DoS Overflow 
CWE ID: CWE-190
Summary: ** DISPUTED ** lighttpd before 1.4.54 has a signed integer overflow, which might allow remote attackers to cause a denial of service (application crash) or possibly have unspecified other impact via a malicious HTTP GET request, as demonstrated by mishandling of /%2F? in burl_normalize_2F_to_slash_fix in burl.c. NOTE: The developer states *The feature which can be abused to cause the crash is a new feature in lighttpd 1.4.50, and is not enabled by default. It must be explicitly configured in the config file (e.g. lighttpd.conf). Certain input will trigger an abort() in lighttpd when that feature is enabled. lighttpd detects the underflow or realloc() will fail (in both 32-bit and 64-bit executables), also detected in lighttpd. Either triggers an explicit abort() by lighttpd. This is not exploitable beyond triggering the explicit abort() with subsequent application exit.*
Commit Message: [core] fix abort in http-parseopts (fixes #2945)

fix abort in server.http-parseopts with url-path-2f-decode enabled

(thx stze)

x-ref:
  "Security - SIGABRT during GET request handling with url-path-2f-decode enabled"
  https://redmine.lighttpd.net/issues/2945