The corruption watchdog said the country's rapidly ageing population meant the care industry was growing very fast - and there was "too little transparency and opportunities for checking things for those concerned."
The system offers many possibilities to "economically exploit the dependence of people who need care," it added.
Barbara Stolterfoht, one of the authors of the Transparency International study into the system, said: "The large number of actors in the system and the statutory rules make it difficult to clear identify who is responsible for what. And this opens the door to fraud and corruption."
The study found cases where care service firms paid doctors to send patients to them - and where some of the most lucrative patients were then sold on to other care service companies, Der Spiegel magazine reported on Tuesday.
The magazine also spoke of cases where companies making care products such as orthopaedic shoes and Zimmer frames pay money to the operating companies of old people's homes to secure their orders for supplies.
Corruption also exists at a level even closer to the patient, Der Spiegel reported, with some insurers paying relevant staff at medical service firms for ranking their patients as needing as little as possible in order to keep costs down.
One major problem is that while some of the larger companies running old people's and care homes operate on a national level, the authorities in charge of checking their practices are organized on a regional, or sometimes state, level.
There is no national register of which firm has broken the rules how many times, making it impossible to expose systematic abuses of the system.
And experts from social support offices say they have only limited legal opportunities to act, and that every complaint of fraud is immediately countered with opposing legal measures such as a complaint of slander.
Transparency International called for a national register of abuses by care home operators and for social support offices to get more powers to punish those who break the rules.
Authorities should also test the economic reliability and quality offered by care service providers by regularly making surprise checks, Der Spiegel said.