int EVP_EncryptUpdate(EVP_CIPHER_CTX *ctx, unsigned char *out, int *outl,
                      const unsigned char *in, int inl)
{
    int i, j, bl;

    if (ctx->cipher->flags & EVP_CIPH_FLAG_CUSTOM_CIPHER) {
        i = ctx->cipher->do_cipher(ctx, out, in, inl);
        if (i < 0)
            return 0;
        else
            *outl = i;
        return 1;
    }

    if (inl <= 0) {
        *outl = 0;
        return inl == 0;
    }

    if (ctx->buf_len == 0 && (inl & (ctx->block_mask)) == 0) {
        if (ctx->cipher->do_cipher(ctx, out, in, inl)) {
            *outl = inl;
            return 1;
        } else {
            *outl = 0;
            return 0;
        }
    }
    i = ctx->buf_len;
     bl = ctx->cipher->block_size;
     OPENSSL_assert(bl <= (int)sizeof(ctx->buf));
     if (i != 0) {
        if (bl - i > inl) {
             memcpy(&(ctx->buf[i]), in, inl);
             ctx->buf_len += inl;
             *outl = 0;
            return 1;
        } else {
            j = bl - i;
            memcpy(&(ctx->buf[i]), in, j);
            if (!ctx->cipher->do_cipher(ctx, out, ctx->buf, bl))
                return 0;
            inl -= j;
            in += j;
            out += bl;
            *outl = bl;
        }
    } else
        *outl = 0;
    i = inl & (bl - 1);
    inl -= i;
    if (inl > 0) {
        if (!ctx->cipher->do_cipher(ctx, out, in, inl))
            return 0;
        *outl += inl;
    }

    if (i != 0)
        memcpy(ctx->buf, &(in[inl]), i);
    ctx->buf_len = i;
    return 1;
}

This weakness involves numeric computation errors, such as integer overflows, underflows, or precision losses, which can lead to miscalculations and exploitable behaviors in software.